Talk to me about rebuilding a motor on the cheap (BMW M10 content)

I've had a spare BMW M10 short block sitting in my garage forever. The idea has always been to rebuild it and stick it in my 2002, not because there's anything wrong with the existing engine, but for the learning experience. I've not rebuilt an engine before, but I have plenty of mechanical experience and most of the tools I think I'll need. I'm thinking the time is approaching when I want to tackle this, so I figure it's a good time to ask some questions. The idea is to do the most basic sort of rebuild (significantly closer to the Roadkill end of the scale than the Barrett-Jackson end) and see what happens.

The block is complete with everything from the oil pan up in place. It's quite grimy but spins freely. I don't detect any significant ridging in the cylinders, and the piston tops look fine. I haven't torn it down to inspect things further. I'm thinking I will replace the pistons with these. They're way cheaper than any other option, and the extra compression is welcome. I haven't found a lot of solid information on them, but as long as they are more or less in spec as far as bore diameter and deck and dome heights they should work, yes? Dingle ball hone the bores, new rings, new bearings, new crank seals and timing cover gaskets, and...what else? I'd like to do the oil pump, new ones are hideously expensive these days.

I've got an E21 head (to match the pistons) that I will also be going through. No idea what the valve guides look like, but resealing would be in order for sure. What do I do about the valves themselves?

What about balancing? I've got a scale and a Dremel. Taking the casting marks off the rockers and matching the ports seem obvious, but should I try to balance the rotating assemblies as well? How close do I need to get the weights?

I'll probably have more questions once I get started, but I'm just trying to wrap my head around the process at the moment and making sure I don't overlook something major and/or obvious to people who have done this before.

I'm not sure cheap and BMW go together. The last time I looked at parts for one of their engines, there wasn't really a kit to do it with. Everything was by the piece and from BMW. Then add $1000+ for machine work and cheap went out the window pretty quick. 

The cost will depend of course on the condition of the engine once it's taken apart, if it's totally clapped out it will need a lot more replacement parts and/or machine work than one with low miles that was well cared for.

I'd be cautious about buying no name parts like those pistons, you often get what you pay for.  I'd check on the 2002 forums to see if anyone can provide positive feedback on them.  There are a number of independent parts suppliers out there who can provide good quality stuff that's often the same as OEM parts for reasonable prices; my personal preference is http://www.blunttech.com  Call them and talk about the project, they are pretty knowledgeable on older BMWs.  

Balancing a rotating assembly takes more than just matching weights of the pistons and rods - a machine shop will have special equipment that spins the crankshaft with bob weights added to determine if any weight needs to be added or removed from the counterweights.  For a stock engine it's nice to do but isn't necessarily mandatory. 

I know that doing it the "right" way isn't cheap - this is really more about going through the process, understanding fully that some of the things I've chosen to do (or not do) would ideally be done differently. If the block or head turn out to be in need of machine work, the project ends right there. If everything is workable as-is, I'll proceed. Bearings and rings are readily available and not terribly expensive. I've got at least a partial gasket set floating around here somewhere.

I'm familiar with virtually all the BMW parts houses (I've had this car for 24 years), so I have a reasonable sense of the costs. I've already checked 2002FAQ for info on the pistons - there isn't much. Since they make pistons for lots of applications I figured I'd ask here.

wait for oldgray320i to come in here, think hes done it before on this engine. doubt he'll know anything about the pistons and such tho...  

ive rebuilt an engine or two, its not nearly as hard as one might think. roadkill has a pretty good engine rebuilding series, and youtube im sure has a million videos out there.  

my suggestion is to get a haynes manual for tq specs, some gaskets that you'll be replacing and a good set of headphones. (add assembly lube if its not immediately going in the car) i had a machine shop hone and deck both the head and block for about 200 euro, dont be too afraid of a machine shop!

not sure if this helps you out too much, hope it answered a question or two!

I've got the factory BMW blue book for the 2002 (and the Haynes manual), so I'm covered as far as specs go.

I've actually thought about picking up a used engine from LKQ on eBay just to rebuild it, since I too have not rebuilt any engines.  LKQ has some for like $120 and if you find one that ships from near you, they'll let you pick it up for free. 

Check end thrust on the crank first.  I have a short stack of useless ones around...

In reply to Streetwiseguy :

An M10 is the first engine I rebuilt. It was just supposed to be swapped from a donor into the project car. Then as it dangled from the hoist, it turned out it had so much crank endplay it was ridiculous. To be measured with a ruler instead of a feeler gauge... So the next step was to find another crank. (It turned out the amateur rebuild in the car had a crank which had been cut back for thicker thrust bearings, which I guess don't exist; all this time later I'm operating partially on recollection and partially based on assumptions based on recollections... It's a downward spiral.)

I didn't have to go to a larger bore, but did have a local machine shop check and hone the bores and look all the bits over. I had the late, much-missed local 2002 guru do the head (which I wouldn't have qualms about doing myself now; you will need a suitable valve spring compressor. IIRC the most basic ones won't reach into the M10 head).

Don't forget the bores need to be checked for not being ovalized as well.

I would think you'd want to replace the valve guides as well as the seals, and that's another one I suspect for the machine shop. At least checking them for status. I'd be more inclined to assume some machine work than to call it the line where the project stops.

Deglaze the cylinders

New rings, bearings, seals etc.

Valve job,  might just need lapping

Things like timing belts/chains/tensioners need to be addressed.

I think I'm going to tackle the cylinder head first, as it's easier to manage. I've got all the tools I need save for a valve spring compressor. Assuming I don't find anything horrible on first inspection, at least I end up with a rebuilt head on the shelf.

Chains are quite robust on these motors, but I'll probably throw in a new one in any case. I'll check the tensioner piston and the guide rail and deal with them as needed, as well as shimming the oil pump if it needs it to get proper chain tension.

Cool undertaking!  I would just stick with the stock pistons because they're tough, I figure the assembly is in balance, and I can't see myself getting much out of a little more CR.  It's a mighty mouse motor in my mind.

Are you going to carb it or KJet?  I had an M20B23 that a previous owner converted to carb with spark controlled by a BMW ignition module, and it was just fine that way.

I would probably be like you and skip getting the cylinders machined.  Unfortunately, that's the situation on the rebuilt SBF I have in my E36 track car and it didn't quite work out.  It's a great cruising motor with good power, but once the RMPs get 5Kish, the new rings just don't manage to seal as well as they do lower down.  Blowby at that point gets ugly, even though the compression ratio measures good in the shop.

I would install valve seals myself and hand-lap the valves, unless the guides felt wobbly to me with the springs removed.  I think those guides are pretty tough though, so I'm betting they're fine.

Have fun!

Assuming I get that far, I'd just stick a 2bbl Weber on it. I've already got the manifold (probably several) and it's an easy setup; I'm already quite comfortable with the pair of DCOEs I've got on the current motor, so one carb shouldn't present any issues.

Curious what rings you used in the SBF - I was just reading something about Total-Seal rings that suggested their sealing was fine for static compression but got worse as the revs rise (this was referring to the M10, BTW). Is that what you used?

Nope just plain old file fit Cleavite or Federal Mogul, I don't 100% remember which.  Installed them right-side-up and used the spreader tool, but still not 100% effective.