The pretty one is my dad's XT500T 4 valve. What it is, is essentially a XT600 2KF, but with a smaller bore (87mm) and the cylinder head off a JDM XT400. This means it has got smaller valves and a smaller combustion chamber than a stock XT600. Whilst in a sense it is good that Yamaha put the effort in to make the engine work decently with the smaller bore, it also created its own set of problems. Whilst engine parts for the 600 are comparatively cheap and definitely plentiful this is definitely not the case for the 500 version as it was pretty much only sold in Austria. This should answer the question why we didn't simply get another cylinder and be done with it. Also yes, you can convert it to 600cc, but that would still mean you'd have to bore out the engine case and quite simply my dad didn't want this. (Remember when I did a frame swap, i.e. registered a new bike, because that was easier than converting Dre-XT-Stück to 600cc? There might be an explanation hidden in there somewhere...)
So the story with the headbolt threads follows a very classic recipe: The headbolts are M8, they are torqued to 30NM and they go straight into the aluminium of the cylinder. The cylinder in question has already been helicoiled three times, but there were two installation failure as I found out during the repair. A more permanent solution was needed and it came in the form of a handful of homemade threaded inserts. They are M12 on the outside and M8 on the inside and the ones I ended up using where 20 and 30mm long.
After taking the head off, it was pretty obvious where the headgasket leak
was.
I (wrongly) assumed that the biggest challenge with this repair would be to
install the recessed threaded inserts... as the first one had spat out its
helicoils this was a very straight forward job. Nerve wrecking, but simple
enough.
On the second one, the helicoil was still in there, but as I found out, the locking tab hadn't grabbed correctly and I just pulled it out.
Installation was simple enough: an M8 bolt, a nut, a spacer and the insert. (Yes the spacer is also made from a bit of turned down M12 threaded rod.
Just to be really sure the inserts were glued in with some high strength locking adhesive.
Test fit with the locating dowels in place worked a treat.
Somewhat inspired by all of this, the next day I decided to upgrade all remaining headbolt threads as well, including the one with the only working helicoil of the previous repair attempt. It did not go well.
As the previous attempt of getting a helicoil out worked rather well, I didn't
think much of it and snapped my drill about 10mm below the surface when it
locked up. Those were two not very funny hours.
... the final one was a case of "more of the same" was every bit as pleasantly
straight forward as one could have hoped for. (And yes, I had made a new
spacer in the meantime.)
The finished product.
What followed next is the "usual" assembly of an XT engine, though I have to admit that there's a distinct difference between a completely sh*gged example and a well-cared for two-owner bike.
Apparently last time I swapped the headgasket I made the same mistake as on the Dre-XT-Stück with choosing the wrong time mark (talk about consistency), so that was rectified in the same go.
... and that one bit where I decided it was time to ruin the purity and replace some mangled JIS screws with some stainless M4x25 allen heads.
The first startup was pleasantly unexciting. Let's hope it stays this way.
Merry Christmas dad. 😌
(Yes there's plenty more work that needs to be done to make it usable, but at least for now it seems as if the head's going to stay where it is supposed to be.)
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