Craftsman air pump/compressor

[hapcj7]

Is anyone familiar with these air pumps?..I bought this used and it has very little run time on it, however, i think it was run with little or no oil, not sure.
It is a 2 7/8" bore twin cylinder mated with a 5hp motor..This pump has been used on many compressors.This unit took on some water at one time or another as seen on one piston top.

The bottom end is knocking badly in my opinion..I took the entire assembly apart.. Saw a good amount of blackish gunk in oil pan..I literally cannot find anything wrong with it...
Connecting rods are good on both ends, the 2 needle bearings are good, piston pin good, crankshaft journal good..Also, pistons and bores are good..

Heres where the knocking is coming from:
Everyone time i get to top dead center on each cylinder, the rotating assembly gets knotchy..Both cylinders, every time, at tdc..The crankshaft by itself rotates perfectly and smooth with out connecting rods..Again, connecting rods appear perfect to me..Everything is cleaned and oil lubed.Thanks

 

[RayH]

You said everything looks ok on the rotating assembly, have you verified rod bearing clearance and wrist pins for movement. Also have you measured pistons and piston bores?

 

[hapcj7]

You said everything looks ok on the rotating assembly, have you verified rod bearing clearance and wrist pins for movement. Also have you measured pistons and piston bores?

Yes, i measured all of the above, the best i could with calipers. Wrist pins are perfect..Pistons and bores are perfect...I just installed an older pump from a 1 Hp Craftsman compressor on the "newer", the one in subject compressor.This compressor has a 5hp motor...Externally both pumps are identical..The swap was literally 20 mins or so.....Maybe this is the nature of these pumps....The 1Hp pump with 1 Hp motor sounds great, nice and quiet, no knocking.... When i put the 1 HP pump (perhaps the same) on the 5HP motor compressor, the pump is much louder and knocking on bottom end...My belief is when these pumps spin faster, more rpm,s, they make noise, which is knocking to me..The 1 HP motor has a smaller pulley, while the 5HP motor has a larger pulley, thus increasing the rpm,s.....Flywheel sizing were exactly the same..Also, the rod bearings are literally the rod caps..All aluminum.......Thanks