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Blow by

Just installed new heads and cam(350), had some trouble with timing at first, but have it straight now. I was going through the break in process and about 25 minutes in I noticed blow by coming from the valve covers. When I reved it up it got worse. Shut it off and let it cool down and fired in up again and there is no blow by. What would cause blow by only at operating temp?

I’ll assume that your rings aren’t due for a refresh. Often tightening up the top end “unbalances” the leakage and you end up with blow by. The hot:cold thing is a bit to wrap around since the leakage should be worse when cold …except that the viscosity would be higher.

I’ll also assume that you have valve cover “vents” of the mesh variety instead of legit fresh air venting and normal PCV system.

The valve covers are aftermarket. I have a pcv system running on it. It didn’t do it until about 25 minutes in. When I would rev it up it got worse. After I let cool down, I started it up an let it run for a couple of minutes, no blow by. I adjusted the valves in the mean time, but I don’t think this would effect it any. Never had blow by before, could this be a sighn of a bad head gasket. No smoke anywhee else. No water in the oil, no oil in the water.

Are you sure it is blow by (passing over the rings and escaping from the crankcase) and not leakage from cracked heads, which would tend to leak more at higher temperature?

What does it look like with the valve covers off? Blowing out of the oil drain down holes?

Get it hot and do a compression/leakdown.

if it was a cracked head, wouldn’t i have water in the oil or vis versa. I just bought the heads from a local speed shop. I don’t think they would be bad.If it was a head gasket wouldn’t it smoke out of the headers also?

There is still blow-by when it’s cold, but since there is no smoke yet, you don’t see it…As the engine warms up, the oil starts burning and you see the smoke blowing out of the crankcase. Until the oil gets hot enough to smoke, there is nothing for you to see…

So you got the “Carb Spitting Fuel” Chevy to start, now it’s all blowing by the rings…

If you are going to build an engine, you have to build the ENTIRE engine. Top AND bottom end…

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Not necessarily. Depends on how it cracked.

The speed shop took used heads and rebuilt them. It’s impossible to know what their quality control procedures (if any) were. Ideally, they should be magnafluxed and UV penetrant tested before machining.