BMW engine rebuild and machine shops - learn me

Admittedly though I've been working on cars for 20+ years, the last thing on my "list" that I've never done is a complete rebuild. I've done "refreshes", HGs, valve adjustments, cams, etc but never messed with the block internals. I've always just dropped known running engines into cars and run them until dead, then just find another used/cheap one.

But time to learn, I think. The engine is a (probably) high-miles BMW M42 4-cyl.

Engine came out of a running car and doesn't show any signs of issues outwardly, has good compression, and doesn't have the profile gasket damage commonly seen on these.

What it does have is a VERY worn crank thrust bearing surface thanks to BMW cheaping-out and only putting a 180-degree thrust bearing on this engine. The bearing surface on the crank is way out of spec (literally worn "in" to the crank when it should have an extruded profile), so the crank is junk. Cylinder bores look to be in great shape visually, as do the other bearing surfaces (though i'll replace all of them).

I've procured another M42 crank with a good surface, and will get the updated 360-degree thrust bearing. So my questions are largely about the block and having the head mating surface checked for level.

A local machine shop has been suggested by some of the more knowledgeable e30 guys, so I'll go there eventually. Note that I am rebuilding this engine to "stock" (except updated timing stuff). So what I'd like some advice on is what to ask the machine shop to check.

Again, since this is my first time going this deep into the engine I'd welcome any advice/thoughts you might have. Or since this is a cheap engine, should I just put the newer crank in with new bearing surfaces and otherwise not bother with the rest. This will not be for a track car and won't be getting "built" or boosted int he future. Just to throw in the e21 for tooling around.

Are you going to drop off the engine to be machined and then assemble it yourself? Or will they assemble it as well? What are you doing with the cylinders? You might need oversized pistons as well.

I paid $1800 to machine and assemble a Toyota 3SGTE block. That only included the machining and assembly using pistons I brought. They shotpeend the connecting rods and supplied the crank bearings.

Slippery wrote: Are you going to drop off the engine to be machined and then assemble it yourself? Or will they assemble it as well? What are you doing with the cylinders? You might need oversized pistons as well. I paid $1800 to machine and assemble a Toyota 3SGTE block. That only included the machining and assembly using pistons I brought. They shotpeend the connecting rods and supplied the crank bearings.

I would assemble. That said, this is a "fact-finding" post to see if it's worthwhile to even bother with this, or if I should just take this $200 engine, drop the newer crank in with new bearings, and call it a day. If it looks like it's going to cost a significant amount to do any of it, I won't even bother. For $1k I can buy an M42 already fully rebuilt...

The M42 in my rallycross car is even higher-mileage and still runs like a champ.

For a "stock" rebuild, I don't really see the point.

z31maniac wrote: For a "stock" rebuild, I don't really see the point.

So you would just suggest to get new bearing surfaces, put the new crank in, and call it a day?

irish44j wrote:

z31maniac wrote: For a "stock" rebuild, I don't really see the point.

So you would just suggest to get new bearing surfaces, put the new crank in, and call it a day?

For your uses, without a doubt.

I'd probably just even try to find another running M42/M44 and move on. No one in the E30/E36 community wants them so you can typically find them cheap.

In reply to z31maniac:

... find another running M42/M44 and move on.

This.

In reply to z31maniac:

The m42 will do fine, he doesn't want an m44.......

I would put in the new bearings and run what you have. I think it would be a fun and rewarding project( and pretty cheap). I think by the time you have much machine work done it would not be worth it for that engine.

Take a bunch of measurements and compare to what the specs are for a stock m42 (cylinder bore, rods, etc.). Chances are you could throw in the new crank, fresh bearings and call it a day. If you want the machine shop to check it out - mostly check the head for level/square or have it skimmed and take a good look at the valvetrain. If stuff is out of spec - go find a runner - m42s are decently cheap. OTOH, if you wanted to add some displacement... that could be interesting...

You could balance/lighten some of the rotating mass (I'm not sure how much grinding there is to be done on m42 rods)...

Rolf Van Os bmw 2.0 m42....

oldtin wrote: Take a bunch of measurements and compare to what the specs are for a stock m42 (cylinder bore, rods, etc.). Chances are you could throw in the new crank, fresh bearings and call it a day. If you want the machine shop to check it out - mostly check the head for level/square or have it skimmed and take a good look at the valvetrain. If stuff is out of spec - go find a runner - m42s are decently cheap. OTOH, if you wanted to add some displacement... that could be interesting... You could balance/lighten some of the rotating mass (I'm not sure how much grinding there is to be done on m42 rods)... Rolf Van Os bmw 2.0 m42....

Not much, from what I've heard. The rotating mass is best addressed by doing the M20B25 flywheel/clutch conversion (which I have done on my M42 in the rallycross car).

If I want to add power to this car in the future, I'll put in a 6-cyl. But for the moment I have a complete M42 sitting here so would rather just have it running and use it, rather than look for another M42.

z31maniac wrote:

irish44j wrote:

z31maniac wrote: For a "stock" rebuild, I don't really see the point.

So you would just suggest to get new bearing surfaces, put the new crank in, and call it a day?

For your uses, without a doubt. I'd probably just even try to find another running M42/M44 and move on. No one in the E30/E36 community wants them so you can typically find them cheap.

How do you think I got the two I already have? lol.

People in the e21 community DO want them, however....

then I stand by just check if it's in spec - throw some fresh bearings in and put it back together.

BTW, how's the stop/go driveability with the m20 flywheel/clutch conversion? My clutch is toast and I'm wrestiling with this - stock vs valeo conversion (thinking this is probably the m20 bits) vs lightened

oldtin wrote: then I stand by just check if it's in spec - throw some fresh bearings in and put it back together. BTW, how's the stop/go driveability with the m20 flywheel/clutch conversion? My clutch is toast and I'm wrestiling with this - stock vs valeo conversion (thinking this is probably the m20 bits) vs lightened

It's fine. I have no issues with it at all. Feels like it could be stock, really. Except the engine revs quicker. Highly recommend.

Then again, I did it when I swapped in the M42 to the e30, so I have no comparison against the stock M42 FW other than driving another M42 rallycross car with the stock setup.. I've never stalled it and don't have to "rev up" to leave a stoplight.

Also, going to rallycrosses I'm towing a tire trailer and a few hundred Lbs. of gear, and haven't had an issue there either.

Even the stock M20 flywheel isn't super light. When I used it in my S52 swap, we had it machined down to 11lbs.

how long do you want it to last???

oldeskewltoy wrote: how long do you want it to last???

It won't be a DD or a track car, just for cruising on the weekends and such. My automotive ADD would probably result in a 6cyl swap a few years down the road anyhow, so "lasting" is all relative. I don't need it to go 100k miles, but I don't want it to blow up in a month, lol.

irish44j wrote:

oldeskewltoy wrote: how long do you want it to last???

It won't be a DD or a track car, just for cruising on the weekends and such. My automotive ADD would probably result in a 6cyl swap a few years down the road anyhow, so "lasting" is all relative. I don't need it to go 100k miles, but I don't want it to blow up in a month, lol.

since you don't intend this to last a long time... I'd say take it on yourself. You seem more than capable, get the specs... and some Plastigage... and go to town.

there are enough of us here who can get you though anything you might not think you can do.......