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Wow, that is so lucky to still be running with valves still in the head!

Maybe its from a previous failure, would have thought without the key the gear would start to move around a bit. Either way its still for pulling it apart.

With the cams on TDC, the timing was only this far out: I have two different workshop manuals, both say you have to pull the head to get the timing cover off. What would they know? I thought I'd give it a go with the head in place, seeing as that $100 head gasket only has about 15 minutes of run time on it. woohoo! it's off. And there it is: It looks like it rotated about 90* - or actually, it came to rest 90* from it's starting point.

The shaft ends have very small tolerances, I was concerned that trying to press off this chewed up sprocket would wreck the shaft so decided to cut it off instead.
With some careful application of the plasma cutter and air chisel: The shaft looks like there was a fair bit of spinning, this mess welded itself on: Very light cut in the lathe, just trying to break off the crap. This really should be turned between centers, but I don't have the tools for that so wrapped the shaft end in paper to save the chuck from marking it: Then pressed on my spare timing sprocket I found in a dark corner of my garage.. Half way through, I stopped to check it and saw this: Obviously, I didn't clean up the shaft well enough

Oh well, better start looking for another idler gear set

maybe try putting the gear in the oven for 20 minutes and put the other one in the freezer ?

I also wonder though, is it the shaft that's too large or is it an old fatigued gear not up to the task ?

I think I was just too gentle on the shaft cleanup, and it was still oversized.

Those gears are cast? I would look at trying to find a billet one with thicker boss as well.

Cast material don't like being stretched lol, too brittle.

The last one held for 35 years until a snapped camshaft locked up the engine (and then it held itself together for another couple of years after being damaged) so I'm not concerned about the factory sprocket strength.
You can't buy replacement sprockets AFAIK, the factory ones just don't break under normal conditions.

The second one cracked because of user error. If I'd measured the shaft after I skimmed it I would have seen it was still 21.31mm at the largest point, and unfortunately a 12t press *will* find a way to force that into a 21.00mm hole

yeah I think thats too big for the hole lol.

I guess you really want it to be nearly no interference, just enough for it to be snug, then once its hot it should be even tighter!

Looked up my chart that came with the balancer, which shows interference fit for a range of shaft sizes.

for 0.750" to 1.000" is a interference of 0.0008"- 0.0012 or (0.02032mm- 0.03048mm)

So 1/10th of the interference you used lol.