View Full Version : max bore for M20

07-25-2004, 05:50 PM
I wonder the maximum bore diameter for M20s. 85mm is used but what about 86 and 86.4mm ? The distance between the centers of the cylinders should be the same as E36 3.2L M3. Can we install 86.4 pistons ?

07-25-2004, 06:50 PM
There is 91 mm between the center of the cylinders..
I've read this on a swedish forum, some tuner said this about the M20B25 block:
"85.0-85.5 is possible, but as we're talking about raising the performance so you should stroke the engine instead, beyond this bore the walls will get too thin and the block unstable for high performance."


07-25-2004, 07:03 PM
if u bore it out u wouldnt want to go turbo eiither. not sure what ur plans are

07-26-2004, 03:00 PM
The top picture is E36 M3 block and the bottom picture is M20 325i block. If we install 86.4 piston, it will be same as S52 block, this S52 block has already very thin cyl walls. I wonder whether it has very special material for cyl walls in terms of heat transfer and stability.

My plan is 2.7 crank + 86.4 piston + 0.6 bar boost.

Most of people say more than 85 mm is risky for M20s but who can say the differences from S52 M3 block ?

07-26-2004, 10:24 PM
Not to hijack your thread, but I was wondering what the difference in bores is for different strokers. I know you use the same crank and rods for 2.8 2.9 and 3.0L , so the pistons and cylinder bore must be different. Anyone have any details on thsi?


07-26-2004, 11:15 PM
The bore size on most 2.8, 2.9 and 3.0 strokers are larger than stock. Oversized piston are used quite frequently with BMW's, I believe they're 84.25 in the e30, both 2.5 and 2.7. The 2.7 is due to the stroke. Going over 2.7 requires bored pistons. I'd stick to BMW specs on the bore size, which have a max spec of 84.25 I believe.

Dan Pinder
07-26-2004, 11:56 PM
2.9 and 3.0 strokers use a different crank from the 2.7 and 2.8 stroker. The crank is from an e36 3.0L m3, and is machined to fit.

Giga Shadow
07-27-2004, 05:42 AM
The secret how to do this must be posted here ASAP :D.

I wanna see somebody turbo a 3.0 m20!

07-27-2004, 06:38 AM
didn't that guy trent over on r3v do that?

dunno if it's a good idea myself, shortened rod ratios and forced induction don't always mix...

07-27-2004, 07:29 AM
I would rather get an M30 block ("big six") as the smallest stock cylinder is 86 mm in diameter, and the shortest stroke is 80 mm in the 2.8 litre version. You get the reliability of a chain driven cam, which can be considered a bonus.


07-27-2004, 12:35 PM
Any idea why they didn't do a chain driven cam on the M20?

Dan Pinder
07-27-2004, 02:21 PM
didn't that guy trent over on r3v do that?

dunno if it's a good idea myself, shortened rod ratios and forced induction don't always mix...

You use the e36 m3 rods, or the 325e rods. I think it would work just fine.

07-27-2004, 07:35 PM
i'm not saying it wouldn't work, but when you use more crank and less rod you decrease the total amount of boost you can run.

if you want to do it, be my guest, you'll have a more usable powerband than pretty much any other m20; i'm just pointing out that when you move in that direction, compromises are made.

Dan Pinder
07-27-2004, 08:51 PM
I just think that if the s50 rods and crank can handle over 600hp in an s50 block, why wouldn't the same crank and rods handle the same power in the m20 block?

07-27-2004, 09:11 PM
i 2nd that so the only diff is the pistons?

Dan Pinder
07-27-2004, 09:14 PM
Yeah. From what I hear you can drop in the m42 pistons for an 8.5:1 ratio.

07-27-2004, 09:18 PM
thats only .3point drop. scew that. im all about the 2.7block on a 2.5head. mad low comp and more displacement. run more boost and shift at 6

Dan Pinder
07-27-2004, 09:25 PM
Isn't the 2.7 stroker about the same compression. 8.4:1 or something. I wouldn't want any lower than that I don't think. It will be dead off boost.

Dan Pinder
07-27-2004, 09:30 PM
I wonder how much power the stock 2.7 bottom end will hold?

07-27-2004, 09:51 PM
i dont think it will be to dead off boost. i wouldnt realy care if i had 400hp. isnt the 2.7 block made out of all the same shit as ours just diff pistons and rods and crank? same material though righht?

1g dsms stock comp is 7.8.

ive seen a few guys drivin around na withthat set up and said it felt stronger then the stock 2.5 even with the lower comp

Dan Pinder
07-27-2004, 10:00 PM
The crank in the 2.7 eta bottom end is not forged, whereas the one in the 325i motor, is semi-forged. So is the 324td crank, which many people use in the 324e bottom end instead, for more strength.

There is nothing special about the 2.7 block. It is just a stroked out 325i bottom end.

You can strap a super eta bottom end from an 88 325e under a 325i head, but the crank is not as strong. You get about 8.4:1 compression from this, which is ideal for a turbo.

A few points of compression makes a big difference.

I think that the stock 325e block would handle 450hp just fine... I mean who has actually had a crank break. There is one locally for a few hundred dollars in good shape. I might just slap it in there and give her shit with 20psi.

Giga Shadow
07-27-2004, 10:45 PM
The E and I hybrid gives you 7.9:1 :D

I've heard from people that it feels like a normal 325i maybe alittle faster.
Ask jay and I think someone else of this board has a hybrid too.

Dan Pinder
07-27-2004, 11:06 PM
Wow, really? Does is that with the Super E 1988 bottom end?

THe Sly
07-27-2004, 11:36 PM
Actually the super ETA bottom end complete with pistons will get you 8.5 to 1 compression on a i head...

And like DonkeyPunch added, The E and I hybrid gives you 7.9:1which might be very good for a turbo aplication...

My Question now is ...Did any of you guys ever got any info on a E36 pistons in a M20

stock 10 to 1 compression on a 325 E36, it is the same bore and same stroke as our M20 325i..

I wonder if it would work in the M20 engine ...the only proplem i would encounter is the valves reliefs..

Back to the real question sorry...Now i think that i have heard that the big 86mm piston can go in the M20 but you would have to watch for the engine cooling system since the cylinders wall are thin....prone to overheating...

07-28-2004, 05:16 AM
Back to the real question sorry...Now i think that i have heard that the big 86mm piston can go in the M20 but you would have to watch for the engine cooling system since the cylinders wall are thin....prone to overheating...

what is your source ? any website any information ? theoretically there may occur temperature problem but people drive even turbocharged S52 ( 86.4 piston ) engines as a street machine. they have already thin cyl walls.

02-20-2005, 06:22 PM
let's go on after a big brake.

any idea about 86.4 piston ?
does it work safely in M20 ?

why or why not ?

I came closer to piston selection for my project.

Dan Pinder
02-20-2005, 06:26 PM
I spoke to metric mechanic and they say a +2mm bore is no problem. Although... the gasket does stick into the cyl a little bit.. they say it's okay.

Doesn't sound good for a turbo motor though. If you used a custom copper gasket.. go for it.

02-20-2005, 06:36 PM
OK, custom made gasket may be solution.

But I want to be sure about cyl walls etc.

Remember that many people drives boosted 3.2L M3 engines with 86.4 mm diameter piston. Does S52 block have any special property for block ? I don't think. Heat transfer and cyl wall is same (except big radiator).

I think 84 mm piston with 1 bar boost is nearly equal to 86.4 mm piston with 0.6 bar boost. So building low boost engine is much easier than 1.0+ bar boost engine.

02-20-2005, 09:19 PM
Thanks for bringing this back to the top! Quite a refreshing topic.

Dan, is there any concensus as to what the optimal c/r is for a boosted 3.0L? From what I have seen 8.0/1 is pretty good, but I am wanting to run in excess of 1 bar so maybe I should go a bit lower? 7.5 or so?

the Ross buy is coming to a close so I have to get my specs all worked out.


02-20-2005, 11:29 PM
It's very difficult to say what boost level is detonation-free. That depends on your fuel, ignition timing, cam, intake temps and exhaust back pressure. I'd say 1 bar is easy even with stock 8.8:1 on a stock 325i on a good setup (intercooled, large turbo, enough fuel, tuned ignition timing). More is no problem if you smooth out the combustion chamber a bit and optimize each part of the system.

As an example of a bad setup :
A small inefficient old T3 turbo, stock fuel injectors with FMU and no intercooler with a bad log type manifold.
Improve just one of those components and it will not be enough to have a durable engine.

02-26-2005, 07:47 AM
Perhaps someone will measure cylinder wall thickness on various engines with ultrasonic measurment gauge. It's possible the blocks for S/M 50/52 are cast with thicker cylinders than M20 blocks.

Dan Pinder
02-26-2005, 07:49 AM
Metric Mechanic assures me they have built m20 motors with the 3.2L bore. So has IE, but they burned gaskets.

Seems like it works fine.. just need a copper gasket.

02-26-2005, 07:53 AM
Copper gaskets can be troublesome with water seapage. Probably wouldn't matter on a track car. How hard would it be to open up a stock gasket a mm/ side with a simple fixture on CNC mill/ water jet?

Dan Pinder
02-26-2005, 05:58 PM
I would say impossible. You would cut past the metal ring.

02-26-2005, 10:06 PM
what about a MLS gasket you could "shave" that and not worry about "cutting past the metal" since the whole thing is steel. I've heard they can also seal a bit better then copper.

However I do not speak from real world experience.

Dan Pinder
02-26-2005, 11:28 PM
when you order a MLS gasket they will make it in any bore you want. that is my understanding.

problem is there is no such thing for the m20, yet.

Giga Shadow
02-27-2005, 02:45 AM
problem is there is no such thing for the m20,yet.

Never. :P

02-27-2005, 03:05 AM
when you order a MLS gasket they will make it in any bore you want. that is my understanding.

problem is there is no such thing for the m20, yet.

well say it was a year or two from now and I had a MLS for stock bore and then did a +1mm later on, I could always "shave" my exsisting MLS or the extra I can see myself having someday. This is all glass is half full type stuff though.....

02-27-2005, 06:11 PM
what is MLS gasket ?

what about stainless steel sheet gasket ? can we use it ? why everybody use copper gasket generally ?

02-27-2005, 06:14 PM


02-28-2005, 12:31 AM
mls gaskets are made just like any other gaskets, to set specs. typically companies that make mls gaskets offer different thicknesses or bore sizes, but you're still stuck with what is produced. i could be wrong, but i'm pretty sure cometic means that we need 50 of the exact same gasket, meaning same thickness and bore size, for the order.

how the cylinder holes are finished on the gasket would dictate if they could be bored or not. if there's a microweld, raides lip, or fire ring, then boring wouldn't be a good idea. however if it was just a plain cut edge, then boring would be fine.

as for bore dimesions of the 3.2 vs the m20, the 3.2 may use liners. i can't tell from the pics if it does or not.

THe Sly
04-08-2005, 06:26 PM
What about a MetricMechanic 85mm head gasket on a 86mm bore instead of trying to use a stock 84mm headgasket...

Would that work??
If it worked with MM....What did they use???

04-08-2005, 08:58 PM
May i ask your definition of semi forged, did they only half temper it? :D