DT125R FORUM

It's a wear groove, seen it many times on cranks etc. your lucky on the DT it's a removable bush/sleeve, that can be replaced. I'd turn it so the seal is running on a smooth section of the sleeve.

I doubt rolls Royce were using hylomar to seal 2 stroke crankcases though. Yamabond, hondabond etc (threebond 1184) as stated in all the official workshop manuals, is the thing to use. Using anything else your just introducing another unknown. With all the issues you've had I'd be trying to illuminate unknowns.

Quite a few bikes run solely AC, kawasaki Kdx200's do. They don't have a battery, just a voltage regulator. And there lights are powered by AC voltage.
Think some honda trail bikes run both AC and DC systems on the same bike. The very early dt's could be the same?

If your riding mainly on the road I'd personally just switch to a full road tyre. I ran Avon roadriders on my DTR. It will improve braking etc as well. I had a second set of wheels with knoblies on, for if I went green Laning.

@SpookDog if your splitting the cases, I'd highly recommend using threebond sealant, on the case halves. Think it's 'yamabond1184' that the Yamaha service manual tells you to use, which is threebond1184.

@SpookDog seriously think your massively over thinking this!
I've used many Athena head gaskets with no issues, shouldn't need to use any type of sealer on the gasket either. It's either something like your piston is hitting the head, or your cooling system isn't pressurising properly. I'd be looking very close at your radiator. Have you checked your water pump actually works?

Reason I ask about the head mount, is my Mick abbey modified head. Needed the bracket holes enlarging, because of how much had been skimmed off the head.

If you've been skimming the heads and barrels, the squish gap will have closed up. It may be a insignificant amount. But with how many times you've been doing it, it would be worth checking.
With how old these bikes are now, you can't guarantee things are to standard specs. Skimming your head and barrel will raise the compression ratio.
Also after reboring a cylinder and fitting oversize pistons, you increase the cc slightly also affecting the compression ratio.
All this will have a slight effect on jetting, add in newer ethanol fuels burn slightly leaner as well.
Not convinced these are the reasons for your issue's but worth ruling out.
Could the piston actually be making contact with the head?
I would pressure test your radiator with compressed air (don't need alot of psi) while submerging it in water, to check for leaks.

What is your squish gap?
Is it easy to fit the cylinder head engine to frame mount bolts, or are they under some strain?
Are you using some form of stock jetting?
What are your exact symptoms of head gasket failing? Pushing water out the expansion bottle? Temps raising into red?
Condition of radiator/cap?

Agree with Calum, you really can't be this un lucky with head gaskets. My own dt had head gasket symptoms, pushing water out the expansion bottle. Turned out to be a slight bit of damage on the radiator filler. Where the cap seal, seals against. It was just allowing water past as the coolant expanded as it warmed up. Also coolant will boil at a lower temperature, if isn't under pressure. I say it's well worth looking at this from a different angle.

@SpookDog or it could just be mixture related........?

@kylescave there's a needle bearing and sleeve, where the swingarm pivots on the frame bolt. Pretty sure its a common size bearing, possibly 22x28x20mm but don't hold me to that.

@jacobx5sv With out doing a proper plug chop, new plug, riding at the throttle position you want to test and then cutting the threads off, to the measure the smoke ring. The problem with just taking a used spark plug out, and checking the colour, is at best just giving you a 'average' reading.
You could be very rich on the pilot say, but lean on the main/power jet. The plug could look dark from riding at the lower throttle openings (pilot/needle) for longer and not show the leaner condition, for the occasional sprint to full throttle.
If your bike is running ok and has been jetted that way for a while. I probably wouldn't be too concerned. It is much safer to run slightly richer.

Yamabond, hondabond etc are all re-packaged threebond. Can't remember if it's 1185 or 1215 I use on 2 stroke cases.

The power valves knock, when the bushes wear. But the valves also seem to wear aswell. Also the right hand power valve cap isn't a great design, having only one fixing point. Probably why Yamaha changed the design of the power valve on the 4fu cylinders.

@spookdog @spookdog a heat seizure is caused by the piston expanding to much/fast. From a lean mixture burning to hot. The piston basically becomes to big for the bore it's running in, and seizes. The fuel in a richer mixture actually takes heat from the piston crown. Lean mixture is backed up by the detonation you've seen on your piston.

@spookdog heat seizure is all to do with your mixture being to lean, nothing to do with how well your cooling is working or not. A lean mixture will burn hotter aswell. Which was probably contributing to your over heating.

I'd raise the front end back up. Lowering the forks through the yokes, will make it more nimble but will make it less stable in a straight line.

You'll raise the compression converting to o ring. It's not hard to get the fueling reasonably good with the stock carb. Only time I've suffered lean issues, it was the base gasket that had gone, leaking in air.

@spookdog yeah they run a 9 heat range plug as standard. Could be a combination of raised compression and lean mixture. Can run lean running to much oil even.

@spookdog that is detonation/pre ignition damage! My dt was running 225psi compression before I lowered it and that suffered no detonation. Normally it's down to a lean mixture.

What heat range spark plug are you running?
Also heard of excess carbon build up causing detonation.