What do you make of this seizure?

[Storkfoot]

Running in an iron 175. Only done 80 miles, or so. Classic case of got stuck on a long straight road were it was difficult to go up and down the gears. Anyway, I had a soft seize and left it for 5 minutes. Kicked it over and started to limp home. One more soft seize before what I thought was a proper hard seize - kickstart absolute solid, even an hour or so later.

This morning, fearing that I had a melted piston all over my barrel, I poured some two stroke oil down the plug hole and left it for a few hours to try and free it up.

After coming back to it late this afternoon, it freed off with little effort. On taking the barrel off, I find virtually no marks on the piston, both ring pegs in place but these “ring marks” around the barrel which would appear to me to have been made by the piston rings.

I am going to clean the barrel up gently with a bit of wet and dry and put it all back together. I don’t have the carb properly set up yet so I am proposing to richen it up throughout the range whilst I do some more systematic plug colour checks.

I have never had a seize like this before - locked solid but no evidence of a melted piston. It’s almost like the piston rings stuck in the bore but not the piston.

Has anyone any ideas what may have happened and why? There was two stroke oil in the petrol.

[Storkfoot]

A random thought. Would these marks be symptomatic of fuel starvation

[Fast n Furious]

Those ring marks look to be concentric with the cylinders fins?
Is it a genuine TV175 cylinder or a bored out 125/150 cylinder?

[Storkfoot]

Those ring marks look to be concentric with the cylinders fins?
Is it a genuine TV175 cylinder or a bored out 125/150 cylinder?

I haven’t got a set of calipers to properly measure the cylinder diameter between 3rd and 4th fins but I am almost certain you have hit the nail on the head. The SX 150 barrel I had rebored and tuned 2 1/2 years ago was, in fact, a 125.

When I have calmed down, I’ll give some thought as to what to do next. It won’t involve trying to use this barrel though.

Thanks for the reply.

[alex_hughes]

That is a really interesting photo and diagnosis!

Having heard of this problem previously this is the first time I've seen it 'in the flesh'

Thanks for posting

[MickYork]

Was it a cylinder/piston seize ? Could it have been the crank or other "connecting" parts ?

[Storkfoot]

Was it a cylinder/piston seize ? Could it have been the crank or other "connecting" parts ?

When I saw that the piston was okay (there are a few scores but nothing bad and no melted aluminium) I too thought of something else. But, it was the two stroke poured down the plug hole that released it so I think it must have been the top end. The crank is fine.

The sidecasing will be coming off, hopefully over the weekend, as I am swapping out the AF 4 speed close ratio gearbox. I’ll see what that shows.

I am minded to get a correct SX150 barrel and just get it bored out to the 62mm Asso Werke piston.

If anyone has an SX150/GP150 barrel (cylinder diameter 69.5mm in the gap between third and fourth fins and a couple of slots in the spigot) and is happy to move it on, please let me know.