Top end knock
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Joined: Jan 2011
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From: Winona Mo.
Top end knock
It looks like this is the day for engine noises so I'll ask about mine. Last year I built a 383 for my 72 Corvette. I've had a lot of trouble with it but I'll skip to the last part.
It has what I think is a top end knock, it's more of a dull sound and is consistent with piston speed. I think it's piston slap. This was a block that I had bored a few years ago and it sat around for awhile. It got surface rust in the cylinders and I stuck a hone through it to clean it up. Then a couple more years went by and I decided to build it, it was rusted again, so I gave it the hone again.
The rotating assembly is correctly balanced by a good machine shop and there are no lower end clearance issues. It's not lifters or the flex plate. I pulled the pan and the crank and bearings are still like new.
I had a 72 Nova that I sold to get this car and it had a similar sound for 10 years but ran great, it did have forged pistons, this one doesn't.
I don't know any way to really tell if it's the pistons or not so I'm asking opinions.
It has what I think is a top end knock, it's more of a dull sound and is consistent with piston speed. I think it's piston slap. This was a block that I had bored a few years ago and it sat around for awhile. It got surface rust in the cylinders and I stuck a hone through it to clean it up. Then a couple more years went by and I decided to build it, it was rusted again, so I gave it the hone again.
The rotating assembly is correctly balanced by a good machine shop and there are no lower end clearance issues. It's not lifters or the flex plate. I pulled the pan and the crank and bearings are still like new.
I had a 72 Nova that I sold to get this car and it had a similar sound for 10 years but ran great, it did have forged pistons, this one doesn't.
I don't know any way to really tell if it's the pistons or not so I'm asking opinions.
Drifting
Joined: Dec 2002
Posts: 1,946
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From: Long Beach CA
Did the noise just start, or has it been there since the rebuild? Does it sound like it's under a valve cover, or down lower? Is it there all the time, or does it get quieter or go away as the engine warms up? Try pulling a spark plug wire one at a time to see if that makes a difference.
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Joined: Jan 2011
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From: Winona Mo.
The noise has been there all along. I hadn't remembered to pull plug wires, I'll try that next time I fire it up. I do have roller rockers but they're only roller tip. It doesn't sound like lifters, the frequency is too slow and it's more of a dull sound. It sounds a lot like a main bearing but I've actually looked at them and they're fine.
It has LT1 rods with dished pistons and 64 cc heads, so I don't think it's a compression knock. I'd just drive it and see how long it lasts but it's embarrassing.
It has LT1 rods with dished pistons and 64 cc heads, so I don't think it's a compression knock. I'd just drive it and see how long it lasts but it's embarrassing.
Racer
Joined: Nov 2004
Posts: 485
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From: Oakville ON
Try this. Start the cold engine and rev to 1500 for 1 minute and shut down. Wait 2 minutes and restart and repeat. Is the noise less after the shutdown and compared to a normal cold start and drive off right away?
