M42 overbore problems.

[rsjon]

Trying to build a 2.1 M42.
Got the correct M47 crank and modified the end.
Asked my engine man to bore the block out to 87mm. Got to the last bore and it broke through to the water jacket. Scrap block. See pic.


My questions are:
Is this common or were we just unlucky?
Are some blocks more prone to this? Perhaps later E36 blocks had thicker walls?
Can you use an M44 block (or even an M40 or M43) to replace the M42 blocks which are getting a bit scarce? I would want to retain the M42 forged crank.

I did notice on a late M42 block there was a boss prepared for a rear mounted crank sensor...I guess they were preparing for the M44.

I look forward to your comments,
Thanks,
Jon.

[rix313]

The picture won’t load for me but the M42 and 44 bore centres are very close together and as such you can’t over do it. IIRC 86 is the most that is safe to do so, even then you are very much on the limit.

Out of interest what length rods and what pistons are you planning to use?

[rsjon]

Thanks for reply.
We were going to use the S54 pistons with machined tops; unsure about rods at present.
Had a close look at stripped bare M42, 43, 44 blocks and they appear to be extremely similar if not quite identical. Mostly some of the bosses have threads on some blocks but not on others. Probably not important. I noticed that the M40 and M42 have 84mm bores as standard and the M43 and M44 have 85mm bores. Whether this would make the latter more suitable for a large overbore or not I do not know....
Jon.

[rix313]

I’d heard of other people trying to run 87 and blowing through all the time. I spoke to a very old school Bmw engine builder last summer about it and he said the bore centres are too close to be able to chew them out too much.

The blocks are all the same as far as I’m aware dimensionally so it’s likely BMW decided they had the space to go out to 85mm to get the extra capacity they needed.

[rsjon]

Interesting. Maybe 86mm would be a safer bet.
I can't find any difference in the cylinder wall thickness between the blocks so maybe 85mm was the max BMW would risk!
Jon.

[rix313]

It might scupper your quest for 2.1L but if it was me 86 would be my limit. Is the engine for a road car or a race car?

[rsjon]

A road car.
I agree, 86mm the max now!

[rix313]

Let us know how you get on. I for one am very keen to see the progress of such a build as I hope to do one some day!