BollAero 18 Problems

[tarboxb]

Hello everyone. I am new here and on the home stretch on my BollAero 18 build. I still have the venturi assembly to construct but thought I would try to prime my engine and see if I could at least get it to fire a couple of times today. I was turning it over with a drill so I know that I had plenty of rpm. I was seeing bubbles coming out from the compression adjustment screw so I believe my contra piston is leaking. I just do not understand how this could be the case. My contra piston is quite tight and I feel that if it were much tighter, the compression would be unable to reset it. Does anyone have any suggestions on what I need to do to fix this problem? I appreciate everyone's help and am definitely hoping that I can get this thing running ASAP. Thanks a lot guys.

 

[gus]

Hello everyone. I am new here and on the home stretch on my BollAero 18 build. I still have the venturi assembly to construct but thought I would try to prime my engine and see if I could at least get it to fire a couple of times today. I was turning it over with a drill so I know that I had plenty of rpm. I was seeing bubbles coming out from the compression adjustment screw so I believe my contra piston is leaking. I just do not understand how this could be the case. My contra piston is quite tight and I feel that if it were much tighter, the compression would be unable to reset it. Does anyone have any suggestions on what I need to do to fix this problem? I appreciate everyone's help and am definitely hoping that I can get this thing running ASAP. Thanks a lot guys.

It is obvious the contra piston is leaking. Same must be lapped to fit it.
The cylinder liner and the piston are also lapped. The liner is lapped with an aluminium bar with grooves. And the piston with a sleeve. Lapping is a black magic skill. Practice make perfect. Lapping compound-------200grit -------400 grit. Good reason why I went over to building i.c.engines with piston rings and tappet valves and spark plugs.Built Webster and Rupnow Engines with running. My luck and skill with compression engines and glow plug engines been bad.
Good luck.

 

[rklopp]

Go read Ron Chernich's ModelengineNews.org tutorial on contrapiston design, fit, and cylinder lapping in general, while you still can. By the way, I have engines with leaky contrapistons that run OK. In those, the leaks are pretty minor. In at least one instance, the leak is due to poor surface finish leaving heavy scratches as leak pathways, even though the overall fit is tight. When I made the cylinder taper, the upper area of the cylinder did not quite clean up.

 

[tarboxb]

Thanks a lot for the suggestions guys. I am going to try and make a new contra piston and achieve a tighter fit and see what happens. I'll keep you guys posted. Thanks a lot for the help. I am very excited to get this project done and hear it run. We'll see what happens.