just wondering about the dramas/pitfalls of using unmatched m20 engine cases ie.war tme drive side half to bm series timing side and also vice versa.
is there any way of telling that you have matching cases to start with?
cheers
dave

email (option): daavem20@gmail .com

Usually they are stamped with matching numbers. I guess the important machining was done with the cases together. You wouldn't be the first to use un-matched cases though. What do you mean by BM series? 1953? Ron

email (option): ronpier@talk21.com

Typical! I have hundreds of pictures of these bloody machines and ths is the only one from this angle!
As far as I know the cases are stamped with a number at the positions shown by the arrows and the numbers should match. As you may be able to see, the decades of corrosion have made the numbers on these (matched??) cases pretty much illegible.







Sorry no experience with using mismatched cases (as far as I know :thinking_face: )

email (option): petercomley@web.de

apologies ..ron bm as in bm20 .....two m20 engines i checked have numbers similar to yours..more numbers on the left than on the right,which has one number matching one of those on the left.
hope that makes sense
so i suppose thats what the term matching numbers mean
be good to know what they really mean
interesting
cheers dave

email (option): daavem20@gmail .com

One number on each crank case half is the 'pairing' number. They should match and are generally in alignment with each other...The other number which is above them is the part number for a set of crank cases....
I've seen many engines run with 'mixed' crank case halves and have used them myself a number of times..B and M Series crank cases have the two halves located against each other by the crank case register. That is the lipped section on one case and the recess in the other that fit together when the cases are joined...

These registers were machined in the same operation as the main bearing housings, so if the register is a good fit then the bearings will be in line and there won't be any problems...It seems the registers were machined to a close tolerance as I have yet to see a set of cases that are not a good fit when put together...Remove any burrs/damage from the lip and recess before assembling the cases...

In two areas the machining tolerances were not so close and there can be a discrepancy between 'odd' halves when they are put together...Namely, the magneto platform faces and the cylinder base surfaces...Due to those tolerances it's a game of chance here and some cases match better than others....The step between the two halves can be pronounced when the match isn't good, perhaps as much as .010"-.015"...Take care to ensure the cases are lined up accurately radially before checking for any discrepancy...

So, if ether of these two surfaces are not in alignment they should be lightly machined to correct the situation, removing the minimum amount of material in doing so....With that done the cases can be used...Ian

email (option): ian@wright52.plus.com

thanks ian and the forum for the information and taking the time to explain it to me
cheers dave :relaxed:

email (option): daavem20@gmail .com