M42 Rotrex (for sale!) + track build

[tim_s]

yes i have!!!
Mine are JE, but there are loads of brands you can use and I have no particular loyalty. I'm happy with mine, however. I'm sorry I missed this thread, I haven't been playing with e30s for a while and not been on here much. I used TEP in hollywood, CA to spec mine as Steve Nelson knows BMW pistons inside out, so I could get right into the details. There are plenty of other pistons that have been used with success in an M42 too, what was wrong with M44 ones? Or S50 ones? Rings for the latter are expensive admittedly. XE pistons also go. XE and S50 need oversize SE bushes, both have 21mm pin iirc. I've used s50b32 pistons in an m42 before.
hth
tim

[GeoffBob]

Ta very much Tim. I'll have a look into those that you mentioned.

Had heard from Appletree with regard to the JE pistons but was lead to believe these were Mitsubishi Evo pistons and required a substantial rebore to fit? Or am I wrong, do JE make a forged piston specifically for the M42? Really helps when someone in the know points you in the direction that you need to go looking

As I recall I did look at the S50B32 pistons but they didn't drop the CR enough. Could be wrong so I'll look again.

No tone intended, sorry if it came across that way.

[rix313]

How much where the JE units Tim?

The idea behind the exercise is to find a simple standard replacement which doesn't cost the earth but meant I could get a little more poke from the system.

[tim_s]

No worries!!
As for the JE pistons, yes they're m42 specific, made based on blueprints of the original piston, but modified to fit my application. They'll make pretty much anything within reason- any bore, any CR, different bowls, dishes, reliefs etc. in a variety of materials and coatings (ceramic crown coating for turbo etc), various grooves and gas ports etc. For example, mine are 87mm with deeper valve reliefs and lower compression height to accomodate my long stroke crank. They'd have no problem at all making a set of low CR pistons for a turbo, you could then also look into all the various options and pros/cons. There's a lot to consider, by manipulating variuos bits on mine I have the CR I want, the chamber volume ratio between head and pistons is pretty much identical to factory, I have nearly the exact valve reliefs I wanted etc. Owing to this, they look very similar to the originals. Having said all of this, there are several other manufacturers who could do the same and even build pistons based on a piston if you sent it to them.
Here are mine (and MLS gasket/ARP bits! )

[appletree]

I did say that you could use the vauxhall XE pistons

Think you can use evo 7 or a vaux calibera turbo XE??? pistons

The low comp custom pistons i'am having made will be custom made by JE and although they wont be added to the JE catologue as they wont have been designed/tested by JE there will be a "Part number" for them so that i or others could order them again.

the company who ive paid to design the pistons will also have the number so he will be able to sell them aswell

Just takes time.....

[appletree]

Tim did you get the "part number" for the pistons you hade made?

IIRC the standard M42 pistons only required a 11cc dish in the to to drop the CR to 8.5:1

[tim_s]

lol no idea on the part no, i might have done matt, was ages ago! you getting them ceramic coated etc?! should be very tasty! They coming from the uk? who designed them?
I must admit I bought mine from the US ages ago cos of the exchange rate (about 2:1 at the time iirc!) etc, I wouldn't have a problem with a different piston manufacturer. Wossner, wiseco etc are supposed to be good, mb even accralite. I'm not sure about the totalseal rings I used to run either, i'll be interested to see how mine goes this time around when i rebuild it with pro seals (prob over christmas - been on a 1.9 for a while as i slightly hung an oil control ring rail on the block and put a little scratch down cyl 2 on the 2.1 so figured i'd do a fresh bore and new set of rings), I can now more or less compare compression numbers between conventional (pro seal) and the gapless (totalseal). Unfortunately I've had about 8 thou off the head too so will get slightly better numbers because of that.
I'd have thought m44 block and pistons with m42 crank would be the cheapest reliable way to get a solid engine and nice low CR. get a good block and you wouldn't have to mess about with anything, just bolt up the new crank and off you go. get a useful little extra 50cc or so too i'd imagine.

[tim_s]

just gave this a bit more thought, my m44 idea will save you probably around £500 over the m54 pistons and should give you better results. Too lazy to calculate accurately, but given m44 CR is 10:1, drop the pistons 1.2mm from the deck and you'll get under 9:1. Repair HG and you've another .3mm.
Budget-tastic

[rix313]

I'm giving it a lot of thought dude hence why we've been discussing it

I'm glad you've found this though, your experience with these engines offers a great perspective on practice Vs. Theory.

Interesting on the M44 bits. Can you just explain why an M44 block over the M42? Also, which rods would you use for this set up? Finaly, how do the M44 pistons differ from the M42?

[tim_s]

I'm giving it a lot of thought dude hence why we've been discussing it

I'm glad you've found this though, your experience with these engines offers a great perspective on practice Vs. Theory.

It's not really practice vs theory. It's just realising what options are out there. The m54 thing would work, i just think there are better options out there!

Interesting on the M44 bits. Can you just explain why an M44 block over the M42?

because then you won't need to rebore, rehone, buy new rings, run an engine in etc.

Finaly, how do the M44 pistons differ from the M42?

Lower pin height, diferent dia. etc.

[rix313]

I know what I meant ;)

M44 block
M44 pistons
M42 crank

Which rods would be required?

When you next in the Brizzle area?

[tim_s]

lol dude, the plan is simple:
take one m44
remove crank
fit m42 crank
done!

Am in bristol over christmas for a bit, will be driving the 330ci i expect though as the e30 will be getting some engine work!

[GeoffBob]

Tim, with due respect, you have presented a number of helpful ideas here, but nothing cast in concrete. All you have confirmed so far is exactly what I knew before - that, with the exception of the various offerings from Metric Mechanic, there is not one manufacturer of forged pistons out there who makes and catalogues a piston to fit the M42B18. If anyone doubts me on this, click here to see the list of BMW pistons made by JE. You’ll find a similar situation with Wiseco and others.

True, there are plenty of piston makers who will custom manufacture to special order, but this will cost more money than it is currently worth Rich putting into his engine. He's said this many time before. And to add insult to injury I understand from Appletree that JE have already once delivered the wrong pistons to him. So you'll have to excuse the fact that I find the idea of finding a catalogued piston available ex-stock both tempting and appealing.

Hence, so far as I am concerned this is an exercise in finding a catalogued ex-stock piston that can be easily replaced at short notice if need be, and is available ex-stock oversize to facilitate a rebore as the engine gets older.

While the M44 engine with M42 crank sounds like an interesting suggestion it is, I am afraid, exactly that - a suggestion. If anyone has hard facts, data, pros and cons, a list of known complications that must be overcome, etc then please post them up. If not, then these are all issues that Rich will have to work out with yourself as he progresses.

I apologise if this message sounds a little harsh, but as someone with a fair bit of experience in engine building, I have become naturally sceptical of unsubstantiated suggestions posted on forums (this is a general statement and not directed specifically at yourself Tim). I tend to deal in hard facts and engineering data, which I cannot help but feel your posts lack a fair bit of. With that said, I accept and acknowledge the spirit that your posts are offered in, but without supporting data I personally won't be investigating the options you have presented any further than I already have.

[GeoffBob]

Rich,

Fitting an M42B18 crank into an M44B19 engine will drop the compression ratio from 10:1 down to 8.93:1 . This option is entirely up to you.

None of the BMW pistons Tim lists for the M42B18 are an option as they have the incorrect crown height and incorrect diameter. Having already once been through the entire Mahle catalogue the only piston with both the correct diameter and crown-height to match your request is from the M54B30.

[tim_s]

Actually Geoff, you're not showing me the respect I'm due at all.
Do some research. If you'd done so, you wouldn't have to question me as you could have quite easily verified everything I've said. I'm not here to convince or prove myself to you, and I don't appreciate your tone.

Fwiw:
-JE can supply BMW forged pistons - they're not listed in their catalogue as they use TEP to distribute them. Have you called either TEP or JE? I even gave you Steve Nelson's name for god's sake.
-MM don't build M42 pistons, they order them in
-An m42 crank fits an m44, no complications. fact. Want to hazard a guess as to what's in my car at the moment? Clue: it's alluded to a few posts above in this thread.

[appletree]

Tim is TEP short for somthing? i cant find them on the net searching for TEP pistons?????

EDIT: found them and they do list pistons for the M42 and M44

http://www.racetep.com/jebmw.html


would be intrested to talk to them.

Geoff, it turns out the engine builder who told me JE sent the wrong pistons was lieing to buy himself some time..... its a long story and i'am not overy happy with the way he's dealed with the situation

[GeoffBob]

Do some research. If you'd done so, you wouldn't have to question me as you could have quite easily verified everything I've said. I'm not here to convince or prove myself to you, and I don't appreciate your tone.

Tim, I’m not the one building the engine here, Rich is. I’m just trying get you to be less vague and more accurate in the info you are posting. So here's some more tone for you . Just how much more "tone" do you require in order that we extract any specific data from you? For example, would you like to state clearly exactly which crank you have (could just as easily be from an M47 as from an M44) or would you prefer to continue, as you put it so accurately, to "allude" to it. Furthermore, Appletree had the decency to post a link to racetep.com - was that too much to ask of you in the first place? Now Rich can at least get on with job of comparing prices and make his own mind up what he wants to buy.

Actually Geoff, you're not showing me the respect I'm due at all.

Put down some hard data and forget the iffy/maybeish type suggestions you've made and you'll get my respect. Likewise, get your facts straight before you suggest pistons that don't fit the proposed application.

If you want to help Rich build a properly decent engine then the onus is upon you to prove yourself right, not us. You do this in polite society by backing up your suggestions with either references or data. Comments like

no idea on the part no.

or

Too lazy to calculate accurately

have no place here. I've been in this business too many years to put up with that sort of BS.

[tim_s]

At least Matt had the decency to post a link to racetep.com . Was that too much to ask of you in the first place?

Geoff, sorry I must have over-estimated your googling ability. It's the first link that comes up if you type "JE pistons BMW" into google. Or indeed if you type "JE pistons TEP" into google. Maybe that's the fundamental issue here, I've just assumed at least a basic level of intelligence and nouse from you?

Put down some hard data and forget the iffy/maybeish type suggestions you've made and you'll get my respect. Likewise, get your facts straight before you suggest pistons that don't fit.

No, actually Geoff you have an over-simplistic understanding of what 'fits' and doesn't. Finding a flat top piston (with rings for an alusil bore!!!) with a short pin height to give the CR you're looking for demonstrates this. Running flat top pistons in a car with a cyl head designed to run pistons with a big bowl is not ideal. Running a forced induction engine with modern NA pistons is not ideal. Running pistons with top ring close to crown on an FI engine is not ideal. Running cast pistons on an FI engine is not ideal. Paying £400 for them is not ideal. In short, they're not the best idea, especially as they cost way more than my m44 idea and give worse results. With your years of experience, I'd have thought you'd have realised this. Indeed, with your years of experience I would have thought that you'd have at least ensured that Rich was aware of the pros and cons of running these so that he could make an informed decision.

If you want to help Rich build a properly decent engine then the onus is upon you to prove yourself right, not us. You do this in polite society by backing up your suggestions with either references or data. Comments like

no idea on the part no.

or

Too lazy to calculate accurately

have no place here. I've been in this business too many years to put up with that sort of BS.

Ok ok I'll try to dumb it down a little bit for you in future. I'm afraid I don't have time to calculate everything for every request I get off internet forums. If I did, it would take a huge amount of my time. Indeed, as demonstrated here, in this case I also didn't need to. My estimate of under 9:1 was spot on as you yourself demonstrated, although the repair gasket will lower it some more.
And the onus is most definitely not upon me to prove myself right! It's an internet forum for god's sake!

[GeoffBob]

Geoff, sorry I must have over-estimated your googling ability. It's the first link that comes up if you type "JE pistons BMW" into google. Or indeed if you type "JE pistons TEP" into google. Maybe that's the fundamental issue here, I've just assumed at least a basic level of intelligence and nouse from you?

Tim, for the last time, I'm not the one building the M42 FI engine here, Rich is!

... In short, they're not the best idea, especially as they cost way more than my m44 idea and give worse results

Tim, I respect your reasons for not liking the M54 piston (indeed it seems I must have really pressed your button that you presented a detailed list of objections). In your own words, however, may I remind you that:

The m54 thing would work, i just think there are better options out there

And I respect your opinion on this.

However, lets be clear on this, I never at any point said a forged piston wasn't a better solution. I merely responded to Rich's request to specifically find an alternative to a forged piston. I respected his request and responded accordingly. By this point in time (regardless of his reasons) he had already clearly identified that he did not want to purchase forged pistons.

To my point, however, I specifically tackled you on the incorrect and minimal detail you presented in your posts of last night. S50 pistons, for example, will not work within this application. neither will M44 pistons.

As for your M44 idea, that's between you and Rich. I've done my bit by calculating the CR and I agree that the idea will work. It's for Rich to decide if he wants to buy an M44 engine.

I'm afraid I don't have time to calculate everything for every request I get off internet forums. If I did, it would take a huge amount of my time.

Do you think it takes any less of my time to help Rich and others out? It doesn't hurt that you back-up your answers with a little detail now does it - its called adding credibility. Believe it or not, it makes it a lot easier to decide whether or not to take your advice.

[tim_s]

Geoff, sorry I must have over-estimated your googling ability. It's the first link that comes up if you type "JE pistons BMW" into google. Or indeed if you type "JE pistons TEP" into google. Maybe that's the fundamental issue here, I've just assumed at least a basic level of intelligence and nouse from you?

Tim, for the last time, I'm not the one building the M42 FI engine here, Rich is!

Come on Geoff, you were the one complaining and insisting that JE didn't make BMW pistons and that I was generally wrong/inaccurate (incessantly).

Geoff, I'll try to give more info as its clear you're a sceptical guy, if you'd try to be a little more open-minded and trusting that would be great.

... In short, they're not the best idea, especially as they cost way more than my m44 idea and give worse results

Tim, I respect your reasons for not liking the M54 piston (indeed it seems I must have really pressed your button that you presented a detailed list of objections). However, lets be clear on this, I never at any point said a forged piston wasn't a better solution. I merely responded to Rich's request to specifically find an alternative to a forged piston. I respected his request and responded accordingly. By this point in time (regardless of his reasons) he had already clearly identified that he did not want to purchase forged pistons.

To my point, however, I specifically tackled you on the incorrect and minimal detail you presented. S50 pistons, for example, will not work within this application. neither will M44 pistons.

no antangonism intended, but I had the advantages of m44 pistons, not forged pistons, in mind when i wrote that about the m54 pistons, and both m44 and S50 evo pistons should work really well for this application from my brief in-my-head calcs (why would you 'agree from your calculations' that the m44 block would work but not just the pistons - it's essentially achieveing exactly the same engine but with different rods (not different length, just different design). You've insisted that I'm wrong or inaccurate and been refuted several times now, is getting a bit silly. A bit pot/kettle there! To use some Geoff-isms, care to elaborate on what's wrong with S50 or m44 pistons?
Blue Peter style, here's one I built earlier with s50 pistons... mmm s50 pistons.

As for your M44 idea, that's between you and Rich. I've done my bit by calculating CR and I agree that the idea will work. It's for Rich to decide if he wants to buy a fonor M44.

Lol, Geoff can you not see that your tone here's really antagonising? 'I've done my bit by calculating CR and I agree that the idea will work'. did you not ever stop to think for a second that I know the CR (I did state it after all), know that it works and that it's a blissfully simple idea not requiring any calculations (if you know how much impact changes have on the CR on a particular engine)? If so, a few secs thought would suffice to give you the necessary.

[rix313]

Geoff and Tim, I very much appreciate the skill and knowledge you both have. I would ask though that before there is much more to-ing and fro-ing we look at the two options which have been presented to me which I want to know more about? I haven't ever looked at engines in great detail (can you tell?) and I am wanting to and hopefuly will be learning alot from this exercise.

The M44 system does intrigue me and as you said Tim, will probably save me more money again which afterall is the reason we've been looking at alternatives.

Myself and Geoff (more Geoff admittedly) have been looking at and putting quite abit of time into the M54B30 piston idea, given that I don't want to just drop it and run.

[GeoffBob]

[GeoffBob]

Myself and Geoff (more Geoff admittedly) have been looking at and putting quite abit of time into the M54B30 piston idea, given that I don't want to just drop it and run.

Rich, if you can stretch your budget to forged pistons and con-rods I'd recommend it, I said this to you a long time ago (assuming you want to keep your M42). But within your budgetary guidelines, and with minimal modifications to your engine, the M54B30 piston was the best I could come up with. Lest anyone think otherwise, I am not advocating the M54B30 piston as the best solution, but certainly as the one which requires the least modification to your engine. I can assure you that you will not be able to achieve the required CR by fitting M44 or S50 pistons without substantial modification to your engine.

If you decide to fit an M44 motor with M42 crank then be aware that the CR will be marginally higher than with the M54 pistons in the M42 motor, and that you may have other as yet unforeseen issues to deal with. I'd check first, for example, that all of your additional fitments (unique to your setup) will locate as you require them to. Your Rotrex and relocated alternator, for example, and all the brackets that you have manufactured. Subtle differences and redundant features on the various castings could make your life a misery - so just make sure ahead of time and you should be alright. This assumes of course that you have no particular attachment of any sort to your current M42 motor (no large wads of cash that you have already put into it ).

[tim_s]

Come on, Geoff! One second you're saying you don't know of any BMW-fitting custom pistons and god knows how much effort you've put into finding some, next second by telling you of one, all I've done is confirmed what you already knew. Right, of course. And your MM info was wrong.

for s50/m44 pistons, you're wrong. again. M44 pistons, obviously, have a lower pin height than m42. I don't need a mahle catalogue to tell you that as I used my brain, nonetheless I'd have thought you'd have checked! (please do!!!) Because you'll accuse me of not spelling it out to you, how can a block of the same height with a longer stroke, the same length rods, and the pistons meeting the face (ok, more or less meeting the face if you're going to be ridiculously pedantic, which I'm sure you will) have pistons with a larger pin height?
As for S50 pistons, you'd have thought you'd have taken heed from my posts above and your own learning along the way that compression height is only one of many factors to consider when choosing a piston. The s50 evo piston not only fits (which you asserted it doesn't), but is also an interesting and quite good choice (with some reservations)!

I have no compunction whatsoever to take your word for it that the CR will be as you say.

I know, pretty dumb too - you'd have thought you'd have worked out that I actually know a little bit about these engines by now. And you've totally missed that you needn't have done so (calculated the CR), you could have used a bit of reasoning to work it out.

lots of unfounded stuff about unforeseen issues with m44s in m42s etc

Rich, come see me over christmas and I'll show you an m44 block next to an m42 one, a car with an m44 block in it etc. so you can see that i've done this lots of times and know the ins and outs. The m44 housing is identical, no packaging issues. Or alternatively just do a bit of research and you'll see several other people who will tell you the same.

Anyway, Rich you can get me on pm.

[GeoffBob]

for s50/m44 pistons, you're wrong. again. M44 pistons, obviously, have a lower pin height than m42.

Yes Tim, the M44 does have a lower crown-height, but not low enough. Likewise the S50B32 US (22mm pin) crown heght is not nearly short enough, and the S50B30 crown-height (also with the 22mm pin) is actually taller! Not to mention that the S50 pistons would require the block to be substantially overbored.

And WTF is this ?

lots of unfounded stuff about unforeseen issues with m44s in m42s etc

You actually have to fabricate a quote in order to argue with me

You can insult me and skirt the issue all you like Tim. All I'm hearing from you at this point in time is

Next time bring a little more technical know-how and a little less ego to the table, then we'll start counting how many engine's you've built.

Bye Bye now

[tim_s]

Geoff, I know you feel aggrieved, but please re-read this thread, I came on in good spirits to lend a hand and from the off you've been nothing but rude, aggressive and antagonistic with me on here. I'm sorry that some of the info that I've provided threatens you.
And tee hee, you're still wrong!
We've just spent ages going over how the M44 solution works fine and going on about packaging issues etc, now suddenly the pin height isn't short enough?
And CC some s50 pistons, calculate the CR on a standard crank and reassess you opinion. Don't believe even having seen a pic of my s50 piston'd m42 engine (that i ran with good results!) you didn't think that i might actually know their specs!
I've stated along your view that pin height is all that matters is way too simplistic, and why you've omitted to realise or highlight several drawbacks with using m54 pistons.

And I wouldn't profess for a second to have years and years of engine building experience, unlike yourself.
It's a shame that now I'm now talking to Rich over PM and not sharing info on the forum, all because of this.
See ya!

[GeoffBob]

We've just spent ages going over how the M44 solution works fine and going on about packaging issues etc, now suddenly the pin height isn't short enough?

Yes Tim - for the M42 engine. As I've said to you at least twice before, the M44 piston is shorter than the M42 piston, but not short enough to provide the required reduction in the CR.

And tee hee, you're still wrong!

So this is your argument in summary? Hardly a very sound technical retort.

Tim, without trying to offend you (and I mean this in earnest and with integrity) I'll explain the fundamental mistake that you are making in assuming that a M44 piston in an M42 engine results in the same CR as a M42 crank in an M44 engine.

The CR of an engine (and I apologise if I am teaching you to sucks eggs here, this is for the benefit of others reading this post) is the ratio of the chamber volume with the piston at BDC to the chamber volume with the piston at TDC. Now, while I fully agree with you that the M44 uses a shorter piston (and I fully accept your argument with regard to block height etc), the M44 piston has a diameter of 85mm compared to the M42's 84mm.

Now while this may not sound like much at the outset, this additional 1mm on the diameter (that must be bored into the block to accommodate the M44 piston) contributes a substantial additional volume to the combustion chamber when the piston is at at BDC due to the fact that it is 1mm extra (on the diameter) over the full 81mm stroke of the M42 engine. This means that while the chamber volume with the piston at TDC has indeed increased (due to the shorter M44 piston, and with a slightly larger dish I might add), so has the volume at BDC, and thus, the total ratio of volumes (by definition the CR) does not reduce by enough to meet the requirements of Rich's FI engine.

The situation is worsened with the 86mm diameter S50 pistons.

Tim, I'll happily discuss technical matters with you in a calm and rational fashion, but because of your ego you took it one step too far. When I challenged the validity and accuracy of the advice you posted you lost your temper and attacked me personally. Right from the get-go you were more concerned with my "tone" than you were with helping the OP.

Now, if you want to prove a point, prove it using sound engineering principles in a calm and rational manner, as I have done above. I can assure you that posting defamatory statements and fabricating quotes does nothing to help Rich find the right piston, and does everything to make you appear defensive and ignorant.

[Gunni]

Math calculations are the defining answer point here.
;)

Talk is cheap, calculations are gold.

[tim_s]

Geoff, we KNOW (no assumption) that M44 pistons in an M44 block with an M42 crank gives an IDENTICAL CR to M44 pistons in an M42 block with and M42 crank. According to you, that CR is 8.93:1. Both engines will have a bore of 85mm and are the same height, with the same length rods. Where's the difference in CR coming in? Where's the assumption there? There is none.

Plus even if this weren't the case, let's imagine a mythical m42 now with 85mm bore/pistons, everything else exactly the same inc bowl (which you seem to have forgotten will also increase in volume and drop the CR, but anyway). The resultant CR would be 10.2:1 instead of 10:1. Pretty negligable huh?
As for the s50s, I'm not going to dwell on them as they're clearly not going to be used here, and there are drawbacks to using them etc so it's a complete waste of my time. But you've still not taken heed that the bowl on them is sufficient to bring the CR down to nearly 9:1 with no otther tweaks. Plus they have a huge top ring to crown height so can be skimmed to drop CR. So going back to the original point, your accusation that I was suggesting pistons that don't fit (my orig question was 'what's wrong with s50 pistons'), it was actually a helpful suggestion and one that could work with some work.

[tim_s]

Oh and lol @ my ego. And what the hell's this about fabricating posts? The posts you;ve been writing have been changing so much it's pretty amusing! I've started writing replies with quotes in notepad (I use win 7 x64 which doesn't work properly with the reply text box) and then noticed that you've edited and toned down so many times! It's funny!

Not quite as funny as this:

I'll explain the fundamental mistake that you are making in assuming that a M44 piston in an M42 engine results in the same CR as a M42 crank in an M44 engine.

but still funny!

[GeoffBob]

Tim,

Both the M44 and M42 have a 10:1 compression ratio stock standard. Agreed?

Except the standard M44 gives a 10:1 compression ratio on an 85mm bore, while the M42 gives a 10:1 compression ration on an 84mm bore, agreed?

Now, the stock M42 has a swept volume of 448.9cc (81mm stroke on 84mm diameter) while the M44 has a swept volume of 470.1cc (83mm stroke on 85mm diameter). Thus, in order that both stock engines have a compression ratio of 10:1, the M44 must have a greater volume over its crown (when at TDC). By the same reasoning the M42 has a smaller volume at TDC than the M44. Agreed?

This is where the fundamental difference comes in. Please, stop thinking in terms of height ratios and start taking the actual volumes into account.

[GeoffBob]

Both engines will have a bore of 85mm and are the same height, with the same length rods. Where's the difference in CR coming in? Where's the assumption there? There is none.

The difference is in the stroked volume, which differs markedly between the two (due to different cranks), and which contributes significantly to the CR.

Math calculations are the defining answer point here.
;)

Talk is cheap, calculations are gold.

Fully agreed Gunni,

As in the manner of my earlier calculation, the CR of an M42 with M44 piston =

(459.63+41.17+13.35)
------------------------------- = 9.43
(41.17+13.35)

where the chamber volume at BDC has increased by roughly 10cc from 448.88cc (with the M42 piston) to 459.63cc (with the M44 piston)

All other values relate to the compression volume above the piston with the M44 piston in the M42 engine.

And Tim, if you think you can reduce 9:43:1 down to 8.5:1 by inserting a repair gasket then good luck to you.

Rich, that’s it for me I’m afraid, I’m tired of arguing with someone who chooses to launch personalised comments as a retort to a reasonable and logical argument, so I'm outta here. No matter how many times I asked you to keep it technical Tim, you just had to come back with insults and BS.

Tim, ignorance is bliss dude. Peace out and chill.

[tim_s]

Let me do this much more simply (and give the correct answer):
M44 piston bowl is 1-2cc more than m42 piston (which makes perfect sense when you consider both m42 and m44 have 10:1 CR). Pin height is 1.25mm shorter (and no, Geoff, I didn't use a Mahle catalogue as I've already told you the m44 block and m42 block data from which you can work this out - but you're welcome to do so and I know you will).

Quick use of plugging in original stats and 10:1 CR, use an online CR calculator, get CR of 8.94:1 with 1cc extra bowl
As even you said all along. that took me less than a minute to work out.
I'll leave you to work out where you've gone wrong.

[tim_s]

I'll explain the fundamental mistake that you are making in assuming that a M44 piston in an M42 engine results in the same CR as a M42 crank in an M44 engine.

Both engines will have a bore of 85mm and are the same height, with the same length rods. Where's the difference in CR coming in? Where's the assumption there? There is none.

The difference is in the stroked volume, which differs markedly between the two (due to different cranks), and which contributes significantly to the CR.

Geoff, M44 piston in M42 engine and M42 crank in M44 engine, in both cases crank = m42 crank - an M42 engine uses an M42 crank, an m44 engine with an m42 crank uses an M42 crank.

[tim_s]

Geoff, just checked this and no reply, so trawled through your sums to figure out where the difference came from.

To get to your figures:
41.173+13.35
=54.52cc chamber vol
m44

versus
49.3cc chamber vol
m42

You must have failed to take into account something along the following lines (as you've not really made it clear what you're basing anything on, it's difficult to pinpoint):
Given that an m42 or m44 block with m44 pistons and m42 crank results in 1.25mm of stroke before the top of the block:

1.25mm*pi*((85mm/2)^2)= 5.67cc

This alone is larger than your total difference in chamber vol, which would mean that the m44 has a smaller bowl than an m42 piston, which is wrong.
That an m44 HG is 1mm larger dia than m42 gives an extra .23cc

5.87

and that for m44 to have 10:1 CR (same vol in head) it requires:

V_stroke=83.5mm*pi*((85mm/2)^2)= 473.821cc

if CR is 10.1:

V_squish=V_stroke/(CR-1)=473.821cc/9.1=52.07cc

according to this there is at least 2cc extra bowl in the m44 piston.

Thus your V_squish figure is out. To get nearer to the actual figure, you need to be talking about a V_Squish of around 7.87.

459.63+57.2
----------- = 9.04
57.2

This doesn't use actual figures that I've obtained from measuring as I wanted to keep it consistent with your figures.
As for the HG, that gives an exta 2cc over factory m42 gasket, giving with the above figures 8.76:1.
This is all way more work than just plugging in the details into an online calculator

I still also don't get the m44 block with m42 crank and m44 pistons being different from m42 block with m42 crank and m44 pistons thing either, I still think they'll be identical in terms of CR, and you've not given me any reason to think otherwise.
Tim