lapping head to barrell and piston bore clearances

When the liquid cooled engines came into play it allowed the port timings to be changed which made more heat to the point the liquid cooling kept the heat under control.

Interesting , so its kinda like a compression fitting ?
If it did have a leak what would be the symptoms ?

lots of hissing noise and power loss is my first guess

loss of compression, power, and throttle response, oil leak and residue under head...maybe even hissing as troy says. the ones ive seen that were leaking had rtv or gasket sealer smeared everywhere, not letting the head seat the whole way

I find a trail of carbon gasses past the first seal ring trying to get to the second outer seal ring as it narrows. The black carbon line is the gasses trying to exit. They don't make it all the way out but it's an indication of a leak starting. I lap every head and cylinder when there apart to be safe. Use the fine lapping compound lightly just to check the seal to be sure it good if the seal shows no leakage it needs a light lapping.

Hi Andy,

Interesting what you say about the fuel. I bought a 165 kit for my modern WR 125 (now 165) and Walt had to replace head twice and still pings. We also had a race meeting in our north east late last year and a number of VMX bikes all seized on main straight in three races and only explanation given by all the guys was the shit fuel used these days.

Mark

Seizing can be a number of things. Bad crank seals and bearings, air leak, jetted too lean on the hairy edge trying to get every bit of performance out of it.
I had this problem in the past and I feel it was a combination of the wrong oil/gas ratio plus the wrong two stroke oil. I was too lean on the oil mix at 40:1 super m maxima. I'm going with the maxima 927 castor oil and 20:1 ratio the original settings by the manufacturer.

you did not seize from 40:1 mix alone, too many people including myself have run that for too long without seizing.

The first bike had this happen. I'm adding more oil with the original ratio. The rest of my bikes were fine I did leak down tests after installing new crank bearings and seals. With the leak down test on the cr125 Honda I found out the PO removed the cylinder with a screwdriver banged into the gasket. One big air leak. Learning how to rejet helps too.

40 :1 is fine bill, you had a mechanical issue you haven't discovered... im running 80: 1 in the desert... flat out all day every day with no issues..keep looking!

I'm going to use the 927 maxima castor oil 2t mix in the cast iron sleeve cylinders. My bikes that followed the problem bike were ok.

I do leak down tests on every newly assembled engine. I figure holding 10lbs of pressure for 12 hours is ok.

The compression passing the first seal ring on the cylinder is on used engines. I lap every rebuild.