Advices needed: m20b20 bad rod bearings, rebuild or swap?

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Nagash
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Thu Dec 13, 2018 5:16 pm

Hi guys,

I used to have a good running e30 320i convertible, bought 4 years ago in Italy.
I invested some time to repair stuff with easy DIY jobs (vert top, brakes, ECU chip and electrical etc..) and of course base maintenance (belt, valve adjustment, engine/gearbox/diff oil etc..).

Unfortunately one of the rod bearings of my loved and hated m20b20 engine decided to randomly snap :|
Because of this, I probably also damaged the crankshaft.

I was thinking to swap a b25 or build an m20b27 using my block and installing b27 crank b20 rods and b25 pistons/head/intake.


After talking with a guy who is known to be an expert bmw preparator in the area where I live, I'm a little bit confused..
He considers the m20 a "shitty" and weak engine (for the head that is prone to crack, and probably timing belt..?), but actually he states that he is an expert for 4 pots bmw engines only (in particular s14..).
We all know s14 is THE Engine, and probably here in Italy we don't have the huge culture around m20 and 6 pots engines in general, so I'll take it easy with his words.

But actually I'm a little bit worried about the head problems, because I would like to have a reliable engine.. and I also talked to another guy who made the same build (b27 from b20) and broke the 885 head.

I know that b20 doesn't have the steam holes that b25 has and I don't understand if this could be an issue for 885 head.
I know 885 head is in general prone to crack more than 731 etc..

I'm not looking for HUGE performacnes, 180HP is actually more than ok for me (I have a vert..), I just need more low-end power/torque (you know.. b20 = no torque) but I don't want to completely lose the high end, because I really LOVE to rev the engine once in a while.

What do you think is the best compromise between power/torque and reliability in my situation?
- Buy used b20 and keep it stock revising the components (cheap ~300€ for a b20 in my area).
- Bore out b20 block to b25 and make m20b27 using b27 crank, b25 pistons and b20 (731) head (Daimlerman 2.7)
- Bore out b20 block to b25 and make m20b27 using b27 crank, b25 pistons and b25 (885) head (Building a 2.7)
- Bore out b20 block to b25 and make m20b25 using b25 crank, pistons and head...
- Start from b25 and keep it b25 revising components (~1500€ for a b25 not in good shape, in my area).
- Start from b25 and make a b27 (Building a 2.7)
- Build a b28 or others..
- Sell the car and buy a 325i sedan/coupé :cool:

Someone with some experience can give me some advice?
Thanks!
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reggid
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Fri Dec 14, 2018 8:56 am

cheaper to swap a nice running B25

alternatively a nice 2.7L (m20b25 slugs and head) from 3rd option from here

https://www.e30zone.net/e30wiki/index.p ... ding_a_2.7

but its quite a bit more work
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Nagash
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Thu Dec 20, 2018 1:56 pm

reggid wrote:
Fri Dec 14, 2018 8:56 am
cheaper to swap a nice running B25

alternatively a nice 2.7L (m20b25 slugs and head) from 3rd option from here

https://www.e30zone.net/e30wiki/index.p ... ding_a_2.7

but its quite a bit more work
Thanks for the answer reggid.

I'm trying to make a work plan and price quotation for the work, from a mechanic in my city.
Bore my block it's cheap (~250€) so I'm trying to follow this path (3rd option from the link).
The base idea is the following:

- Bore my block (80mm to 84mm )
- Get an used m20b25 head + inlet.

- Get a m20b27 crank OR a new custom crank (coupled with pistons/rods)
- Get a new 130mm rod (I have 5 goods from m20b20 and a broken one) OR a full set of 6 rods (coupled with pistons/crank)
- Get m20b25 pistons OR custom pistons (coupled with crank/rods)


Actually my mechanic said that we should use new pistons, because old ones are coupled with the cylinder in which they where running (because they tend to deform and "ovalize").

Do you think he is right?
What about using old pistons with a little bit larger bore and fit a set of larger piston rings? Is this a bad work?

Which new pistons should I look for? (I don't want to fit turbo, I'm looking for reliability and I don't need HUGE compression ratios)


Edit: I find out that it's possible to buy mahle m20b25 pistons (I think they are the original ones) from sites like this for ~£100 each.
And for about 1000$ I can found sets of forged pistons, like this.
Best choice? Forged or original? :D
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reggid
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Fri Dec 21, 2018 3:11 am

the decision on new pistons should be based on measurements. nothing wrong with new ones but could be $$$ unnecessarily. people reuse pistons all the time but depends on the condition of the pistons and bores you have. if you are re boring and honing from 80-84mm then a nice used set of pistons is all you need as the bores are going to be fresh and hopefully sized correctly

people have bore 80 to 84 mm id start with a 84 if i could depends what is available and cost

custom crank is out, unless you mean a 24V based crank with custom pistons

If you want custom the these http://www.iemotorsport.com/bmw/item/M2 ... stons.html from mahle MS are 85mm pistons for a M52b28 84mm stroke crank to make 2.9L (avoid generic pistons with an improper OE style dome) its is better but depends what you want and how much you want to pay things snow ball quickly when building engines

there is an alternative M20B28 option 84mm OE cast pistons, m52b28 84mm stroke crank 130mm rod thats an economy version of the 2.9L
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flybynite
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Fri Dec 21, 2018 9:24 am

I think with building an M20 it is going to come down to what you have to hand and what you can source easily. Along with what you can get done. The problem with the 2.7 combinations is that cranks are getting old and (at least in the UK) good ones are not overly common, whereas the later cranks are newer and now more plentiful.

Boring out a fresh 84mm is a good idea, however I read that some of the early 80mm blocks do not take to this well. I do not know for certain if this is the case or when it changed. I will know for sure when I get round to looking at the early 80mm block I have.

My pet hate is 130mm rods, after the experience I have had with the B23 I personally would not build an engine with 130mm rods and a stroke over 75mm

I'm only looking to build a M20B26 because I have the bits already. If I get any doubts about it I will just get it to be a good running M20B25 and be done with it. The M20B25 just runs so well.

And a lot of those new $100 pistons from those sites are not in stock and probably never will be
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Nagash
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Fri Dec 21, 2018 11:36 am

reggid wrote:
Fri Dec 21, 2018 3:11 am
people have bore 80 to 84 mm id start with a 84 if i could depends what is available and cost
I want to use my 80mm block for two main reasons:
- Cost: can't find m20b25 engines under 1500€ in Italy.
- Law: shitty Italian bureaucracy don't approve any modifications to engines or other big modification to cars :| keeping the original block code is a big benefit.

reggid wrote:
Fri Dec 21, 2018 3:11 am
If you want custom the these http://www.iemotorsport.com/bmw/item/M2 ... stons.html from mahle MS are 85mm pistons for a M52b28 84mm stroke crank to make 2.9L (avoid generic pistons with an improper OE style dome) its is better but depends what you want and how much you want to pay things snow ball quickly when building engines
That was an option (iemotorsport kit), but I would like to stay at 84mm. BTW I think they can make custom bore pistons as they say in the site.
The full kit is also an option ( http://www.iemotorsport.com/bmw/item/m20-29.html with mahle pistons and forged rods) but $2,670 it's a lot of money.

flybynite wrote:
Fri Dec 21, 2018 9:24 am
I think with building an M20 it is going to come down to what you have to hand and what you can source easily. Along with what you can get done. The problem with the 2.7 combinations is that cranks are getting old and (at least in the UK) good ones are not overly common, whereas the later cranks are newer and now more plentiful.
So m52b28 crank is the best option in your opinion? They also are for sure more common in Italy than m20b27 engines.
flybynite wrote:
Fri Dec 21, 2018 9:24 am
Boring out a fresh 84mm is a good idea, however I read that some of the early 80mm blocks do not take to this well. I do not know for certain if this is the case or when it changed. I will know for sure when I get round to looking at the early 80mm block I have.
Very interested on this. I hope to have a "late" block (I have motronic 1.3, engine produced in 1988).
Do you know the reason why some blocks should not take so well boring out to 84?
flybynite wrote:
Fri Dec 21, 2018 9:24 am
My pet hate is 130mm rods, after the experience I have had with the B23 I personally would not build an engine with 130mm rods and a stroke over 75mm

I'm only looking to build a M20B26 because I have the bits already. If I get any doubts about it I will just get it to be a good running M20B25 and be done with it. The M20B25 just runs so well.
I'm having a look to your topic (m20b26 build). It's interesting. Actually I would like to have a reliable engine more than everything (with of course some more torque in the low range and, why note, a little bit more power).

So your problem with 130mm rods was piston slap? What about building 2.7 with 135mm rods? (e.g. iemotorsport pistons/kit).
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flybynite
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Fri Dec 21, 2018 1:21 pm

The problem that I have heard is that some early 80mm blocks were too thin in places to bore to 84mm. It is what I have heard not what i have seen (yet) in practice. I am a way off stripping the B23 so do not know for sure. An '88 block should be OK though.

Lots of people have done 2.7, 2.8, etc etc but it is a personal thing. The B23 slapped and BMW could not stop it at the time even with new design pistons. It did not surprise me the next itteration went 84x75 with 135 rods.

I personally would not build anything with a 130mm rod and long throw, that includes the 2.8 from the M50/M52. Many obviously have but I would not.

I am only considering the B26 because i will have a good, known 76.8mm crank and it uses 135mm rods. my gut feeling at the moment is stick with M20B25

If you could find a good B27 81mm crank then it may work with 135mm rods but it seems an awful lot of work/expense over a good B25.
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Nagash
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Fri Dec 21, 2018 3:21 pm

flybynite wrote:
Fri Dec 21, 2018 1:21 pm
The problem that I have heard is that some early 80mm blocks were too thin in places to bore to 84mm. It is what I have heard not what i have seen (yet) in practice. I am a way off stripping the B23 so do not know for sure. An '88 block should be OK though.

Lots of people have done 2.7, 2.8, etc etc but it is a personal thing. The B23 slapped and BMW could not stop it at the time even with new design pistons. It did not surprise me the next itteration went 84x75 with 135 rods.

I personally would not build anything with a 130mm rod and long throw, that includes the 2.8 from the M50/M52. Many obviously have but I would not.

I am only considering the B26 because i will have a good, known 76.8mm crank and it uses 135mm rods. my gut feeling at the moment is stick with M20B25

If you could find a good B27 81mm crank then it may work with 135mm rods but it seems an awful lot of work/expense over a good B25.
As far as i understand, with b27 81mm crank I need 130mm rods OR 135mm rods + custom pistons (For example these https://www.tzr-motorsport.de/epages/61 ... cts/KE320M or the one from iemotorsport).

In the case of 130mm with BMW b25 pistons, I have to remove ~2mm from the block and use an adjustable camshaft pulley.
In the case of 135mm + custom pistons I hope I can use the block and the original camshaft pulley.

My main problem now is to find a good crankshaft (from b27/b25/b23 or maybe m52..).
And I don't know if it's a good idea to buy an used one online with the risk of having to rectify at a machine shop (assuming that we can rectify a crank..)

Actually this slapping problem is now terrifying me.. I don't want to build an engine for thousands of €€€ only to find out I have to open it again etc..

But do you really think that 5mm of rod length change so much the slapping problem?
Maybe your original b23 pistons were much more prone to slapping respect to 84mm pistons? In the end the b23 is 80mm bore, maybe the pistons are also shorter?
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Fri Dec 21, 2018 6:22 pm

Nagash wrote:
Fri Dec 21, 2018 3:21 pm
Actually this slapping problem is now terrifying me.. I don't want to build an engine for thousands of €€€ only to find out I have to open it again etc..
That is why I am not building one. A lot of people who have built strokers with 130mm rods may tell you it is fine, I have no experience of or with them. I would not risk the time, expense and effort when a standard B25 runs so well. An M20B26 was built stateside and by all accounts did not make much more power. I am only looking at it because I have the parts and a B26 is a bit more in keeping with the old chromie I have.
Nagash wrote:
Fri Dec 21, 2018 3:21 pm
But do you really think that 5mm of rod length change so much the slapping problem?
Maybe your original b23 pistons were much more prone to slapping respect to 84mm pistons? In the end the b23 is 80mm bore, maybe the pistons are also shorter?
It is a combination of everything including the pistons, but yes 5mm can make the difference, I don't have the answers but to be fair in 1985 neither did BMW when they unsuccessfully redesigned the pistons, but the longer the stroke and the shorter the rods the more sideways forces. Some slap, some just wear the bores oval, I have seen both. Having gone through it before I will not chance it again when there are plenty of better options.
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reggid
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Mon Dec 24, 2018 2:16 am

Nagash wrote:
Fri Dec 21, 2018 11:36 am
reggid wrote:
Fri Dec 21, 2018 3:11 am
people have bore 80 to 84 mm id start with a 84 if i could depends what is available and cost
I want to use my 80mm block for two main reasons:
- Cost: can't find m20b25 engines under 1500€ in Italy.
- Law: shitty Italian bureaucracy don't approve any modifications to engines or other big modification to cars :| keeping the original block code is a big benefit.

reggid wrote:
Fri Dec 21, 2018 3:11 am
If you want custom the these http://www.iemotorsport.com/bmw/item/M2 ... stons.html from mahle MS are 85mm pistons for a M52b28 84mm stroke crank to make 2.9L (avoid generic pistons with an improper OE style dome) its is better but depends what you want and how much you want to pay things snow ball quickly when building engines
That was an option (iemotorsport kit), but I would like to stay at 84mm. BTW I think they can make custom bore pistons as they say in the site.
The full kit is also an option ( http://www.iemotorsport.com/bmw/item/m20-29.html with mahle pistons and forged rods) but $2,670 it's a lot of money.

flybynite wrote:
Fri Dec 21, 2018 9:24 am
I think with building an M20 it is going to come down to what you have to hand and what you can source easily. Along with what you can get done. The problem with the 2.7 combinations is that cranks are getting old and (at least in the UK) good ones are not overly common, whereas the later cranks are newer and now more plentiful.
So m52b28 crank is the best option in your opinion? They also are for sure more common in Italy than m20b27 engines.
flybynite wrote:
Fri Dec 21, 2018 9:24 am
Boring out a fresh 84mm is a good idea, however I read that some of the early 80mm blocks do not take to this well. I do not know for certain if this is the case or when it changed. I will know for sure when I get round to looking at the early 80mm block I have.
Very interested on this. I hope to have a "late" block (I have motronic 1.3, engine produced in 1988).
Do you know the reason why some blocks should not take so well boring out to 84?
flybynite wrote:
Fri Dec 21, 2018 9:24 am
My pet hate is 130mm rods, after the experience I have had with the B23 I personally would not build an engine with 130mm rods and a stroke over 75mm

I'm only looking to build a M20B26 because I have the bits already. If I get any doubts about it I will just get it to be a good running M20B25 and be done with it. The M20B25 just runs so well.
I'm having a look to your topic (m20b26 build). It's interesting. Actually I would like to have a reliable engine more than everything (with of course some more torque in the low range and, why note, a little bit more power).

So your problem with 130mm rods was piston slap? What about building 2.7 with 135mm rods? (e.g. iemotorsport pistons/kit).
you cant get the good pistons in 84mm from IE. they give you some other generic custom junk i recommend you avoid this at ALL costs.

130mm rods work fine, if there is piston slap issue then its somewhere else and obviously the piston to wall clearance is wrong, a 5mm longer rod wont magically fix that some basic mathematics tells you the difference in angle is about 1 degree
.
if you get custom pistons then ones to suit a 135mm rod makes a whole lot more sense of course
Last edited by reggid on Mon Dec 24, 2018 2:24 am, edited 2 times in total.
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Mon Dec 24, 2018 2:19 am

flybynite wrote:
Fri Dec 21, 2018 1:21 pm
The problem that I have heard is that some early 80mm blocks were too thin in places to bore to 84mm. It is what I have heard not what i have seen (yet) in practice. I am a way off stripping the B23 so do not know for sure. An '88 block should be OK though.

Lots of people have done 2.7, 2.8, etc etc but it is a personal thing. The B23 slapped and BMW could not stop it at the time even with new design pistons. It did not surprise me the next itteration went 84x75 with 135 rods.

I personally would not build anything with a 130mm rod and long throw, that includes the 2.8 from the M50/M52. Many obviously have but I would not.

I am only considering the B26 because i will have a good, known 76.8mm crank and it uses 135mm rods. my gut feeling at the moment is stick with M20B25

If you could find a good B27 81mm crank then it may work with 135mm rods but it seems an awful lot of work/expense over a good B25.
B23 i had was a beautiful bullet proof silky little minx. still you only use the 130mm rod when it suits the parts you have
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reggid
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Mon Dec 24, 2018 2:28 am

Nagash wrote:
Fri Dec 21, 2018 3:21 pm
flybynite wrote:
Fri Dec 21, 2018 1:21 pm
The problem that I have heard is that some early 80mm blocks were too thin in places to bore to 84mm. It is what I have heard not what i have seen (yet) in practice. I am a way off stripping the B23 so do not know for sure. An '88 block should be OK though.

Lots of people have done 2.7, 2.8, etc etc but it is a personal thing. The B23 slapped and BMW could not stop it at the time even with new design pistons. It did not surprise me the next itteration went 84x75 with 135 rods.

I personally would not build anything with a 130mm rod and long throw, that includes the 2.8 from the M50/M52. Many obviously have but I would not.

I am only considering the B26 because i will have a good, known 76.8mm crank and it uses 135mm rods. my gut feeling at the moment is stick with M20B25

If you could find a good B27 81mm crank then it may work with 135mm rods but it seems an awful lot of work/expense over a good B25.
As far as i understand, with b27 81mm crank I need 130mm rods OR 135mm rods + custom pistons (For example these https://www.tzr-motorsport.de/epages/61 ... cts/KE320M or the one from iemotorsport).

In the case of 130mm with BMW b25 pistons, I have to remove ~2mm from the block and use an adjustable camshaft pulley.
In the case of 135mm + custom pistons I hope I can use the block and the original camshaft pulley.

My main problem now is to find a good crankshaft (from b27/b25/b23 or maybe m52..).
And I don't know if it's a good idea to buy an used one online with the risk of having to rectify at a machine shop (assuming that we can rectify a crank..)

Actually this slapping problem is now terrifying me.. I don't want to build an engine for thousands of €€€ only to find out I have to open it again etc..

But do you really think that 5mm of rod length change so much the slapping problem?
Maybe your original b23 pistons were much more prone to slapping respect to 84mm pistons? In the end the b23 is 80mm bore, maybe the pistons are also shorter?
if youre going to use the m52b28 crank with aftermarket forged pistons it would be a mistake to not use the IE 2.9L mahle MS 85 mm pistons. you can get similar custom pistons in 84 mm but its a bit trickier to order as you need to know who to speak to and what to ask for to get something that's going to be suitable (last years of trouble free service) and actually be better than stock.

if i was you id do the 84 mm bore OE 325i piston, 130 mm rod, m52b28 crank. you wont have slap issues if the machine shop is even half competent as you are boring and honing to suit a piston than can operate with very tight clearances.
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Mon Dec 24, 2018 5:27 pm

reggid wrote:
Mon Dec 24, 2018 2:19 am
B23 i had was a beautiful bullet proof silky little minx.
What did you have it in? was it an E30? if so was it a high power or low power 323i? Mine slapped like a bitch when cold and left witness marks down three sets of pistons.

Mine went all the way to BMW tech in Germany and when the revised pistons failed to cure the slap the only solution they suggested was turn it into a B20 or wait for the soon to be announced B25 which they said would cure the problem through a change in geometry. Which it did
reggid wrote:
Mon Dec 24, 2018 2:19 am
130mm rods work fine, if there is piston slap issue then its somewhere else and obviously the piston to wall clearance is wrong, a 5mm longer rod wont magically fix that some basic mathematics tells you the difference in angle is about 1 degree
Yes they work fine (in the right engine) in the B20 they were fine but that was with a 66mm stroke. it is the combination of longer throw with shorter rod that I do not like.
Andyboy wrote:
Thu Dec 16, 2010 2:56 pm
And yes, I meant to say less, not more! The longer the rods the better - less side thrust, less friction, less wear, more power. That's why BMW lengthened the rods on the 325i - the 323i had a longer stroke and the 130mm rods and they were buggers for piston slap.
This was my experience and I was not alone when I had this problem in the '80s, it was a common problem in an uncommon car.

Each to their own but personally I won't be building an M20 stroker with 130mm rods anytime soon. Too many better options
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Tue Dec 25, 2018 1:12 am

it was a 1983 version so the 102kW version not the 110kW updated one.

there is no argument that a longer rod produces less side loading and if you can use the longer rod then its preferred. its the claim that the short rod caused the slap and that an extra 5 mm would have stopped it that is being alluded to that is questionable.

the slap will be caused by the piston and bore interaction, the rod length is a 2nd order effect so would barely even exacerbate it. you are basically implying that if you were able to put the 135 mm rod in your engine that the slap would have never happened ? why would you even blame the rod when its clearly a piston problem and you admit that it works fine elsewhere?

if there is a common problem with the M20b23 then its probably an issue with the piston design itself where they didnt get the correct piston barrel shape and ovality to compensate for the thermal expansion so had to run more piston to wall clearance to avoid scuffing. this is common with some aftermarket forged pistons which can rattle when cold.

lets look at the con rod peak angle in a few different engines

M20B23 = 16.5 degrees (15.9 degrees with 135 mm rod)
M20B27 = 17.3 degrees
M52B28 = 17.3 degrees
M54B30 = 18.4 degrees
S54B32 = 18.1 degrees
B3 3.3 = 19.2 degrees

if the short rod was causing the issues there would be major issues with almost every modern BMW engine.

the actual less on to learn from is use a good piston design and this involves passing on many of the forged ones available. the OE m20b25 piston is a good piston you wont have any issues with a good set with good bores no matter what rod you shove in there. i know someone with 85.8mm stroke and 130mm rod.....
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Wed Dec 26, 2018 12:34 pm

reggid wrote:
Tue Dec 25, 2018 1:12 am
it was a 1983 version so the 102kW version not the 110kW updated one.
Mine is 1983 but 150bhp version and that is where the problem started.
reggid wrote:
Tue Dec 25, 2018 1:12 am
there is no argument that a longer rod produces less side loading and if you can use the longer rod then its preferred. its the claim that the short rod caused the slap and that an extra 5 mm would have stopped it that is being alluded to that is questionable.

Putting a 135mm rod in it and shortening pistons was one option explored by BMW as a cure. They said it worked.
reggid wrote:
Tue Dec 25, 2018 1:12 am
the slap will be caused by the piston and bore interaction, the rod length is a 2nd order effect so would barely even exacerbate it. you are basically implying that if you were able to put the 135 mm rod in your engine that the slap would have never happened ?


Indeed, it was done, and worked but it caused other problems. They would not authorise it as an official fix and I could not go it alone and run a ‘modified’ engine at the time. I could not easily get pistons that would work either back then.
reggid wrote:
Tue Dec 25, 2018 1:12 am
why would you even blame the rod when its clearly a piston problem and you admit that it works fine elsewhere?
Because I know the effort BMW put into re-designing a piston to cure this problem. Nothing they could do with the piston design in this engine cured the problem. Redesigning the engine with completely different geometry did.
reggid wrote:
Tue Dec 25, 2018 1:12 am
if there is a common problem with the M20b23 then its probably an issue with the piston design itself where they didnt get the correct piston barrel shape and ovality to compensate for the thermal expansion so had to run more piston to wall clearance to avoid scuffing. this is common with some aftermarket forged pistons which can rattle when cold.
As above they did a piston, I ran it and it did not work. They are still in the car and it doesn't just slap when cold. They admitted they went as far as they could go with that format. Their cure was the M20B25 325i. I even spoke to one of the people involved when I collected an M3 from the factory.
reggid wrote:
Tue Dec 25, 2018 1:12 am
lets look at the con rod peak angle in a few different engines

M20B23 = 16.5 degrees (15.9 degrees with 135 mm rod)
M20B27 = 17.3 degrees
M52B28 = 17.3 degrees
M54B30 = 18.4 degrees
S54B32 = 18.1 degrees
B3 3.3 = 19.2 degrees

if the short rod was causing the issues there would be major issues with almost every modern BMW engine.

the actual less on to learn from is use a good piston design and this involves passing on many of the forged ones available. the OE m20b25 piston is a good piston you wont have any issues with a good set with good bores no matter what rod you shove in there. i know someone with 85.8mm stroke and 130mm rod.....
Lots of strokers may not slap but they are hard on their rings and some bores get oval after very little mileage. Many don’t do the mileage to matter but lateral forces either cause or exacerbate problems in some of those modern BMW engines you listed. I have run some of those listed above too. Offset gudgeon pins can only do so much

But getting back on topic :thumb:

The OP asked what to do with a broken M20B20 to make it reliable. I gave him one opinion based on stripping a 323i engine down three times and running a M20B25 for many years thereafter.

Never said there was anything wrong with the rods per-se just not combined with a long throw in this engine. My opinion is there are better options for a reliable M20

I think it is great that people push these engines to get what they can, (including yours) and I am a huge fan of parts-bin engineering but in this particular case it is not for me, and I still think there are better options for the OP for what he wants. Some people do not want to push the engine to the max and damn the consequences.

Nothing I have read to date changes my opinion, but we could argue the toss over this till the cows come home so we will have to agree to disagree, the OP has some opinions to help him decide and it is Christmas after all …..there is still drinking to be done. :beer: and I gather it is warm down under ....better beer drinking weather. :beer: :beer:
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reggid
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Thu Dec 27, 2018 2:18 am

alot doesn't make sense, for example why the early 323i with same piston and rods doesn't have the problem....
why BMW after apparently designing the 325i with long rod to stop piston slap would design a new piston for the 1988 eta to fit the 325i head but not use the longer rod like alpina 2.7 engines if there was an issue. to me it all leads to the root cause actually being something else....

in the end people will do what they want anyway, even if they ask a forum for an opinion many times you see they have already made their mind made up.
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Nagash
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Fri Jan 04, 2019 4:18 pm

Guys thank you very much for your replies, you are helping me a lot.
The alpina 2.7 engine is actually the base idea behind the 2.7 builds, I think, and I don't think it suffered of unreliability (but idk actually!).

Maybe the smaller 80mm bore of the b23 has something to do with the piston slap? Because there is less surface in the piston skirts countering the angled force?
I'm not a mechanic eng.. so it's just a random supposition.


So @reggid, tell me if I understand correctly your opinion:

- m52b28 crank is the best crank I can use for the build:
this also let me avoid the vernier pulley for what I read in the wiki.


- The stock b25 pistons are ok.
Should I avoid used b25 pistons? Used ones probably have worn skirts.. in this case the piston slap problem is more likely?
I hope to find new b25 pistons somewhere, maybe at BMW, but I don't think they have.


- The only custom pistons you advice for a reliable stroker build is this one: http://www.iemotorsport.com/bmw/item/M2 ... stons.html
but it's 85mm only (84 not produced). I don't know if pushing the b20 bore to 85 it's dangerous.. actually it worries me a little, but it's just a feeling... I don't know the distance of water jacket from the cylinders in late b20 engines :? If I could find these numbers I could make a more thoughtful choice together with my mechanic.
But at least they are NEW pistons, and another benefit is that with those pistons I have to use 135mm rods instead of 130mm.


Thanks again to both of you :clap:
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reggid
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Sat Jan 05, 2019 4:11 am

- best would be subjective, for a 2.8L an M52b28 is probably the best;) but you want the early ones without the big counterweights

- you should avoid worn B25 pistons. getting new OE pistons from BMW would probably cost more than half a set of custom forged. a good used set is fine, may take some time to find.

- those are the only off the shelf forged piston i would consider, however these are not custom pistons as that is what you get there are no options. you can get custom forged in 84 mm from other suppliers but the correct dome, alloy among other things need to be specified to end up with a quality product so its a more involved process.

to be honest going from 80-84 or 80-85 might be much of a muchness IDK
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Nagash
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Mon Jan 07, 2019 2:03 pm

Thanks a lot reggid for all the info.

reggid wrote:
Sat Jan 05, 2019 4:11 am
- best would be subjective, for a 2.8L an M52b28 is probably the best;) but you want the early ones without the big counterweights
What about this crank? I have to look at the m52b28 NOT TU crank? Or maybe there are two different m52b28 not TU cranks..?
Is it easy to recognize the difference with a picture?
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reggid
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Tue Jan 08, 2019 8:01 am

you want the top one. the bottom one needs the counterweights trimmed

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Nagash
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Wed Jan 09, 2019 12:59 pm

Thanks, good picture!
I'm in touch with a guy who has two m52b28 cranks (the right ones) and should get a set of good used pistons and good 885 head!

All euro pistons are ok for the build? Or I need one specific (I think there are two or more kinds with different CR, depending on cat/no-cat).






edit:
from realoem I find out that there are a lot of different pistons:

Motronic 1.0: http://www.realoem.com/bmw/enUS/showpar ... Id=11_4320
Motronic 1.3: http://www.realoem.com/bmw/enUS/showpar ... Id=11_4320

So we have


Oldest (motronic 1.0):
  • Mahle pistons - up to 09/1987 - 1125170886 1/2/3/4 ENDED
  • Pistons (????) - up to 09/1987 - 1125171444 4/5
  • Pistons (????) - up to 09/1987 - 11251720865 ENDED

Mid (motronic 1.0) - used for both low compression and high compression engines:
  • Mahle pistons - up to 06/1988 - 112517148 07/08/09/10

Newest (motronic 1.3) - "weight improved":
  • Mahle pistons - from 06/1988 - 1125173575 5/6/7/8
On the web I can find some of the not-ended pistons (from BMW and other sites) in stock.
I found NOT-mahle oldest pistons (1125171444) for ~100€ each, but I don't know which kind of pistons is.. :? (Made by bmw? or by another Manufacturer?)

Anyone know which piston should I look for? Thanks!


Edit Edit:
Sorry, from the wiki I read that with m52b28 crank I should shave the skirt of the 325i pistons, especially the high CR pre 9/87.

IR Mahle pistons with 85mm bore could be the best options but they are really expensive (1200$ + shipping + import taxes).
So I think I can buy a set of new 11251735755 pistons from BMW or other online shops in europe.
Cool :mrgreen:
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reggid
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Fri Jan 11, 2019 8:04 am

the later ones are shorter skirt weight optimized probably KS (kolbenshmidt) brand if not mahle with profiled skirt.

there are different sizes (oversizes)
atleast two different manufacturers Mahle and KS
the different part numbers have short skirt, long skirt, flat bottom skirt, profiled bottom

the far right i think is best, its a KS and gives best clearance to the crankshaft counterweights. no idea on PN

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Nagash
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Sun Jan 13, 2019 3:35 pm

Any experience with Wiseco forged pistons ( https://www.tzr-motorsport.de/epages/61 ... cts/KE317M ) or other new pistons available in europe/UK shops?

I would buy a set of new pistons from IE motorsport, but import taxes almost double the price.. so it's impossible to buy.
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reggid
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Sun Jan 13, 2019 11:57 pm

i would not use those pistons, too many unknowns and the compression height is wrong if you want a 135 mm rod which is what you would want if you were getting expensive pistons, a good condition used OE set is fine
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