Could be a long shot but could the head bolts be bottoming out? If the block / heads have been decked? Or stripped head bolts? Don’t some of the bolts go into water jackets on sbc’s?
I've purchased an leak down gauge. Left head is still on,(driver side) . Attached leak down gauge , air leak coming from intake port on head.i am going to other head back on and leak test. Thanks trucke
If you have a dual plane intake, the commonality between #5 and #8 is the intake runner. The pistons in those two cylinders travel in the same direction at the same time, but on different strokes and fire 360* apart from each other. With no compression on #8 and low compression on #5, I believe you will find the primary problem on #8 which is then affecting #5.
If you lay the removed head on a workbench and pour some alcohol in the combustion chambers, you should be able to tell if the valves are sealing. Might want to add a little food coloring to the alcohol so you can see it better.
The Jalopy Journal
