m20b27 crank in a m20b20

[JamEsDavi3s0]

Hello! Recently joined this forum and newbie e30 owner.
I’ve got a 1990 320i, having recently picked it up and driven south to north island here in New Zealand I’m over the moon with the old girl and kinda in love with its little 2ltr! So... my original plans of wanting to go big stocker engine or charged induction have changed.

So the new plan to make the most out of the original engine as possible, trying to do mostly ‘bolt on’ mods.
Here lays my question.. will a b27 crank fit the b20 block without interference?
On top of this will it use the same length rods of 130mm like that of the m21 engine that uses the same stroke and bore sizes of what I’d like to build.
I’ll do other mods to match like bigger cam, lightened rods, gas flowed head maybe skimmed to raise compression to try increase power and pickup abit. I know I won’t see some serious gains but I’ll just enjoy the build and this is the path I’d like try.
I’ve read though a lot online about all the builds you can do but very little does anyone actually come out with advice on M20B20 it’s always swap it or turbo so any help would be greatly appreciated on this idea.

Thank you!

[steve_k]

welcome to the zone,

if you wish to build a 2.7 stroker then have a look in the wiki, there's a "how to" in there plus there's plenty of threads on here where folk have built their own,

one thing though, will you be going for BHP? or torque? whichever you go for will dictate which head yo use, 731/200 (b20/eta) head(s) for torque & 885 (b25) head for BHP.

like i say have a look in the wiki & in the m20 section, there's loads of info there & don't be affraud to ask either.

if you build a 2.7 you will notice the difference straight away, i've got a heavily reworked hartge 2.7 in my sport & i can the difference between that & i lightly modded 2.5 easily.

[steve_k]

forgot to add, yes the b27 crank will fir in the b20 block.

[JamEsDavi3s0]

Hey Steve_k
Thanks for the quick reply, I’m glad the crank will fit as is like to try and build a 2.4ltr.
Do you think the stock 130mm rods would be fine or need different lengths to prevent hitting the head? Would also hopefully assume that standard b20 pistons would be good for clearance?
I’m just wanting to give it a little more torque low down and mid range. Just make the engine a little more lively?

I have had a good read through all the wiki pages in here about the 2.7 builds and 2.8. Also a good read on other sites too. I do want to try make the most out of all the stock parts including the head, at least for time being.

Second reason for that is parts availability here in NZ isn’t so great, I haven’t found a single eta engine or b25 to work with. So I come back to using what I have while not really wanting to bore the block out as then I need pistons ect to match.
I’m willing to buy a new crank but the rear would be salvaged/ reused. From old more available engines , like the rods from an m50.

[steve_k]

never hard of a 2.4 before, i did build a 2.6 a while back.

as for which pistons to use? i've no idea sorry, mine are custom one's so i could use the 135mm I beam rods i had in for ages.

one thing to remember is that if you use 130mm rods the block will need decking,

also, the head makes a big difference, port, polish & gas flow can play a big part along with cam choice,
also depends on how you want to use the engine, a well built stroker can be a hell of a weapon on the street.

i think you can use the rods from the m50 as well.

[steve_k]

when it comes to an m20 build the worlds your oyster.

iain, one of the admin on here (can't remember his user name right now) has a hartge h27 cabby with a 3.1 m20 on itb's, also a memberin either OZ or NZ (reggid) has a 3.1/3.2 m20 as well.

so it just goes to show what can be done.

[JamEsDavi3s0]

I’ll try look those guys up! thank you for your help on this matter!

Will the block need decking a lot or something like a couple mm?

[steve_k]

I’ll try look those guys up! thank you for your help on this matter!

Will the block need decking a lot or something like a couple mm?

no worries, another good build thread is simon13, alpina c2 2.7, it's a long one (100+ pages) but a fair bit of info in it.

as for the block, if you use the 130mm rods it would need decking about 2mm IIRC.

[reggid]

this is more difficult than it seems. what pistons are you going to use?

If you retain the 80 mm bore then you CANNOT use ETA or M20B25 pistons as they are 84mm so all the wiki stuff is irrelevant. you also CANNOT use b20 pistons as they are de4signed for the much shorter crank so will stick out the top of the block a mile with the longer 81mm crank

You are better of using the b23 crank with 76.8 mm stroke if you must retain 80 mm bore as it should be a direct fit. You might be able to use b23 pistons with the 81mm stroke and cut the tops of the pistons down [(81-76.8')/2] = 2.1 mm or so.

Obviously best bet is boring the b20 block to 84 mm and using the either a 81 mm stroke and following one of the wikis based on the 731 head…..

[JamEsDavi3s0]

That’s what I was trying to work out if stock pistons on a 2.7 crank would work... answer being no unless custom pistons or maybe off chance a set of b23... like you say at that point may as well bore the engine out and make the most of it...

Hmm lists to think about.

Cheers guys!!