"boreability" of m30 block

[daicos35]

Guys/Gals.
The late M5 (s38b38 engine) had a 94.6 mm bore. With only 5.6 mm web spacing, this would appear to be the maximum BMW was willing to take it from the factory on an m30 block.

My question is- Is there an ultimate difference between the earlier m30 93.4/84 block, and the later 92/86 block , in terms of maximum safe bore? ie Can both of these blocks also be taken to 94.6 mm?

 

[Devinder]

You need to consider the thickness of the entire cylinder, not just the web between the cylinders. I just bored an 89 mm 733i block to 92.5. Before boring, I checked two blocks with an ultrasonic thickness gauge to find the best one to bore out. The thinnest spot was actually on the right rear of cylinders 5 and 6. One of the blocks could not have been bored out.

The core used to make the water jacket in the sand mold can shift and create variations from block to block. I suspect the larger-bore blocks like the M90 and S38b38 were just selected based on core placement. You should have the shop use an ultrasonic gage to see if the block can be bored out.

Devinder

 

[paul cain]

blocks are totally different

the Euro 3.8 block has Siamesed cylinders i.e. no water flows between the cylinders. This is the same basic block as the M5/M6 and the early euro "L" blocks in the 1979 -81 Euro 635csi's.

The US blocks (3.3 and 3.5) are not siamesed and are limited to (as I recall) 92 mm at the outside. Metric mechanics old manuals are great resource for this for more detail.