Hi, been lurking on the forum a while, but finally got round to registering. Over last 6 months or so been returning a H reg dt125r to the road. It sailed through it's mot back in february, but the engine is currently apart, being rebuilt to bring it up to the standard of the rest of the bike. I also own a K-reg honda nsr125rk, so like my 2 strokes Will start a project thread when I get some photo's worth posting, John.

You normally check the squish by feeding a small length of solder down the spark plug hole, so the edge of the solder touches the edge of the bore. Then turning the engine over by hand then measuring the squashed end of the solder with a micrometer or similar. The squish on my dtr was 0.6mm when I bought it, which seems to be on the tight side.

From experience it tends to be running too lean that kills two strokes. Though a 2 stroke makes more power the leaner it runs. At 57mm you often have to modify the power valve for clearance. And the head gasket will need opening up. When the barrel 1st seized they might have replaced it with a 'good' second hand barrel that had already been taken out a few sizes.

If yours has got the reed switch in the speedo it wont have the black/green wire. Pretty sure fitting the zeel would do away with that restriction anyway.

Freshly powdercoated parts going back together

starting to look more like a bike again next to my nsr125rk

and after a little light offroading

Piston rings have a hard life in a 2 stroke travelling over the open ports. I've seen the pins that stop the rings rotating on the piston move, allowing the ring to spin and open end catch on a port. With it being under load at time it failed (going up a steep hill), I wouldn't be surprised if piston expanded due to heat build up and pinched the rings. May have been running on the lean side aswell.

Welcome, your not too far away (NW Leicestershire), often over that way doing a little green Laning.

@gilerasc125 at 57mm bore it could well be catching powervalve and/or head gasket. I'd whip the powervalve out and check for any witness marks.

Tdr 250?

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.

@zx125 put your finger on the exposed pulley end of the powervalve. With the engine running, you will know straight away if it's the powervalve rattling.

@spookdog if you look at the under side of the cap you'll see a little metal washer that you can pull down, that's spring loaded. When your coolant cools and contracts, it creates a vacuum pulling down that washer allowing coolant to flow back in to the rad.
Not had great success with copper gaskets myself, tend to be alot thicker as well, lowering the compression.

Check your radiator cap and where the cap seals on the radiator. Had a rad that with a small bit of damage where the inner part of the rad cap seals against. Which meant water would flow to the expansion bottle as soon as it warmed even a little.