Jared,
Overhauling a small compressor pump is basically very similar to overhauling a Wisconsin or similar small engine. The principles are pretty much the same as working on any engine, for that matter.
There are three likely causes of a knock in a compressor pump, which usually track back to use of the wrong oil, failure to change the oil periodically, or oil leakage til the pump runs dry. The three common knocks are a rod bearing failure, gudgeon-pin bushes, or the pin-bore in the piston, wearing loose, or a build-up of carbon on the piston-tops.
The combination of worn rings and low-quality or deteriorated oil will cause dramatic carboning-up of the pump......I remember one Quincy which was neglected til the entire head and valve cage assemblies were solidly choked with carbon build-up.....it would run, but didn't move much air.
A rod bearing failure from bad oil or no oil is obvious, same as any engine.
Call the works for a specs/parts sheet, and ask for a set of assembly drawings.
Clean the exterior of the unit, and work in a clean location.
When disassembling, mark adjacent parts with matching number stamps. Note the lengths of bolts/capscrews, and bag them, noting the places they came from. Bag and tag the parts as you remove them....take photos of the disassembly process if you like.
Look for wear spots on the parts as they come apart, and reason out whether those wear spots are 'normal wear' as compared to 'abnormal wear' on misaligned parts or parts which came adrift with the machine running.
Match-mark the low pressure rod/cap, if it isn't marked already, pull the cap off, and remove the piston/rod assy.
Likewise, mark the high pressure rod/cap, have the cap off, then remove the capscrews holding the cylinder casting to the crank-case, and remove the cylinder casting with the h.p. piston/rod assy.
Carefully inspect the 'crankpins' or rod journals on the crank-shaft. Look for any loss of the high finish original grind, such as noticeable wear or scoring.
If the crankpins are scored, or out of round as much as .001/0015, don't try to 'polish' the pins, send the crank out for re-grinding. Look in the 'Yellow Pages' for a specialist crank grinding shop in your area, or call a local Diesel engine shop and ask who they use.
Carefully inspect the pistons for scoring, ring groove wear, or other damage. Check the fit of the gudgeon-pins (wrist-pins) in the pistons....there should be no appreciable play. Sometimes, particularly on the h.p. piston, the pin bore will have worn oval....if so, replace the piston.
Inspect the cylinder bores for wear/out of roundness, per dwg specs. Have the cylinders re-bored and fit oversize pistons if necessary. I've gotten by with honing bores round/straight as much as .007-.008 over, and knurlising up the pistons to match, when oversize pistons weren't available, but I'm not recommending that you do that.
Any good automotive machine shop can fit the new bronze pin bearings in the rods, assemble the pistons to the rods, and check piston/rod alignment.
Have them hone the cylinder walls lightly to 'de-glaze', leaving a nice 'cross-hatch' pattern. Be sure to trial-fit the new rings in the bores, and adjust the ring-end gap if necessary, per assy dwg specs.
Its unlikely that the 'big ends' of the rods will need to be 'reconditioned' by grinding the caps a few thou and honing the big-end bores accurately round in the Sunnen hone, as is commonly done for engines.....but have them check the big-end bores, just to 'play safe'.
If the big-end bores show any damage, take the crank and the new bearing shells to the shop, so that they will be able to establish the correct i.d. for the big-end, for the correct bearing clearance.
Clean the the oil pump and all oilways carefully, using acetone or lacquer thinner, (put a small fan behind you to blow fumes away) and lint-free rags. Inspect the oil pump for worn or damaged parts, and fit new components as necessary.
Assemble the pump much as you would an engine, in a clean environment, using loctite on the threads and new capscrews or at least new lock-washers. Note the torque specs in the assy dwgs, particularly for the rod caps. Use a good grade of 'assembly lube' on the bearings.
Replacing the valve discs/springs per the drawings will be obvious.
After disassembling and cleaning the valve cages, lap the valve surfaces against which the discs bear, using 400, then 600, emery paper on a known good flat surface, such as a small granite surface plate (buy an inexpensive small granite 'toolmakers flat' for the job, if needs be)
Clean/gently polish the little pins for the unloader, and other unloader parts, as needed, so that they work freely....apply a little bit of 'assembly lube' at assembly.
When re-assembling the pump, leave the head assy off.....put the pump back on the tank, belt it up to the motor, fill it with best quality 'compressor oil', not 'motor oil', and run the headless pump for a few hours, as 'break-in' time.
After the 'break-in' run, drain the crank-case, refill with new compressor oil, put the head on, taking care to torque the head-bolts carefully, and run the pump to pressure, noting unloader operation and pressure switch operation. Re-torque the head-bolts, and snug up the bolts holding the valve cage retainers/unloaders.
Yes, I know, all that seems like a lot of work.....but its not a bad job, actually....much easier than any automotive engine.....
There's nothing whatever mysterious about rebuilding small compressor pumps.....just follow the works manual or assembly drawings exactly, keep everything perfectly clean, use ordinary common sense, and you'll do just fine.
cheers
Carla