The problem (spinning circlips) won't be the circlips,
but the thrust washers behind them. There is a bulletin that is accessible from
this site on the fix, a thrust washer that has locking tabs in its ID to stop
the washers from spinning with the gears, and cupping the circlips.
These should be used on 4 of the 5 washer positions, not just for 3rd gear.
Part number for the updated circlips is in the bulletin, 24mm thrust washer with stop tangs on its ID.
They can be used on all but the dead end of the output shaft, because the cuts for the gears on the shaft curve upwards and the washer tangs won't go all the way onto the shaft.
YES, that is the reason the circlips cup. No tangs to stop the washer from spinning with the gear, and thrusting against the circlips. The gear/washer just side load against the circlip and bend it outwards, gear won't stay in engagement.
Although I have reused circlips for transmissions, I
feel that the on-off cycle of them is twice, max. I have bent them back tight,
but haven't really found them to set back into the grooves as well as new ones.
if they won't stay tight, new ones, PLEASE.
A suggestion, might try McMaster-Carr on the web, they may just have those circlips available in metric size, for not too much price.
On these types of clips, you will find them to be "punched" from both sides, the center ID from one direction, the OD the other. This gives a rolled edge and a sharp edge on each cut side. If a person would mount the circlips with the sharp edge ID away from the gear, the sharp edge of the OD would face the thrust washer, and this would make the stopping ability of the circlip greater than if it were reversed. Kinda-sorta digs 'em into the shaft that way, less chance for them to ride out of their grooves.
As far as the thrust washers go, they are punched from only one side, gives hem the same sharp/rolled over edges, only same for each side, sharp/sharp vs rolled/rolled. Sharp side towards the gear, please.
The only shafts these tanged thrust washers will work
on have a 24mm diameter, as in H and KH500, NOT KH small engines.
If it;s a small engine, please measure and post the shaft diameters where the gears ride on them, PLEASE.
I don't remember if the KH small and S series transmissions had the toothed washers, but I'd figure they would if they were built after the H series bulletin was issued.
I do know the S series engines use a 24mm shaft diameter and more than one A7R has the toothed washers in place, because I set them up that way.
Also, when I set a trans up, I load the shafts onto their thrust bearings, as they run in the engine. This can be done by adding the clutch setup to the input shaft, and the sprocket setup to the output shaft, but I just make up a sleeve of steel tubing and a flat plate for the end with a hole for the threads through it, then put it on and set the nut to hold the shaft in its correct side loading relationship. Just an extra step to get it right.