When I got back I decided to strip it down again to find out what was wrong with it. There was damage to the top edge of the exhaust port and the rings were stuck in their grooves. It then occurred to me to look in the box where I had got the rings from, it was then i realised I had used old worn out rings that i had changed a few year before, the new set of rings I knew I had were still in a bag in the box.
The worn rings must have snagged the exhaust port because they allowed the piston to rock more in the bore. The piston was ruined and the exhaust port needed some attention.
I decided that as I needed to do some work to the exhaust port anyway i may as well change it a little while I’m at it. The blow down on the barrel was only 25 degrees which isn’t enough to allow it to rev out properly so I decided to raise the port by 1mm which would give me 27 degrees blow down and widen the top 50% of the port to increase the blow down area. This was something I had wanted to do for a while but hadn’t because the engine had worked so well.
The first thing I did was order a new Wosnner piston in the same 71mm size, I didn’t want to re-bore the barrel even though it needed a re-bore due to the scoring that had been there for years. Then I set about making traces of the barrel to decide what metal I needed to take out to get the port shape I was after.
This is a comparison of the new port shape against the old shape. The top of the port is now 33mm below the top of the barrel!
Next I had to do the modifications to the piston