Knock knock

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I have a 1941 sedan .original flathead. When i first start up it knocks for a bit say around 4 minutes .i usually just drive away and the knock disappears quickly. So while i am driving the engine is quiet. Upon stopping for a bit and restarting it knocks agin then goes away as i drive ,pretty much immediatley. The knock never gets worse. I have replaced the original fuel pump with a 6 v electric. Could it be the mechanical pump is knocking . Any ideas about this. I cant help but think that if this is a connecting rod knocking that it would get loader and loader as i dive more and the engine gets hotter,but this does not happen, thank you oil pump?

[scrapiron]

If the noise isn't heavy enough to be a rod it could be cracked piston or loose pin,,,Maybe?

[makemc]

in my experience, that sounds like a main bearing knock,
it knocks until the oil gets to it and fills the gap.

[Cecil/WV]

Originally Posted by makemc

in my experience, that sounds like a main bearing knock,
it knocks until the oil gets to it and fills the gap.

I agree! That's where I would look.

[oldford2]

Maybe just piston slap. Once they warm up and expand it diminishes. I would not pull the engine to check mains and rods. Just drive it. If it gets worse then you might have a bearing problem. Use 20/50 oil if the engine is getting tired.
John

[Lawrie]

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I agree with John,Piston slap.
I had the same noise ,it would take till the end of the street go away,got worse in the cold weather,
Pulled the eng and found some pistons with .006 clearance.
Lawrie

[CWPASADENA]

Originally Posted by Cecil/WV

I agree! That's where I would look.

I ALSO AGREE.

Classic situation with too much clearance in the pistons.

Chris

[40 Deluxe]

Not likely a main as suggested. Four minutes of knocking! Doesn't take that long for oil to circulate (I hope!). I agree with it being pistons.

[1932tub]

Piston slap, drive it