The TR21 is simple to work on. I just bought a used one cheap to test some modifications on and found the insides were pretty gunked up from the pump inhaling oil in the two short months since it's production date. So I stripped it down, cleaned it, saw that no permanent damage was done, lubed it, reassembled it and it works flawlessly once again. I think that with regular cleaning and maintenance the pump can be reliable enough to not necessitate parts replacement and short rebuild intervals. Proceed at your own risk, of course(!!). To see how things are looking inside, remove the metal fan shroud and plastic wiring shroud by simply taking out the seven philips head screws attaching those parts to the pump. Then remove the handle/valve cover via the (3) 3/16" allen head bolts. You will see the four brass valve spring access plugs in the head which can be loosened with a flathead screwdriver to inspect and clean the ports, valves and valvesprings once the head is removed, if you so desire. The brass plugs seal with an o-ring which should also be inspected for damage prior to reassembly. You will also want to check the o-ring seals around the brass inlet and outlet fittings as these are poorly implemented and prone to wear from over-tightening of the fittings themselves (make sure your inlet screen is clean and undamaged as well). I did some machining so I no longer use those parts, but that was just for the sake of experimentation. Unplug the pressure sensor wiring before removing the (4) 3/16" allen head bolts holding the head on, at which point you can remove that portion of the assembly for cleaning. There are two o-rings that seal the cylinder liners to the head - inspect these for wear and clean/replace if necessary. The cylinder liners simply pull straight out for inspection and cleaning, leaving the piston and rod assemblies protruding from the crankcase. Now you can determine to a fuller extent the damage done internally, if any. After a thorough cleaning of the cylinders, pistons and rings/seals, I found that my cylinder walls had no scoring or wear which is important because the rings need to seal perfectly to avoid butane vapor entering the crankcase. The two piston rings per piston, one of which can be gently removed, can be cleaned and lubed. Check for uneven wear and replace if necessary. Check for play in the rod bearings in the direction of piston movement. If you have a bunch of oil and goop running down the rods towards the bearings like some pictures I've seen floating around...you may want to disassemble the unit further to lube the rod bearings or replace the assemblies entirely if need be. Worn rod bearings/eccentrics will definitely contribute to an unpleasant knocking sound. The piston/rod assemblies can be purchased for $120, which in addition to the $45 valve rebuild kit, are all you should ever really need unless your pump is already messed up from neglect and misuse. If that's the situation you find yourself in, the master rebuild kit is the way to go. I'm sure I left out a few things but that's a start for those who like to tinker with stuff. I used blue loctite on the head bolt threads during reinstallation, and tightened them incrementally in a criss-cross pattern. I don't have any exact torque specs...perhaps someone else could find out that info. I cleaned the non-rubber parts with alcohol and the o-rings with warm soapy water. I used food grade silicone spray to lube all rubber o-rings as well as the piston rings prior to installation, which is done simply in the reverse order of removal. Also, if you remove the piston rings, be sure to clock the gaps properly before installation. CPS positions them one way (remember how the looked when you took them out?), I chose to clock them averaging a 45 degree angle away from the thrust loads of the piston. Good luck! If you break something don't blame me
