I posted this to the perf-list and got some really good suggestions as to the problem, but you can never have to much information..

Engine: 1970 351W with approx 70K miles on a 8-10 year rebuild..

History: Castrol GTX 20-50 oil used since rebuild (I got the engine from the person who had it rebuilt and put it in my '64 F100 4 months ago)

3000 mile oil change due.. I elected to use Castrol Syntec BLEND in the 20-50 weight and put on a new Fram TOUGH GUARD filter..

Problem: Within 10 minutes of changing oil and letting the engine idle, it starts to knock at idle (like a ballpeen hammer tappin on the block).. Oil pressure at idle is 20lbs when hot and 70lbs when cold.. prior to the change to the synthetic blend the pressures were 10lbs at the same idle hot and 60lbs cold... (understandable since the synthetic is "thinner" albeit the same viscosity)...

The knock is only heard at idle... 100 to 200 rpm off the 800 (in park) rpm idle and the knock is gone.. and otherwise the engine is smooth and performs as before...

Suggestions, ideas?... (yes I intend of rebuilding the engine anyway, just wanted to wait a little while)

 

Man put some 10W30 or 40 and get that 20W50 out, you aint got a race engine there. That should settle it's stomach. You haven't been over revving have you?

John

 

Nope... havent seen the high side of 5000rpm since i have had it... gonna change the oil back to a 10-40 and change the filter to a "normal" one... let ya know the results

 

The better filter should be OK.

John

 

Went back to a 10-40 conventional motor oil, a puralator filer (saved the Tough Gaurd Fram for someone who might need it).. knock still there... but.. when the oil pressure drops to 10lbs at idle the knock goes away... but now I also notice oil smoke on crank up after a short (but hot) drive... weekend after next the engine comes out and gets delivered to the speed shop for a complete redo... thanks anyway ...

 

Pulled the engine.. delivered it to my trusted speed shop... complete rebuild... better not knock when I get it back...

 

If you kill an engine in 70,000 miles I don't want you driving anything of mine.

John

 

'tweren't me that put the first 70,000 miles on the engine.. 'tweren't me who took the engine to a "back yard" rebuilder for it's initial rebuild.. I was "given" this engine with 70K on it.. all I did was change oil and drive it... albeit somewhat harder than the origianl owner, but nothing it should not have handled... my truck is lite, no towing, no hauling.. rpm never got over 5000.. so, shucks, maybe, just maybe, sumptin' mechanical went wrong other than this nut behind the wheel???