Can you do the penny in the clutch/ring gear trick? I may have a TDC tool from my 2 stroke Kart racing daze, I'd have to scour the garage to find it. It was, as you suggest, a spark plug threaded sleeve with a long nylon center post, but intended for a 2 stroke head, where the plug is centered on the top of the head. I doubt it would contact the piston in your XT500.
I took an old spark plug and broke out the porcelin and tapped it to run a bolt down.
Getting the porcelin out of that plug was a bitch though.
PS. I know this probably doesn't apply...but using a TDC stop to lock the crank for work is a bad idea...at that point the crank has tremendous leverage on the piston...good way to tweak something.
To lock the crank for work I usually use the penny method.
If you just want to find tdc why don't you use a dial indicator or even a hand held screwdriver in the plug hole and turn the motor untill you get it the screwdriver where it isn't moving. Dial indicator is a bit more accurate but not by a lot.
I've used the screwdriver method, and frankly, after seeing someone with a dial gauge, I realized VERY quickly that the degree of error is VERY large using the screwdriver method. It is when the piston is at the top of the stroke . . . . oh . . . never mind.
I often use the screw driver method. It's accurate if you do this....mark on the degree wheel where it stops moving...keep turning the crank the same way....mark where it STARTS moving again...split the difference. That usually gets you within about one degree of TDC.
Problem with a piston stop is technically it can't stop the piston at TDC...there is no movement of the piston at TDC....the stop actually stops the piston a couple of degrees before TDC. With a piston stop to find TDC you have to turn the crank one way...mark where the wheel stops...turn the crank all the around the other way..mark where the piston stops....split the difference.
Actualy you have to use this method even with a dial indicator too. There is always a couple of degrees of crankshaft movement at TDC without moving the piston.
Well I guess so because I did say it first. JB said it better but then he has Mary to edit his posts. Cuz I know he can't put that many words coherently together without help. Of course it may be that he's back on the meds. That must be it because I haven't heard that many rants lately. What's up John, not staying up for 48 hours straight anymore?