Engine Rebuild M70 (V12)

Photo's & comments by Cor Verlinde, story by Sean750
Document history:
# 12 August 2004 initial version

Someone we (Johan and I) know through our website contacted us that he had started an interesting project. He was planning to build a Cobra replica. Not powered by the average cast-iron Mouse or Rat V8, nor or a slainth six or obscure 4-banger. He (Cor Verlinde) decided to use a REAL engine, that mighty alloy hightec short-stroke 5 liter beloved & trusty BMW V12 (we are not biased ;-). So he bought himself a tired '90 750i and used it as a donor for his project. The idea was to pull the engine, give it some cleaning and painting, replace common seals and shoe-horn it into the engine bay of the Cobra.

BUT: the former owner of the 750i was at a certain point where he could barely afford to pay the fuel. When he couldn't spend even that money anymore, he decided to sell the car. Now the v12 is a forgiving engine, but not when it comes to neglecting and lack of maintenance. Cor faced some unpleasant surprises when disassembling his engine...

This is the story about partially rebuilding a tired v12 with 270.000 km and getting a second life in a cobra. The photo's are taken by Cor, some additional comments (based on what I saw on the photo's and Q&A's with Cor) are made by Sean. Cor, thank you very much for this contribution.

Pulling the engine:
The engine of the old 750i. You can tell that the engine hadn't been running for quite a while by the slightly corroded alloy parts. One distributor cap looked like its been replaced, the coolant reservoir seemed somewhat contaimed. It's the first model v12, with the oil filler cap at the right. Someone messed around with the plugged vacuum hose connections of DK motors (the bolt with a hose and 2 hose clamps are not original):

Cor about the car: 'the car had not been running for half a year. After that, it wouldn't run on 12 cylinders anymore. After replacing some leads and spark plugs it fired up and blew lots of dirt and water right out of the exhaust. After that, it was running smooth(!). A 300km trip with the car showed no problems whatsoever and my intention was to remove the engine, clean it and bolt it into the Cobra. Oh boy, did I make a mistake.'

Dismantling the front of the car, to make removal easier. At last there is some easy access to the ABS pump:

And from the right. The 2 loose alumium pipes right in front of the engine are from the a/c condensor connection. Notice the heavy shock absorpers of the front bumper:

Actual pulling of the engine, together with the transmission. Now you can see how unreachable that alternator is. The oil filter canister is still attached to the engine, I personally wouldn't do that. A part of the exhaust (probably just the downpipe) is still connected:

Notice the absolute lack of rust behind the left fender:

The engine and transmission. Wiring loom already removed. Notice the engine hoist brackets (not original):

Empty engine bay:

The removed wiring harness/loom. You wouldn't say it, but this is the complete wiring harness of the engine shaped like its more or less attached in the engine room:

A removed (more or less) fuse/relay box:

Unbolting the hydraulic pump from the engine. Notice the hose connection next to the right of the pump, that gave me some thinking. This is actually the hose connection for the drain hose from the oil filter canister to the upper oil sump. Usually impossible to reach:

Removed cover of the oil supply and return connections to the oil filter canister:

Disconnected leads to the starter. The starter is located behind the heat shield at the bottom-right. Behind the leads the fill-tube for the tranny:

Removed airco pump. Lower and upper oil pan still attached:

Cor about this: 'When I pulled the engine, I was shocked to see how full of oil and debris the outside of the engine was. There was a good chance the engine suffered in the past from a small engine fire because of some small melted parts. Probably because the thick battery terminal wire from the alternator shorted within his metal housing.'

Unbolting the intake manifolds. Here you see you have excellent acces if you remove the noise supressors (located underneath each manifold) first. At that point you don't need to use swivels, just some extensions (1/4"):

Removed and probably cleaned intake manifold in all his beauty. Did you know that left and right are 1:1 identical (and thus interchangable)? Excellent castings:

This is where the trouble began after removing the valve covers. A sheared off head bolt laying around at the bottom of the picture. The empty hole just above it is the place where it originally was located. Notice that these are torque-to-yield head bolts, recognizable to the windings from top to bottom:

Cor about this: 'it never was my intention to pull the heads. I just removed the valve covers to clean the engine. I was really worried and disappointed when I saw the contaminated inside of the engine, worn cams and the sheared head bolt laying around. The broken bolt surprised me, because I never experienced symptoms of a blown headgasket (also engine temperature during testrun was normal). After I saw this mess, I decided to tear the engine apart.'

Also both cams where in bad shape, probably due to insufficent oil supply (loose banjo bolts of the oil spraying bar) or inferior oil quality:

Upper timing covers removed, revealing the camshaft sprockets and timing chain:

Definately sludge/crud buildup and deposits in this engine. Probably the previous owner used el-cheapo oil or didn't bother to change the oil at all. I just hate it when people f*ck up their engine this way. This is the reason why you should use a good brand oil and change the oil frequently:

The removed waterpump. Luckily the PO used coolant and not plain water, considering the overall condition of the engine. The unbolting of the lower timing case cover has started:

Removal of the exhaust manifolds with an air wrench, behind the air wrench is the oil filler tube. Notice that the upper timing case cover leaked severly, a common problem with early V12's (there is actually a BMW service bulletin about this problem):

Removed exhaust manifolds. At the right, the big bolt sticking out next to the lower timing chain cover, is the timing chain tensioner (also infamous for oil leakage). Notice that 1 stud stayed in the cilinder head:

The other side with removed exhaust manifolds. If you look closely, you'll see the actual exhaust valves. Again, 1 stud stayed in the cilinder head (by accident). The central bolt below exhaust port number #10 (third from right) is the coolant drain plug of the engine itself:

Old versus bead blasted exhaust manifolds (more on bead blasting later):

Starter removed, but torque convertor is still attached to the flywheel:

You don't see the back of the engine very often. The intake manifold gaskets are still attached. Between them is the plumbing of the coolant system. Now you have a good look at the coolant cross-over pipe (aka coolant collector) between the cilinder heads. Normally very hard to reach. Also you can see the 3 coolant sensors (from left to right: 1 for both DME's, 1 for EML and 1 for your temp gauge)

All bolts of the lower timing chain cover removed, crankshaft pulley and damper removed. You now see that it isn't necessary to remove the big crankshaft bolt to remove the crank pulley. And that is a good thing. The upper crankshaft position sensor is removed, the lower one still attached:

But this was a tough one, the big crankshaft bolt. Its bolted on with a staggering 440 Nm and you can't reuse this bolt, it's torque-to-yield and a beast to remove. Cor about this 'even an air impact wrench didn't do the job. Only the largest hammer I could find in combination with an incredible whack did it. Normally we use these kind of brute force for agriculture machines, not lightweight alloy v12's':

Draining the engine oil, black as the night:

For removing the upper oil pan you need to remove tranmission, torque convertor and flywheel:

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If you would like to add any comments, remarks and/or corrections to this procedure, feel free to email Mike Oswald and we'll put it on our site. Share your experiences with us so others may benefit from it.