BollAero 18 Work in Progress Problems

[tarboxb]

Hello everyone. I am new to the forum and model engine building. I have built a BollAero 18 and I cannot get an adequate cylinder and piston seal for it to run. It leaks slightly out of the ports. I have made and remade every critical component numerous times and cannot get the seal I need. I have lapped the cylinder several times without luck. This time I used a FlexHone on the cylinder and attained a much better cylinder than previously. I have polished the piston to fit the cylinder and regardless I am unable to get the quality of sealing that I need. I need to get this project finished ASAP for a school project and do not know what to do. I am honestly contemplated adding o rings to the piston and contrapiston but I do not know if this is even possible on a ported engine. Any ideas? Thank you all ahead of time for your assistance!

 

[rklopp]

Have you searched Ron Chernich's site on cylinder and piston lapping? http://modelenginenews.org/ He presents the proven ways to make these things. Search for "How to Make and Use Cylinder Hones." Ron should have used the word "laps" rather than "hones," but otherwise, the info is spot on. I think you're wasting your time on FlexHones. You can make an o-ring work on a contra-piston, but I doubt on the main piston. I also suggest you search GailinNM's writings on the subject on this forum.

 

[digiex-chris]

I wouldn't bother with a flex hone, it's not nearly as controllable for the tolerances we need for these little engines. It'll also tend to take more material off near the ports because it can expand into them.

I haven't done the bollaero specifically, but I've done a few cylinder liners and pistons for very similar commercial engines. Here's how I do it. You need a lap expanded by a screw, so that it can correct both taper, out of round, and machining marks at the same time. You're ideally aiming for a tapered liner, so the piston jams at the top of the liner. It's only 0.0002-0.0003" taper per inch. Boring cylindrical is fine, you'll make the taper with the lap.

Lap the cylinder to the piston. Start with a piston slightly larger (0.001" or so) than the bottom of the cylindrically lapped cylinder. Then start expanding the lap slightly, and start lapping spending more time on the the bottom of the cylinder than the top so you get an intentional taper, just till the piston can just barely enter the bottom, cleaning the cylinder carefully before each piston try. Then continue the same lapping technique to expand the taper so that it starts to get tight right around the ports. You're aiming for a fit such that there's a little drag at the ports, and progressively more drag until it jams right at TDC. You're close enough to clean all of the lapping compound off of everything, oil up the piston, then install it in the engine and turn it over by hand a bit.

The contra-piston is easier. Tight fit so you need to use the screw to move it is fine, as long as it's light enough that compression can push it back up.

 

[creast]

You have my sympathy as I too have been struggling to get my 4 stroke to run reliably and I still suspect compression problems . Have made the pistons 3 times and rings too but it still just doesn't feel right.
I think this is one of those things that would be easier if you know someone locally who can give their judgement.
Good luck and hope the patience stays Thm: