To ring or not to ring

[Jkrych]

Looking to build a small(ish) nitro engine, but I'd like to get some real performance out of it. I don't have access to real heat treating equipment, so steel/steel sleeve and piston is out of the question, so steel/cast iron is where I'm leaning unless I ring the piston. This will be multi cylinder, so the bores will be less than 1/2inch (12mm) and I think rings aren't much use when the engine is that small.

I'm also not clear on how far the piston should go up the bore after lapping. Some say just above the top of the exhaust port, some say not until just before TDC.

Any wisdom someone can lend me?

 

[lohring]

cast iron in mild steel is probably the easiest piston and sleeve system. The below suggests a cast iron liner but will work for a steel liner as well. Be sure to clean the lapping compound off very thoroughly.

Lohring Miller

The best possible combination is a cast iron piston and a cast iron cylinder. There is enough free graphite in cast iron to let the two surfaces slide over each other without galling. Never use steel on steel. For 1/2" bore you don't need any rings. Lap the piston into the cylinder to make an air-tight seal. Leave a cylinder wall of 0.060" thick. Make the cylinder first. Make the piston close to size but not quite. Use a sanding strip and work the piston down until it just starts to slid into the cylinder but will not slide in. Coat the piston with 600 grit carborundum paste or diamond paste and with one part turning slowly in the lathe, work the other part into it very slowly until you have a complete fit from end to end.




Brian Rupnow
Design Engineer
Barrie, Ontario
Canada

 

[WOB]

This is the correct process to fit a cast iron piston to a steel cylinder: AdriansModelAeroEngines.com :: Piston/cylinder finishing
It requires building an internal lap for the cylinder and an external lap for the piston, but nothing beyond a hobby machinist's capabilities. You cannot use the piston for a lap if you want a high precision, durable fit.

WOB

 

[BobsModels]

I agree with WOB. On my engines the combination is cast iron / cast iron. I did one stainless steel cylinder and aluminum piston for a hot air engine, used same procedure.
Hone/Lap the cylinder as perfect as you can get it. Then make the external lap. My goal is with the cylinder on the table. the piston sits just starting in the cylinder and stays put, ie air compression holds it up. Lift the cylinder up and the piston falls through. Sometimes I screw up and get to make a new piston which teaches me patience's when lapping the piston.

good luck

Bob

 

[lohring]

Unfortunately, the model engine news site has died. There's a ton of engine building information there. You can still access it through Wayback Machine Take a look at Model Engine Development under the engines tab. An even better reference for beginners is Building Model Engines that Run under the projects tab.

Lohring Miller

 

[minh-thanh]

Jkrych

This will be multi cylinder, so the bores will be less than 1/2inch (12mm) and I think rings aren't much use when the engine is that small

It's a really small and really hard multi-cylinder engine! You will need a lot of patience and effort to do it.

I'm also not clear on how far the piston should go up the bore after lapping. Some say just above the top of the exhaust port, some say not until just before TDC

When I made cylinders and pistons for my engine (cylinder diameters : 13 and 16 mm): Pistons just passed the exhaust port and tightened from the exhaust port to the TDC (without oil) and with oil : the piston moved a little smoothly .
For cylinders with a diameter of 16 mm: I follow both ways and the engine runs in both ways
I am not sure which way to make the engine more efficient according to your wishes (real performance).

 

[KenC]

Unfortunately, the model engine news site has died. There's a ton of engine building information there. You can still access it through Wayback Machine Take a look at Model Engine Development under the engines tab. An even better reference for beginners is Building Model Engines that Run under the projects tab.

Lohring Miller

The Model Engine News site has not died it is still accessible but it cannot be added to nor modified. The "Members Only" plans are only accessible to those who paid Ron for membership, but I have compiled a book containing all those drawings, which is available for free download at Adrian's Model Engines or at rclibrary.co.uk. The other thing I would add is that I often see references to lapping a piston inside a cylinder. This is very misleading. If you simply from the start, lap an unfinished piston into to a cylinder you may produce an engine that will run, but that is about all. Pistons and cylinders are best lapped to size separate to each other until the fit of the piston is correct. Only as a final process is the piston ever lapped inside the cylinder, and that would be using a very fine abrasive compound of perhaps 2000 or smaller size, and only for removal of an almost immeasurable amount of material. This is very rarely done, so do not believe experts who make beautiful engines that sit on shelves, to tell you otherwise. It is never done in high performance engines where piston shape and cylinder shapes are very important, highly controlled, and simply cannot be achieved by lapping a piston within a cylinder. And as already been said in this thread, you can read my article on piston and cylinder fitting on Adrian's Model Engines web site.
And finally, it depends what kind of engines are under discussion, but I know of no current small capacity IC engines ie less than 10cc displacement, that use a cast iron piston in a cast iron cylinder. In the early days of small engines for model aircraft iron pistons in iron cylinders were not uncommon, and in the 1950's some manufacturers like Allen Mercury (A M engines) tried but failed with that combination and abandoned the idea, reverting to iron pistons on hard steel cylinders. So these days iron on steel is common, steel on steel is not, iron on iron is not.
KenC