M20B25 Rebuild

[Malcolm206]

I'm picking up an m52 crank from a 97 328i tomorrow, so I guess in the fall I'll find out if it fits or not lol.

Thank you for the pictures digger, that's very helpful! Just starting to get a good understanding of how it all fits together and that's just what I needed.

Realistically, I don't plan on doing much more on the motor after this, so the 272 would probably work well for me. It's for my daily driver and if I can get ~175 whp out of it, I'll be happy.

[ForcedFirebird]

Digger, in those pics, that piston appears to be modified already. I found the m52 crank has larger weights depending on cylinder (US versions are different?), so the clearance on them *almost* works on a few cylinders, but all need modification. I have done it both ways. Cut and re-balance the crank, as well as putting notches in the piston skirts to clear the bob weights (then of course weight matching back the pistons) with no ill effect. I feel the most important thing is making sure the clearances are all well, and the parts are balanced. About .038" is about all you can squeeze the combustion before piston to head clearance becomes a factor - you will start wearing rod bearing prematurely, this is with a 7500rpm cap. I have had the pleasure of pulling apart a few virgin m20's, and was pleasantly surprised with BMW's weight matching. We found a +/- .5gm on most examples. Stock pistons are pretty stout. They are a hyper alloy and have thick crowns and ring lands. We have seriously abused fresh +.5mm Mahle pistons on turbo builds.

The 272 cam will work ok for a street car. Can't wait to get an after dyno on 284 cam vs 272 with a locals' 2.8 itb car. PTV is close already at .045/.073 intake/exhaust respectively. We will have to rig a valve pocket cutter to the engine in car, but feel the 272 is restricting the 2.8 at 9.8:1 +1mm valves and ported head. If you plan on itb's, then go with a larger cam than anticipated IMO.

[digger]

Yeah I should have said that I hacked those Pistons so the skirts were flush with the bottom of the piston bosses. It is also only cyl number 1.

[Malcolm206]

So I got a m20b27 for $500 today... Rebuilt 885 head with 272 reground cam, IE HD rocker arms, bottom end has eta pistons and rods in it currently with m52 crank that's had the counterweights skimmed but needs to be rebalanced. I figure I can pull the crank, have a shop rebalance it, then put the rebalanced m52 crank back in with eta rods and i pistons after I deck the head ~.5mm.

[digger]

perfect

[ForcedFirebird]

Deck the block not head. ;)

[Malcolm206]

Deck the block not head. ;)

Dammit. Again. Yes, thank you lol.

[redlightpete]

Deck the block not head. ;)

I realize that decking the block maintains the squish band better at the outer edges than cutting the head 0.5 mm, but do you think this is significant? Or is there some other problem with cutting the head? I ask because I've already assembled a 2.8 bottom end, but didn't deck the block. I was planning to just get the head skimmed 0.4 - 0.5 mm along with a valve job. Thanks

[ForcedFirebird]

You will be shaving the squish band off the head with a large cut like that, make the head thinner (they are already prone to cracking), and you will be creating hot spots on the edges of the bore when the combustion chamber starts to overlap the block deck. Can it be done, sure, but so can an 885 head on a flat piston eta block too ;)

[digger]

one of those things that not ideal but will it affect anything significantly probably not. the trend with modern performance pistons, and chambers is reducing crevice volumes.
what affect in real terms it will have is hard to say. id say it would be better than doing nothing ie not decking either of the head or block

if you had of had the head machined or need the engine on the road in a day or two id say go ahead but since you havent machined the head. use some clay and measure the squish clearances and work out how much needs to come off and dissemble the block?

[redlightpete]

I'll check the existing clearance. I may still have the clay from when I was checking the valve clearance.
Isn't a 0.5 mm head cut standard practice with spec E30 motors?

[digger]

they may very well skim the head as much as possible to build more compression to the limit of the P to V and head to piston clearance

[ForcedFirebird]

In stock configuration, the pistons will hit the head before the valves will. No, we don't just take off .020" in Spec racing. Generally we want to skim as little as possible to save for future rebuilds. Do you know how many heads you can go through on the flow bench to find the best stock core you can? ;)

We had a customer bring us a 2.8l bottom end to install under the head we already built for him (no porting, just fresh and a 272). He bought it from someone and the block was not shaved since he has plans for turbo. Well, it made 180whp nd the torque wasn't far off. I would have to go through my dyno files to find the exact numbers.

[Malcolm206]

We had a customer bring us a 2.8l bottom end to install under the head we already built for him (no porting, just fresh and a 272). He bought it from someone and the block was not shaved since he has plans for turbo. Well, it made 180whp nd the torque wasn't far off. I would have to go through my dyno files to find the exact numbers.

If you happen to find those dyno files I would love to see them, since that's about what I'm doing.

[redlightpete]

In stock configuration, the pistons will hit the head before the valves will. No, we don't just take off .020" in Spec racing. Generally we want to skim as little as possible to save for future rebuilds. Do you know how many heads you can go through on the flow bench to find the best stock core you can? ;)

We had a customer bring us a 2.8l bottom end to install under the head we already built for him (no porting, just fresh and a 272). He bought it from someone and the block was not shaved since he has plans for turbo. Well, it made 180whp nd the torque wasn't far off. I would have to go through my dyno files to find the exact numbers.

Thanks for the direction, guys. I was 'mis-remembering' the factory service limit as 0.5 mm instead of the actual 0.3 mm.
My old clay shows the stock motor at 1.5 mm piston to head clearance with a full thickness head, so I'll be looking at 2.0 mm with the current setup. I guess I'll have to decide if it's worth it to me to pull the bottom end apart and get it machined for 0.5 mm of squish/compression. This project has already been dragging on for years and I'd be pretty happy with 180 whp!