BMW M20 Engine Rebuild, Part Two – The Teardown Begins

In my last installment, I told you a little bit about the BMW M20 motor I recently purchased to rebuild for the Spec E30 race car. As it is a spec car, it must be built to factory specifications.

Before digging in too deeply, I consulted with fellow Spec E30 racer and owner of Offset Motorwerks in Southern California, Mitchell Dyche. A little tutelage from an experienced engine builder would go a long way toward making this a successful endeavor. Mitchell told me that the engines are pretty stout, but recommended the replacement of some key parts to ensure longevity. Things like rockers, valve keepers, valve springs, and the camshaft are all wear-items that might need replacing.

Once I took everything apart and could take a look at the condition of the bearings, pistons, and cylinders, I’d have a better idea of what I’d need to purchase. So, I poured a cup of coffee and headed into the workshop.

First thing I did was find a place in my garage with a lot of light and space. Without much counter-space in my old-timey garage, I ended up laying out a couple of large pieces of cardboard where I’d place and organize the various parts of the motor. It’s not elegant, but it really helps to think of things modularly and keep nuts and bolts together with their respective parts. In addition, I made sure to take tons of photos to help me remember how things go back together. Warning – if you have dogs that like to find random objects and hide them in the yard, this may not be the best way to organize for you.

Starting with the front end of the motor, I removed the distributor assembly, water pump, as well as ancillary stuff that had been left on it like engine mounts, exhaust manifolds, and the crank position sensor. Take your time with removal and do not force anything. The last thing you want to do is to have to machine out an old rusted stud that you broke because of using brute force.

The distributor, crank pulley, thermostat, crank position sensor, and exhaust manifolds all needed to come off.

With most of that stuff removed, I was then able to remove the timing cover and get a good look at everything.

The timing cover keeps all the spinny bits inside.

Various pulleys and cranks cover the front of the motor. A high-speed impact gun is really helpful here to spin nuts and bolts off without rotating the entire assembly.

Blurry image, but you see the oil pump sprocket, crank dampener and tensioner here with the timing belt routed through.

Next, I removed the valve cover and used a six-point star socket to remove the head bolts, separating the entire head from the block.

When removing the head bolts, I made sure to work carefully, slowly increasing pressure on the wrench until the head bolt gave way. Sudden pressure can sheer the tops off bolts and leave you with a messy drilling operation to back the bolt out. Once the head bolts were out, I separated the two parts of the motor.

After speaking with numerous engine builders and confirming with Mitchell, I decided to keep the head in one piece for delivery to a machine shop. The shop will disassemble the head, look for wear and advise me on which items to replace. They will also reassemble the head so that tolerances and clearances are to spec. I don’t have the knowledge nor the tools to do this kind of precision work in my garage.

Disgusting. Months of sitting around outside have introduced rust and other nastiness into this poor motor.

Carbon build up – the refreshed pistons will look good as new.

With the head removed, I was able to turn the crankcase over and start to disassemble the connecting rods and remove the pistons.  I ended up needing to put the crank damper back on in order to turn the crank over to get the needed clearance to remove all of them. Mid-stroke some of the connecting rods were really cantilevered and would not fit through the cylinder opening. I also took the opportunity to remove the rest of the timing components including the tensioner and pulleys. Unfortunately, I didn’t take a good picture of the front with the timing cover removed as I was trying to troubleshoot removal.

Here’s a good view of the bottom of the engine, including the oil pump which I will also service and check for metal shavings.

Same as the head bolts, take your time here when removing the crank journal bolts.

A good view of the crank journals as we take the bolts out.With the pistons removed, I can now safely back out the crank bearing bolts and remove the rear and front seals. The rear cover was a little tricky to remove as the engine was connected to the engine stand. I simply backed all the bolts out and lifted the entire assembly out as one piece.

With the front cover completely removed, this is what remains.

There she is…now to remove that grungy old gasket.

With everything apart, I was able to take a closer look at the bearings. Lots of wear and tear, it’s time for new ones to go in!

This crank bearing shows a fair amount of wear.

The bearing on this connecting rod has noticeable wear and needs replacement.

This old bearing has seen much better days.

As you can see, there’s somewhat of a method to my madness with the way I’ve organized these parts.

Next step will be to the send the head, crank, and block to the machine shop for measuring, honing, and replacement of bearings. I have to make a parts list of stuff to purchase which I’ll share with you in the next installment. I’ll also apply some degreaser to the parts and clean up what I have laying around, readying them for reassembly. Hope you found this helpful, and see you next time!

About the author

Kasra Ajir

Having competed, raced, or trained with SCORE, SCCA, NASA, BMW CCA, and more, Ajir is well seasoned in the various series that pollinate the grassroots and professional auto racing world.
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