!st of all, my apologies for the slow response to your query, but I seem to have problems when trying to put lengthy answers onto the forum--when I try and put them through, they just "blow out"--very frustrating.
The fact that one of your cars is a Belvedere means that it must be a 500C, with the OHV engine (as against the side-valve engine in the 500A). Which of your 2 cars is giving you the problem? What does the oil-pressure drop to when you rev the engine? What oil are you using in the engines? Castrol used to recommend (for overseas use) Castrol XL for when the ambient temperature was over 50deg F, and Castrolit for below 50deg F. The sump capacity of the side-valve engine was 4 pints/2.27ltr. The fact that you have good pressure initially (should be about 35psi//2.5kg/cm when engine running normally) would normally indicate that your bearings are in good condition. Overfilling the sump with oil can, strangely produce a similar effect (ask M/Benz about their 2.5ltr 5 cylinder diesels) because as you rev up, the crank thrashes the engine oil and put lots of air bubbles into it. Another cause could be worn teeth in the oil-pump, but again, you are getting good initial pressure. I honestly don't think that whether you still have 'white-metal' bearings or have converted it to 'shell' bearings will have any bearing (oops,) on the problem.
I have a 'pukka' factory workshop manual for the 500a/500B and 500C--sadly in French as that was all Fiat England could find for me--many years ago when I was an apprentice, my first car was 2 500As, stripped to bare chassis and then built into 1 car--I have a very soft spot for the Topolino. keep us informed as to how you get on.:bang:
