Updated: Feb 9, 2017
The 903 is still in process, but I wanted to share with you all what I am up to.
In order to install the 903cc engine from a Fiat 850 into a Fiat 600 you have to reverse the rotation of the bottom end. The top ends rotate the same direction. Chris Obert wrote a pretty good guide. The easiest way, I think, is to replace the timing chain with timing gears. The gears can be purchased from Exotic+Raceparts here or from Fiat Plus here. You will also need a spacer because the timing gears are thinner than the chain sprockets, which you can have a machine shop make or purchase here. Also, you’ll need to replace the front and rear main seals with non-directional or clockwise seals.
The other way to go about this is to leaving the timing chain alone and installing a reverse rotation camshaft from a Fiat 127 or 1050cc motor and use the distributor/oil pump drive gear from the 600.
I’m reversing the oil flow and using an external oil filter. This process is fairly common and well documented: Paul’s guide or in Fiat and Abarth Tricks by Greg Schmidt or Cosentino’s Fiat 850 Competition Manual. The latter two you can find for download on my literature page. I’ll say more later.
The Weber is now rebuilt and on the Alquati intake. It’s topped off by the Bayless Racing air filter.
If you have any questions or comments, contact me using the contact page or email me at firstname.lastname@example.org. Cheers.