I have a shop that will modify my stock 600 crankshaft for longer stock, they weld one side and offset grind it, is this worth doing ? and what size stroke is best
or is it better to start with a 903 crankshaft and increase the stock of the 903 crank ?
There are several issues to note when stroking the 600 crank (or swapping in a Fiat 850 843cc 63.5mm or 903cc 68mm crank).
The original 633cc 600 block can theoretically take up to a 74mm crank with modifications done to the block for clearance issues (ala Abarth). However, the maximum safe overbore size is 63mm, so piston selection & con rod choice (length & bearing journal size) is a concern depending on the crankshaft being used (the 600 con rods have smaller-diameter crank journals than the later 600D/850/903/etc. con rods).
If using an early 600 (not 600D) block, parts-compatibility dictates that a good choice would be to use the Fiat 850 843cc 63.5mm-stroke crankshaft, the 600D connecting rods, and your choice of Fiat 600 633cc type oversized pistons (generally sized up to 61mm) or Fiat 600D 767cc type standard 62mm pistons (or 600D oversized pistons up to 63mm). While 850 rods would be nice, they require a minimum bore of 64mm for clearance issues. The 600D rods share the same crank journal size as the 850, and CAN be used with under-64mm bores, hence the deferral to the 600D con rods.
Using this combination of parts in the 600 block, you can basically get up to 792cc displacement.
The 600D engine block has a maximum overbore of 65mm, though the cylinder walls get thin at that size. A good "upgrade" parts combo would be the 850 843cc 63.5mm crank, 850 843cc con rods & 850 817cc 64mm (or 64.?mm oversized) flat-top pistons. With this, you get 817cc-843cc out of a 600D 767cc block.
These upgrades are both done using
STANDARD FIAT 600D/850 PARTS. Of course, more cc's could be gained using the longer-stroke 903cc 68mm crank, but pistons &/or rods would have to be custom-made depending on the block used.
A very good page for reference on block/crank/piston/rod use can be found on Mahlon Craft's website at
http://users.ntplx.net/~kinukoyc/Pages/tech_pgs/engine_how_to.html As for your question about stroking the crankshaft, the 903 crank is your best choice to start with, and can be stroked to 70mm or 74mm quite well. Stroking a 600 55mm crank is really a lost cause when other options are more readily available.
Cheers,
-JS.