Duc,
There are 2 basic block families-
850-997-998-1098 (small bore)
970-1071-1275 (big bore)
850 has only 1 cam bearing (front) and main bearings are wider than the others.
997 (rare now), -998-1098 are pretty much the same block.
They can all be offset bored to 68.0mm (1220cc with 1100 crank) if you are lucky.
850 blocks & bearing caps need machining to fit a 998 or 1098 crank in. I used to do it, years ago. Not worth it these days.
1275 block is taller than the 970 and 1071, which are both rare now.
1275 S block has side covers, the other 1275s (1100S, 1275LS, 1275A+) have a solid wall block. They can all be offset bored to 1380 (73.5mm) but the S block is a bit thinner in the walls. My spies tell me an A+ block is less reliably overbored than the earlier ones- more crap in the casting.
Any 1380 motor is supposedly a bit thin in the walls if you are going to turbo or supercharge the motor. Better to keep it to 1310 bore, stroke it if you want more cubes.
Bore centres for these 2 groups are totally different. The small bores have 1-2 and 3-4 siamesed, with a water passage between 2 & 3.
The big bores have them more or less equal spaced, a little thinner between 2-3. On all these there is no water passage between any of the cylinders.
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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.
