Anyhow, I decided an engine rebuild might be necessary, as I suspected the oil seals were getting worn. When I removed the cylinder and piston this is what I saw! Even though I had fitted new rings - quite some time back now - the piston was badly discoloured. As far as I can judge, though, the piston fits quite snuggly in the barrel and the rings show no sign of wear.
This morning I took the barrel and piston for my friend Hughie to take a look. He runs a garage on the edge of town and not only is an excellent mechanic but also a Bantam owner himself. That's when he's not helping Alan, his father, on the local farm!
Hughie took one look at the discolouration and said it was quite serious. After checking the piston rings - which he realised had lost much of their tension - he measured the end gaps. At .032" top and bottom, they are worn out and because of scoring in the bores and a ridge at the top the barrel must be rebored! The present piston has .020" on the crown, so the D7 had been bored out before I bought it in 1991. I guess it's done quite well.
I've also noticed the needle-roller little end is buggered (technical term!) and there's too much side play on the con-rod. All in all, sound a bit expensive. Never mind, you can't take it with you.