BMW M20 Engine Overhaul, Rebuild, Restoration, and 2.7 Stroker

June, 2014: This BMW M20 engine was offered to me and the deal was too good to pass up. Supposedly it was taken from a parts car with 88k miles. It's from a 1988 325is. I know it's an 88 because it has the later style coolant system with the metal coolant pipe across the front. And it's an "is" because it has a oil filter housing set up for an oil cooler. I got lucky!

Or did I? As I dismantle it I'm finding someone has been through this engine quite a lot. Nearly every bolt I touch is loose already. The timing cover is held on with one bolt, there are hoses missing, and the exhaust manifolds are already removed. And the kicker - the cylinder head is new/fresh. There's not a smudge anywhere inside and half the rocker arms are new! But I can't be so lucky - the timing marks between the cam sprocket and the crank pulley are off. It's off by 1.5 teeth on the cam mark. Since the M20 is an interference engine this can be BIG trouble. I'm hopeful that someone just reassembled the engine hastily and left it off a tiny bit.

Update: after removing the cylinder head and inspecting it and the pistons, I think I have the whole story on what happened. As is too common on the E30 325, the timing belt probably broke, which sent the valves into the pistons. There are grooves in the exhaust valve reliefs on the pistons that shouldn't be there (the intake side looks good). So the head was pulled and rebuilt but was not milled. Then the pistons were cleaned up but not removed from the block. And the motor was reassembled with all new gaskets and driven for a little while. Then the car was sold. The last owner of the engine started taking it apart but took a different direction. That's why so many parts are removed or loose. I got very very lucky with this deal!

Update, August, 2015: first drive with the new engine. A few small issues but running great!

Update June, 2019: I have about 3,000 miles on the rebuild so far and it's been a joy to drive. There have been some setbacks from things that should have been done with everything off the car. But that's Murphy's Law for you. You can find the series of minor setbacks at the bottom of the last page.

To see more about M20 and E21 life, check out my blog at http://www.e21323iblog.com.

All images publicly shown are mine and I reserve all copyright.

as found. nearly everything is loose, like it's been taken apart and quickly put back together.
as found
throttle body heater line. if you don't want to run warm coolant up to the throttle body you can remove this hose and replace the fitting with a 12x1.5 plug (BMW makes one). And then plug the other end or join the hoses at the thermostat.
throttle body coolant lines
wiring loom
a few pounds of grease...
somewhat de-sludged!
intake manifold removed and a debris blocker put in place.
camshaft sprocket. the cam is retarded by 1.5 teeth for some reason.
crankshaft aligned on TDC mark
cyl 6 detail
cleaned with KBS Klean - worked amazing! spray on, agitate with brush, spray off.
cyl 1. wiped clean with a rag. the pistons are so clean that I suspect they were removed and media blasted during the previous rebuild (or just cleaned well with the head off).
"83.98" for the bore - the original size. if the block wasn't bored for the rebuild there should be enough block material for my new 85mm bore.
cyl 6.
you can make out the valve-piston contact in the left (exhaust) valve relief. there shouldn't be a 'step' or lip in the relief. I suspect the valves hit the pistons and the head was rebuilt but they re-used the pistons.
rebuilt head with very low miles. it would appear that the incorrect timing had no effect or it was hastily assembled before it came to me.
I also checked the height of the head per the Bentley manual; it's 125.5mm tall which is what a new, un-milled head should be.
cyl 1
crankshaft timing marks
front timing cover with crank and intermediate shaft gears
bottom-end
oil pan gasket damaged - the source of the sludge all over the front of the engine.
bottom-end
cleaning with KBS Klean and a scotch-brite pad.
cleaning with KBS Klean and a scotch-brite pad.
cleaning with KBS Klean and a scotch-brite pad. the KBS works amazing with only a little effort. I'll probably hit it with a mild media blaster to get the really stubborn crud off.
intermediate shaft.
intermediate shaft and oil pump shaft.
cyl. 1& 6
cyl. 2 & 6
cyl. 3 & 4
connecting rod cap orientation. the picture is taken from the front of the engine. I was confused on this because the Bentley manual says they should all be installed in the same orientation. but as I pulled them out they were not all facing the same way. I think it's more important together the bearings lined up.
hammering out the wrist pin with 14mm deep socket.
The wrist pin clips were a big pain to remove. There's two and only one side has the split accessible with two tiny screwdrivers.
KBS Klean at work again!
valve cover after 10 minutes in the sandblasting booth
first few coats with VHT Wrinkle paint. I used the same on an M50 valve cover but not over such a wide area. The nozzle spray is narrow which made some problems. You can see the narrow streaks where the paint hasn't "wrinkled" yet. The instructions say to apply more coats until the surface is uniform. The good news is that it will wrinkle over 24 hours, no baking necessary.
intake manifold after KBS Klean and 10 minutes of sandblasting
fabricated crank holder and 3ft breaker bar to loosen the front crank bolt. the crank holder is forced against the workbench. pushing (rather than pulling) on the breaker kept the engine stand from lifting. it came loose with only a little effort.
engine block stripped and hosed down again. still a lot of sludge in spots. the block will be thermal cleaned at the machine shop.
crankcase
bearing journals