Hi,

I have a 235 engine with the splash oiling in my 53 Chevy Bel-Air.  My engine has mechanical lifters, and I swapped the fiber timing gear for an aluminum one and I don't think I have experienced a dull thud noise from that swap.   

The engine is very noisy sounding due to the thin steel valve covers propagating all of the noise, but it's hard to say what is going on. 

It could be valves that need adjusting, have you confirmed that there within spec?

Wish you the best of luck!

The Australian Holden red engine (and probably others)  originally had the fibre cam gear but some aftermarket manufactures made a alloy replacement that was a little noisier from 1st start but did quieten down somewhat after a while when the gears settled in.

Tony

Bill describes that the "thud" occurs when timing is "correct" but subsides when retarded slightly.  In my experience, combustion rumble occurs when timing is over advanced for a given engine/combustion chamber design.  Since the engine does not exhibit audible knock/pinging, I'm assuming combustion rumble which is the result of the combustion process starting too soon before TDC which means the air and fuel mixture is being compressed as it burns. That means peak cylinder combustion pressure occurs before TDC. This is a classic phenomenon described in basic IC engine text books. Examine your spark plugs.  Are there any tiny balls deposited on the porcelain center electrode?  That is a sign of Pre-ignition or pinging which may not be audible in this engine.  Has your cylinder head or block been milled which would increase compression ratio?  Pre-ignition can result in spark plug, rod & main bearing damage or piston damage.  Also, what gasoline are you using?  Does the rumble disappear when using premium?