TP800 are that takes me back. right hear we go...
as over people have sugested remove the 4 screws and the black plastic cap from the top of the machine. you should be able to see the con rod and the crank gear, and in front of the crank gear you should see the small gear which is the top of thr armature, you may have to scoop out some grease in order to see it. Check that the con rod is not broken. earlier machines had aluminium cod rods but the later models have plastic ones which were prone to breaking. this isn't to say that the aluminium ones never broke because they did and wow what a mess may made when they did. if your con rod looks ok, try this... keep your hands clear of the open gears, and put some glasses on. turn the machine on and make sure that the crank is turning. look at the small gear on the armature to see is it is wobbly. the small roller bearing at the top of the armature use to callapse causing them to wobble. this would strip both the gears. a good sign of this is metalic parlicles in the grease. this fault is very expensive as you will have to replace the armature, bearing, seal, and the crankshaft. ouch...BUT DONT DESPARE. if this looks ok, it must be the piston rings these are cheap, fairly easy to change and available, as they are still used on more current models. the part number for these piston rings (o-rings) is 13772. want to know how to change them?.. read on...The hardest part is removing the chuck as you must remove the retaining clip at the tip of the chuck. hilti make a special tool for this, but it is possible with a good set of circlip pliers, 2 flat screw drivers and 4 hands. top tip...Remove the rear handle assembly, just remove the 4 screws and pull it off, now the machine can be stood upright on a workbench. Wear safaty goggles and put a chisel in the chuck, this will help you not to loose the clip when it flies across the workshop, I hate it when that happens. once you have the clip off its easy from now on. and don't worry, there is an easy way to get it back on. ok, the clip is off. remove the washer and the plastic chuck cover. remove the circlip, remove the chuck sleeves, dont loose the 6 balls or the roller. now there is another large circlip which holds on the black plastic cover. remove them both. now you can see the 6 bolts which hold the machine together. remove them. remove the chuck.you will see a large o-ring around the cylinder, remove it. Now with a screw driver carfully split the cylinder at the point where the o-ring was sitting. carefully pull out the inner cylinder. dont loose the small plastic balls. if they fall out dont worry. put them to one side. inside this cylinder you will find the follower piston, just tap the clyinder on the workbench and it will fall out. replace the o-ring, smaer with grease and replace. The drive piston is probably still inside the red casing. Simply pull the con rod off the crank and tip it out of the casing. on early models this piston had 2 seals. 1 teflon ring over a green rubber O-Ring. on later models these were replaced with an o-ring, the same as the follower piston, the earier type may not be available in which case you may have to replace the pison as well in order to fit the newer o-ring system. once the seals have been replaced, smear with grease. place the piton inside the cylinder just far enough so the conector pin cant fall out of the drive piston. Replace the plastic balls into the detents in the cylinder. use grease to hold them in place. Replace the cylinder into the caseing but make sure that the cut out at the end of the cylider is facing down. This is where the armature gear sits. as you push the cylinder in ensure that the con rod is hooked onto the crankshaft. reassemble in reverse order. use grease to hold the chuck balls and roller in place. dont fit the rear handle yet..now for that clip that caused all the pain at the start of the job. PuT YOUR GOGGLES ON. stand the machine upright on the workbench and get somone to hold it. place the clip on top of the chuck body tube. get a block of wood and place it on top of the ring, and hit it with a hammer. this will knock the ring on 80% of the way, now tap it on with the hammer. refit the rear handle. your done,,,if you removed all the grease replace with a similar type. 70 grams (3 oz) in the crank case under that black plastic cover..
Well I hope that helps, its not as bad as it sounds.
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if you have any more questions please email me.
if you want a parts diagram i can email you a drawing of a TE804 which is virtually the same machine with a slightly different chuck.