1972 CS M90 Conversion

[Deano1]

Hi Everyone.

Thought I'd start a project thread here, following my previous post in the technical section.

I partially disassembled my M90 engine yesterday. Much to my surprise no bolts were broken, and no nuts were rounded in the process. Witnessed signs of someone having been in there before. The combination of 12 and 13mm nuts on common fittings was annoying.

The valve train looks to be in very good condition. Very little sign of wear.

I removed the head. The gasket was in good condition. I haven't cleaned the cylinder head surfaces yet, so my measurements aren't 100% accurate but it looks like the cylinder head is 128.6mm thick. It had a 1.72mm M90 gasket fitted. I need to get the machine shop to assess the head for hardness, cracks, and the flatness of the head surface.

There's no lip on the cylinder bores, faint cross hatching lines, no obvious signs of damage to the cylinders. I haven't measured them yet. My initial thoughts are hone and new rings, bearings etc. however I'll get the machine shop to advise.


Regards.

Dean.

 

[ScottAndrews]

Did you do a leak down test prior to disassembly?

My M90 with 150K miles was solid in tems of cylinder bores and bottom end.

The valves were all bad, and the head gasket leaked between cylinders. Rebuilt the head and off she goes. Runs strong as ever..

A truly great engine!!

If there are no obvious reasons, I'd leave the bttom end alone. If you did not to a leak test, then you really have no idea about the rings, so probably best to redo the bores. Although that's a big step that may not have been necessary. ALWAYS do a leak test...

 

[Deano1]

Did you do a leak down test prior to disassembly?

My M90 with 150K miles was solid in tems of cylinder bores and bottom end.

The valves were all bad, and the head gasket leaked between cylinders. Rebuilt the head and off she goes. Runs strong as ever..

A truly great engine!!

If there are no obvious reasons, I'd leave the bttom end alone. If you did not to a leak test, then you really have no idea about the rings, so probably best to redo the bores. Although that's a big step that may not have been necessary. ALWAYS do a leak test...

Hi Scott.

The engine came as spares with the car so no, I wasn't able to do a compression or leak down test. It had been sitting for some years so the leak down test may not have been successful. I put an M30 engine with 300,000km on it in my E32. Compression 175 to 180 across the cylinders. Didn't do a leak down test but it runs sweet.

Regards.

Dean.

 

[ScottAndrews]

The biggest bugaboo wit the M90 is the very short gap between the cylinders. This area almost always results in cylider to cylinder leakage. And, as I said above, exhaust valves.

The bottom end is basically bulletproof, so unless the cylinders show signes of serious wear, I'd leave it alone.

 

[Deano1]

The biggest bugaboo wit the M90 is the very short gap between the cylinders. This area almost always results in cylider to cylinder leakage. And, as I said above, exhaust valves.

The bottom end is basically bulletproof, so unless the cylinders show signes of serious wear, I'd leave it alone.

I did see a company online yesterday that make head gaskets that cut into the head/block thus in theory eliminating failure between the cylinders. Dropped off the cylinder head at the engine shop today. Fingers crossed.

 

[ScottAndrews]

I did see a company online yesterday that make head gaskets that cut into the head/block thus in theory eliminating failure between the cylinders. Dropped off the cylinder head at the engine shop today. Fingers crossed.

It isn't a big an issue as it may seem. My M90 had significant leakage between two cyliners at 150K miles. SInce being re-habbed, it has run strong for 15 years...