Put together my 330 10.5:1 comp and had between 150-175psi in 7 of my cylinders and negliable compression on the last one (#2 cyl) did a leakage test and it had almost no leaking air. Pulled the pushrods to find one to be maybe 2-4mm too short.

Is this enough for the cylinder to not make compression?

 

I don't think so.

 

That is .157" of an inch short. The valves should still open enough. Did you replace the pushrods? My sure your test didn't crap out on the last cylinder.

 

Only switched one pushrod and it happened to be a bit short. Tested all the cylinders multiple times and this wasn’t the last one i tested. Just changed the head gasket but i can’t imagine it would leak enough.
and the leak test was positive with around 10-15% like the rest of the cylinders

 

Troubling

 

my thinking was that if it was too short there would be less and less air for the piston to compress compared to how much it can push out

 

If you think the short pushrod is the problem, swap it to a different cylinder and re-run the test. If the new cylinder is now the low one, you have found the problem. If not, it isn't the pushrod.

 

I am astounded i did not think of that myself, thank you

 

For any future readers, the problem was not the pushrod. However, the exhaust valve had gotten slightly bent by the stem letting air out in what sounded like a blowoff valve. Spotted it by taking the head of and shining a light into the exhaust just to see it between the valve and valve seat.

 

Glad you found the problem; but what caused the bent valve stem is the question!

 

there has been so many problems with this engine since i picked it up from the previous owner who «rebuilt» it. so i believe it was bent from the moment i got it just didn’t notice due to all the other problems like the piston with a hole through it.