Current progress so far
Links to parts and information websites
Where I will be building my diablo
Where most of the important bits come from
The stuff it all bolts to
What makes it go
What makes it pretty
The cars nervous system
Where you sit
The gubberment bit
Who really cares?
Stuff that will not fit anywhere else
Other diablo replica websites


Due to the size of these pages, I have split them over a number pages for quicker loading. You can click on some of the images to link to a larger version:

» Index Page Introduction, About the BMW M70 V12
» Page 1 Engine removal
» This Page Fault finding, condition checking
» Page 3 Stripping and cleaning
» Page 4 Re-assmebly, Timing chain & heads
» Page 5 Ancillaries

Fault finding & Condition checking

Here is the side of the engine that had the problems with the HT leads. As it was on the crane, I decided to do a compression test. To do this I hooked up the starter motor back on to the engine, removed all the spark plugs and placed a drip tray underneath to catch all the oil that will pump out from the block as it turns over. You should not turn over the engine excessively as this stage, to prevent excessive wear from poor lubrication.

Compression results are as follows:

 Cylinder 1  135 psi  Cylinder 7  135 psi
 Cylinder 2  130 psi  Cylinder 8  125 psi
 Cylinder 3  130 psi  Cylinder 9  110 psi
 Cylinder 4  125 psi  Cylinder 10  122 psi
 Cylinder 5  125 psi  Cylinder 11  130 psi
 Cylinder 6  125 psi  Cylinder 12  125 psi

Cylinder number 9 showed a marked decrease in compression compared to the others. It was this cylinder that had the broken HT lead, causing the spark plug not to fire. It is likely that as the cylinders were wet with fuel, the rings on this cylinder are more worn than the others. I have yet to decide if I will replace the piston rings. I will consult an engine rebuilder for advice on this aspect.

Here is the empty engine bay. The exhaust headers on the drivers side are still installed as they don't actually foul anything, and the bolts on the down pipes were rounded and would require cutting off. It was just easier to leave them where they were. If you get the opportunity to remove the exhaust system, I would do it before any other job in future. There isn't much else in here I need in here, bar a few cables and useful fuse boxes etc.

Here is a good example of why its necessary to tear down an engine before refitting it. The banjo bolt feeds oil along the camshaft via a spray bar which delivers this oil along the cam lobes. Here one of the banjo bolts has undone from vibration. This is shown exactly as it was after I removed the cam cover. This is a known fault with the M70 engine, and something I will be making sure does not happen again. A total of four of the bolts were either loose or had undone to such an extent, the only thing holding them in was the cam cover itself. Fortunately, the camshafts showed no appreciable wear along any of the lobes or bearings, so it looks like disaster was averted in the nick of time.

This picture shows the engine with all the ancillaries removed. Its an awful lot smaller like this. I have yet to drop the sump and investigate the bottom end. I will need to do this as there is a lot of congealed oil present in the timing chain covers. This phenomenon is caused by the temperature differential between the engine at the hot end of the block, and the cold end of the timing chain covers, which is blasted by cooling air from that huge viscous fan. The oil sets into a hard black paste, that becomes more and more difficult to remove, the longer it is left. Infrequent oil changes and contaminated oil can also cause this problem. Rover V6's suffer from this, it is affectionately known in the trade as 'Black Death'. All of this black sludge will need to be removed to ensure that none of it makes its way into the oil ways where it could block an oil feed, causing the automotive equivalent of a heart attack. You should be careful handling this stuff, especially if you are sensitive to oil, as this stuff is basically concentrated crude.

The basic moral of the story is that the BMW M70 engine requires frequent oil changes with good quality oil.

With the engine almost stripped and a compression test completed, I had to decide how far I wanted to take the rebuild. Undamaged bores mean that any compression problems are in one of three places, piston rings, valves or head gasket. The head gasket was undamaged and a chemical test of the coolant showed no sign of exhaust gas contamination. So I have decided to change the piston rings and regrind the valves.

With the sump removed I can check the condition of the bearing shells for damage. The engine was very quiet so its unlikely I will find excessive wear here, but it wont hurt to check.

With the engine in this state, its time to start thinking about what other parts to purchase. An engine gasket set is essential, and at £211 for the BMW original parts, its not cheap. Note that this kit does not include head gaskets or inlet manifold gaskets. The head gaskets come out at £71 each (you will need two!), and you will also need two sets of head bolts as the original ones are stretch bolts which cannot be reused.

A known fault on M70 engines concerns smoke from the exhaust under hard acceleration which dissipates after a few embarrassing minutes. The valve stem seals crack and deteriorate, causing oil to become mixed in with the fuel air charge. If you are regrinding the valves (and you should be thinking of doing this if you have stripped down the engine this far), you would be daft not to fit a new set of seals at this time. Prevention is better than cure.