Inliners International Forums General Discussion Engines 250 cold engine (knock/noise)

My chevy, 250 (not stock), has a noise, kind of a knock at start up, until it's warm. Then it goes away. I've use my stethoscope at the top end and bottom end, all around and can't find the source. It runs and sounds great after about 3 to 5 minutes. A good friend, mechanic / restorer of older exotic cars, agrees that it sounds fine - he is unfortunately about 6 minutes away so he never hears the noise.
Any ideas? Maybe an exhaust leak that swells and closes after temperature rises?

Piston slap?

Did you build the engine?

Was it bored out or just honed and re-ringed?

Did you 'scope the fuel pump?

Did you replace the cam bearings?

Piston slap. Would that stop after it's warm? My friend suggested that but he can not hear any evidence of it when I take it to his shop.

I rebuilt the engine, for the second time after my cam went flat (no zinc in the oil, I now use EOS and Valvoline VR1), it didn't make the noise when I first built it.

The cylinders are .080 over, 307 pistons, new rings. It has a Crower cam and lifters. New cam bearings, Clevite, I think.
I did not `scope the fuel pump. I not familiar with that phrase

Yes, thermal expansion, as the pistons heat up they grow larger. Do you know how much piston to cylinder wall clearance the block had when it was freshened this time.

'scope is shorthand for stethescope...


You need to have your friend come over to listen to it, or leave it at his shop overnight so that he can hear it when its cold.

No I don't know the clearance. And when I think about it, I didn't put new rings in it this time. They only had about 4000 mile on them from the first rebuild.

I would check the fuel pump pushrod.

Thanks I'll check it this weekend. Am I looking for anything in-particular? Like a stuck rod?

Well, after I posted about the pushrod I looked at the 250 I have on an engine stamd but haven't taken apart yet. There probably ISN'T a pushrod - most likely a lever running right on the cam. But, it could still be a source of noise.

V8's use a push rod.
Stethoscope the fuel pump...