Hi Chris. Here's a few things that I can tell ou off the top of my head....
The 903cc block is taller than the 817/843cc and 600 blocks - 7mm if I remember correctly. The 850 blocks (817, 843 & 903) use larger diameter head bolts than a 600. The 600 water pump arm is a little shorter than the 850 blocks. The 903 water pump does not bolt directly onto an 817/843 block, but will fit with a simple strap adapter. 817/843 rods are shorter than the 903cc ones, and the pushrod length is different too. The A112 pump is a little different, and as I remember, does fit, but needs a bracket adapter as well. You could use a PBS water pump.
If you are trying to build a 1050cc engine and want it to look like a 600 engine, you must use the 903cc block (because it is taller like the A112 block). Use the A112 connecting rods as they are stronger. You should get high tensile rod bolts (Scuderia Topolino has them) as the stock ones are too weak for performance use. The main journals on the A112 74mm crank are larger in diameter, so you need to align bore the bearing saddles in the 903 block to make it work. The snout of the A112 crank is not drilled for the clutch pilot bushing, so you need to get this done. Scuderia T. has roller bearing type ones. You need to offset bore the block for the A112 pistons. You can get a copper head gasket from Scuderia T. as a template. You can use standard A112 pistons, but you should think about the compression ratio. The A112 head has a slightly larger combustion chamber volume compared to the 903 head. You may need to deck the block for zero deck height. I recommend using an A112 head as they have valve guide seals. You can put the A112 guides in a 903 head though. I highly recommend getting a good 903 or A112 that has not had much metal milled from the surface to ensure that you don't have too high a CR. Use the A112 distributor driveshaft with the gear on it as it has the correct reverse rotation (if you plan to use a CW rotation camshaft) or get a reverse rotation cam gearset and use an 850 cam.
A lot of 903 (Fiat 850) blocks have badly worn oil delivery sealing ring surfaces on the block. The sealing rings are very difficult to find, so you might want to consider the reverse oil flow and pressurized center main bearing modification. Also, I think the easiest clutch setup is to use a Fiat 850 clutch disc and pressure plate on a 600 flywheel. You need to get a machine shop drill the flywheel for the 850 mounting hole and locating dowel locations. Forget about using a 600 clutch setup.
There is lots of good reading on Mahlon Craft's and Scuderia Topolino's websites. If you don't have a copy, you should try and locate (ebay) a copy of Greg Schmidt's old books "Abarth" and "Fiat and Abarth Tricks."
Gil N.
