Cheers Nick

Still a bit along the learning curve to go with this engine.
To days lesson:- I made the piston from PTFE it seemed a good idea at the time, I know PTFE has some strange properties and that it has a high figure of coefficient of linear expansion, but I though that shouldn't pose any problem, I measures the width of the piston and the cylinder up and made them so that I had about 0.2mm clearance fit, I reduced this down to 0.05 to try and get a better run, but as I was doing this I was having problems getting a repeat size for the piston, this morning I stripped the engine down and measured things up the clearance was size for size, great i thought, tried it for fit and it all rolled over smoothly I fiddled about a bit, tried it for fit and things had tightend up, measured up again and I had an interferance fit
Time for my morning coffee

so stuck the piston in the fridge for 1/2 hr measured up again and it had shrunk by 0.15mm.
Just done a google search on the properties of PTFE and as far as I can tell it has coefficient some 5 times greater than brass.
Time for a change of material i have some nice shash weight cast iron that John gave me I'll give that a try.
Stew