I have a 1987 dodge 150. The engine has been rebuilt with .040 pistons and .10 under rod bearings. I built the gm hei conversion and it has a fat spark. The previous owner had a radiator leak and it ran hot on a few trips. It had hard starting problems low power and quit. It won't start at all. I did a leak down test and all the cylinders were 0 to 20% Air was heard in the pan..I suspect the rings lost sealing power from overheating. I put in rings and rod bearings and it won't turn over, with the starter or a bolt in the crank pulley. The cylinders are oiled and the bearings were greased.

is this the first time that you are trying to turn the engine over since it was re built?

if yes, did you spin the crank after installing the mains, then again after installing the rods and pistons? Prior to installing the cylinder head.

If yes and it seems to roll over with a just a bit leverage,, you can assume that you have a straight crank and the correct bearing installed correctly.

If no, then you may have an incorrectly sized bearing or journal or piston or bore and that has it locked up
another possibility is that something has fallen down the intake tract ( a bolt or a nut) and is now on top of the piston preventing it from rolling through TDC.

on a never ran pistons, rings, bores,, I would not put much faith in a compression test,,

The truck would not start when i first bought it. It would turn over. It needed a choke repair and bare wire fix on the coil to start. Then i found a stripped bolt on the carb throttle plate. It ran but stalled and hesitated in stop and go traffic (bad). I built and installed the gm hei conversion. It was still hard to start. The carb was a rebuild, i installed a rebuild fuel pump, timing gears and chain. All the ignition parts were new as part of the hei conversion. When it did run it ran about 7 miles and quit with lost power. I cut the cat out to no avail. I find the leak down test vs the compression test to be more informative. All the cylinders read 0 or <20%. I had the head reworked, finding the pistons to be '40 os and rod bearings '10 us. thereby meaning it was a rebuilt block. Then on to rings and rod bearings which look to have little wear. I install rod bearings from 1 to 6 and turn over the engine to about #4 to #6 and it won"t turn over, spark plugs out with plenty of lube,it doesn't spin fast even with two rod bearings installed. Nothing fell in the intake manifold.

The engine now turns over as i had the rod caps on wrong. The caps are numbered and match with the rod #'s on the right side, tang to tang.

The engine now turns over as i had the rod caps on wrong. The caps are numbered and match with the rod #'s on the right side, tang to tang.

good find, that should get you back on track.