UPDATE: She Lives!!!!!!
After a few more hours in the shed.... success!!!! Thanks for all the advice.
So I had to go backwards to go forward and ended up re-making the cylinder liner and piston. This time I used carbide grinding paste after having had the bad previous experience with the diamond paste (although I'd probably try the diamond paste again in the future after I've got my hands on an ultrasonic cleaner as it removes metal a lot faster). I tried making one of the coper laps
This time I tried using a copper lap and getting the final fit by lapping the piston to the liner with 1000 grit paste (thanks Ramon for the suggestion) very very cautiously, cleaning and checking fit many times before the 'pinch' at TDC. The last 1mm or so was actually done without paste and just a good bit of WD40 lapping the cast-iron to the EN1A with a gentle rotation by hand. In the end, I didn't use the copper lap as I struggled to get the final size right after taking out all the imperfections after hammering and silver-soldering. I think I will try these laps again when I'm dealing with a slightly larger bore (1/2" on the Boll Aero is v small to be making one of these copper thingies!)
I also used this technique for the last mm or so of the contra-piston. Mounted on the mandrel used for the turning on the lathe, i gently lapped it in to the top of the liner and contra-piston until the the fit was right.
After the compression was sorted, I got back to the original issue again; the engine would fire up on the primer when squirted into the exhaust but then no more. Also, there didn't seem to be a lot of suck through the carb or 'pop' when turning the engine over with the cylinder head and contra-piston removed. Re-reading the thread here I eliminated one potential problem at a time.
- Spray-bar hole at 90deg to the air flow.
- New cylinder liner has a push-fit with the crank case (needs heat to remove now) so should make a good seal.
- Fuel hight in tank just below venturi
- Correct-size port holes
Still no joy. Then the last the last ting I tried. A bit of paper!!
A postit note cut out on my DIY laser soaked in oil and installed as a gasket in the front and rear of the crank case. That seemed to be the issue. My crank case rear cover was just not machined well enough and wasn't air tight. Installed the gasket, gave it a few flicks and off she went. I'm on my 5th 1-minute break-in run and the revs seem to be going higher every time.
I am over the moon. Thanks all for the help and bring a noobie like me one rung up the ladder. I am off to rest my arm and put some plasters on my fingers.
Patrick
