What could cause this? 360 problems ####update####
How do you even turn the engine by hand after installing the valve gear?
Should be easy with the spark plugs out. With spark plugs in, you have to fight each cylinder, which is a poor-man's test of "compression". If you have to wait 5 sec for each cyl to hiss down (over 2 crank revs), you know the engine is good. A quick test on a used car where you can't run a compression test with a gage. BTW, don't totally trust a gage test since several ways to get invalid readings. On a stand, you can use a socket wrench on the crank bolt. That isn't often easy in the car, especially with a many-groove crank pulley installed, radiator shroud, and trans cooler tubes. I got spoiled on my 1985 M-B diesel engine which has a hex head on the power steering pulley up top I use, so started using the alternator pulley on my 1965 Dart, using a pipe-wrench on the front nose disk. I also have to tighten the belt and push down on the belt to not slip, but much easier than fussing with socket extensions and working a wrench down on the crank which keeps falling off the bolt. Chews up the end of the alternator pulley, but I have spares on the shelf and could file off the nicks and paint if ever presenting the car at a show (not paying a fee so judges can diss my work anyway).
Re what compression values you should measure, with factory camshaft and unmilled block and OE heads, you can use the Shop Manual specs, which I recall is 145 psig for my 1965 273 2bbl (not HiPo). But, few have the OE camshaft today, so most sources just state "less than +/-10 psig variation between cylinders". I would think if any cylinders measure <100 psig, there are sealing problems, which you can go onto a "leak-down test" to determine. Apply ~10 psig with piston at TDC compression stroke. Might need to lock the crankshaft so it doesn't fly down if not perfectly TDC, or at least don't leave a wrench on. If bad, you will hear hissing, so try to find the source. If you hear at end of exhaust pipe (or flutters tissue paper), bad exhaust valve seal is likely. If coming past throttle, bad intake valve. If flowing out of oil fill stick, new getting past rings. If you pour a bit of oil down the spark plug hole and it minimizes the flow, that verifies flowing past the rings. Easier to distribute the oil over the rings in a Chevy/Ford six with vertical pistons. With V-8 on a stand, rotate so cylinder under test is vertical. Slight leak past the ring gaps is normal. With some racing camshafts, you may measure >200 psig compression, and can depend on how fast the engine is turning, due to air-pressure dynamics. Prop throttle open when testing so no pressure drop there. If you ever need to change the little Schrader valve in the tip of the compression gage, don't use a normal bicycle/car one. Needs to have a very weak spring, a special type usually with a white plastic collar.
