M20B25 assembly

[plmichal]

First time rebuilding an engine and I need to take my block to a machine shop for some machine work. I want to build a bullet proof, reliable stock m20 with all new parts and I want to assemble it myself. But I'm not sure what to expect from the machinists end. I want to ask for...

1. Fresh bore and hone for new Mahle oversize pistons
2. Slight skim of deck surface
3. Rods and crank inspection
4. Rods and main journal measurements for bearings

How important are additional services like align honing, balancing the bottom end, crank polishing, etc on a stock rebuild? Is it as simple as trusting the machinists work, measurements, and slapping it together with new bearings, seals, and assembly lube? How important to measure actual clearances from the large list of specs in the Bentley Manual like piston to wall, main and rod bearings, etc? I want to be thorough but reasonable. Thanks.

[mach schnell]

Interesting. I was considering this myself.

I wanted to do some modern rods and pistons for a little higher compression (like ~9.5:1).

Are you going to do that too?

[TobyB]

From what I've experienced, you can match the piston weights yourself.
Then IF you're not building a motor that's not running at high revs a lot
(like at the track) BMW's factory balancing is pretty good.

Honestly, you'd want to ask your machinist about things like crank finish-
they'll look at your parts, and recommend what it needs. For example, the
rods often need polishing, while the mains don't.

I DO measure bore size after boring- I caught a mistake once, and that's made me
gun- shy. Plastigauge is a good overall check of everyone's work for bearings,
as is feel and common sense. Ring end gap is very much worth checking, too.

hth

t

[plmichal]

I DO measure bore size after boring- I caught a mistake once, and that's made me
gun- shy. Plastigauge is a good overall check of everyone's work for bearings,
as is feel and common sense. Ring end gap is very much worth checking, too.

This makes me just want to spend the few hundred dollars on a bore gauge and micrometer and do it myself... I have a hard time trusting people anyways.


This won't be a racing engine. Mainly as a daily driver. Although I occasionally enjoy a HPDE.

[Andre3127]

This makes me just want to spend the few hundred dollars on a bore gauge and micrometer and do it myself... I have a hard time trusting people anyways.


This won't be a racing engine. Mainly as a daily driver. Although I occasionally enjoy a HPDE.

BMW didn't go nuts with balancing, crank journal super polishing, or anything like that. Slap it together and run it for another 200k miles.

If this was a race engine the story might be different, but for a daily driver rebuild I'd think you're more than fine.

[digger]

First time rebuilding an engine and I need to take my block to a machine shop for some machine work. I want to build a bullet proof, reliable stock m20 with all new parts and I want to assemble it myself. But I'm not sure what to expect from the machinists end. I want to ask for...

1. Fresh bore and hone for new Mahle oversize pistons
2. Slight skim of deck surface
3. Rods and crank inspection
4. Rods and main journal measurements for bearings

How important are additional services like align honing, balancing the bottom end, crank polishing, etc on a stock rebuild? Is it as simple as trusting the machinists work, measurements, and slapping it together with new bearings, seals, and assembly lube? How important to measure actual clearances from the large list of specs in the Bentley Manual like piston to wall, main and rod bearings, etc? I want to be thorough but reasonable. Thanks.

align honing not usually required with the factory bolts if the engine has not had a catastrophic failure. ARP mains will create different clamp and needs to be honed.

work to the journals will depend how good of a condition they are in.

going over the top is not required for balancing, to BMW's specs is fine

get the machine shop to check things then use plastigauge as a final check at your end for bearings. you can do some checks on the bore as a check that nothing is out to lunch

[redlightpete]

You can use your pistons and feeler gauges to check the piston to cylinder clearance. That way you don’t need to buy other measurement tools.

[TobyB]

Inside mic is pretty important-
you are going to have to work pretty carefully with feelers and
deck height to catch belling right below ring top height.

t

[Cayoncrusier93]

When i got mine done, I had my block bored and honed for 84.50mm mahle pistons, line honed, new brass wrist pin bushings in rods plus re-sizing to new wrist pins, re ground rod ends, crank+rods+pistons+flywheel+clutch all balanced, new intermediate shaft bearings, block drill and tapped for turbo drain line, o-ringed deck, crank lightly polished, and i think that’s it.
New bottom end gaskets, new rod bearings, new main bearings.
I’m assembling the block myself.
And yes check clearances with plastigauge to ensure nothing is wonky.

[plmichal]

I need a source for Mahle +0.50mm oversize piston ring set M20B25, one of the oil rings broke during install.

Someone suggested installing a Hastings ring set, but the thought of mismatching my new piston with a different ring set doesn't sit right with me...

[digger]

https://www.realoem.com/bmw/enUS/sho...diagId=11_0354

https://www.turnermotorsport.com/p-3...uine-bmw-part/

https://www.schmiedmann.com/en/11251713195MO-mahleoriginal-pistonringkit

[plmichal]

digger

The schmiedmann link is what I'm looking for but shipping across the pond and out-of-stock *thumbs down*

It wouldn't bother you running one piston with a non-Mahle set of rings? I assume they're manufactured to the same spec but my OCD...

[digger]

if they are the same style/material and size (axial and radial) then wont matter.