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Chevy 302 engine – History - The heads are the "186" cast iron heads and a 1969 cast iron, ”010” block. The engine is fresh from the machine shop. The cleaning, valve job, and magnafluxing were performed by the machine shop on both heads and the block. The machine shop has performed many custom jobs for me in the past with no problems. This is a rebuild and the engine has not been running nor has water or fuel been through the engine.

I’m adjusting the mechanical lifters (cold) without the intake manifold installed. I’m using the starter to turn the engine over. I notice a puff of air coming from the heat riser port, in the middle of each head, from one of the two center cylinders, on each side of the riser port, every time I rotated the engine. Normal because one of the center exhaust valves, on each head, pass exhaust gases to warm the intake.

Problem - But then I noticed, as I was turning the engine over again, that there was a second puff of air, not as strong as the first, coming out of the same heat riser port. Also, while one or the other exhaust valve is open and the air pressure is flowing, there is no airflow/pressure on the cylinder next to it. As if the block is cracked or bad head gasket bad. So that's good!

I looked into the heat riser port, I can see the exhaust valve’s stem and that the riser port leads directly to the exhaust port. The other cylinder exhaust port does not have the same opening or design as the first. It looks closed to the riser port.

I’ve checked the torque on the heads and their good. The compression check is good. Then I decided to pressurize each cylinder. I shot a 125 psi of air through the spark plug holes to try to trace the second puff of air. Of course, the correct exhaust valve to the heat riser port passed air as it should. Then I tested the other center cylinder. When the exhaust valve started opening, air passed through to the heat riser port just as the first exhaust valve did.

When both intake and exhaust valves are fully closed there is no airflow from the cylinders through the exhaust valves to the heat riser port, a good seal. But as soon as the pressurized cylinder exhaust valve starts to open, on either head, air starts flowing out of the heat riser port. And there is no bleed over into the non-pressurized cylinder.

Just in case, I pulled one head off yesterday to just double-check and make sure that the new Fel-Pro gasket was not broken nor damaged. And it was good. I could see the copper stray coating that had stuck to both the head and the block between cylinders.

I’ve looked at an old set of Chevy “462” heads and they have the same design as my “186” heads.

I’ve heard that some of the small block Chevy heads do have both exhaust valves open to the heat riser. But I don’t see that on these heads and if so, the flow paths to the riser port must be different.

I’ve built many of these older engines and have never run into this problem before but they have all been hydraulic and hydraulic roller cam engines. This is my first solid lifter cam engine. On the hydraulic lifter engines, I get a good feel for the lash while the engine is on the stand by turning the engine by hand then put it in the vehicle for the final adjustments and I have ran an engine on the engine stand.

This is my first time adjusting a mechanical cam using the starter and no intake. Could this be why I’ve never seen this before? The engine is turning much faster. I don’t see or hear it when the engine is turned by hand.

Would it really matter since all the exhaust gases are going out of the exhaust port and the heat riser head ports? Maybe a little extra heat. The intake does not have a heat riser on either side because they are blocked off from the factory.

Is there another port that I’m not seeing? Or I’m I missing something here? Or is this normal and that I would not have seen it before when turning the engine by hand?

Any help would be apprehended!


True Hotrodder
1,955 Posts
Personally I wouldn't be using a starter to turn the engine over when setting lash on a mechanical lifter cam. Just turn it to each cylinder with the piston on TDC by hand and set that cylinder's valves to the correct lash + .002.

More for Less Racer
20,929 Posts
That second smaller puff you are so worried about is normal, it is happening because of the engine turning faster from the starter than turning it by hand.

Spinning on the starter traps more air in the cylinder on the intake stroke, it is compressed but not fired, then the exhaust valve opens with the piston just a little past 2/3 of the way back down the get that initial puff just as soon as the valve breaks seal with the seat due to that partial pressure in the cylinder from the air still being half compressed....then you get the regular expected full puff after the piston reaches BDC and starts back up the bore shoving the exhaust out through the now wide-open exhaust valve..
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