Couple of questions about glow engine cylinder lapping techniques

[jpeter]

Wouldn't you want to make the new piston out of aluminum fitted with a cast iron ring to go along with your new steel cyl? A cast iron piston would be too heavy to match up with the original crankshaft counterweight and IMHO the engine would shake itself off the airplane!

Pretty good point.

Just an idea but put c/i rings on the original piston and make a new c/i cylinder to fit.

 

[mu38&Bg#]

The cast iron and steel was an idea for a diesel piston liner set. Rings do work, but that's the traditional setup.

 

[digiex-chris]

ah, yes, good point on the piston weight. I'll put a ring on the existing one.

Next problem: the wall thickness is pretty small. My 3 jaw is definitely going to distort it. I'm planning on finishing the outside first, then making a fixture with a hole that fits as close as I can with a split in it so clamping the fixture closes the hole slightly. Clamp it that way in the 3 jaw, then finish the inside.

Is that a good way to do it? Is there a better way?

 

[jpeter]

If you were to leave an inch or so of extra stock on the sleeve for a spigot you could bore the cylinder without ever having to put the sleeve in the chuck. If you had to you could slip the tube on a mandrel for turning the outside.

 

[digiex-chris]

Is adding a ring to my current piston going to change my timing enough to worry about? Should I make a new piston who's height between the wrist pin and the top of the ring is the same as between my current piston's wrist pin and top of the piston?

 

[dalem9]

You have to at least leave room for the land ,Space from the top of ring to top of piston.Dale

 

[mu38&Bg#]

The stock piston won't take a normal ring, there just isn't enough material. It might be worth thinking about a Dykes type. Piston rings have no effect on port timing.

 

[dedaddy007]

Be careful trying to put a ringed piston in a sleeve designed for a lapped piston. If the sleeve does not have "bridges" across the intake and exhaust ports, the ring will catch and break causing major damage. (I know....!)

These engines turn above 10,000 rpm and balancing is pretty good from the factory. I have seen engines turning over 30,000 in speed model airplanes and cars and the builders go to fantastic efforts to reduce the vibrations. In Europe there are some model cars with .09 CI engines that turn close to 40,000 rpm. They use in some cases, lapped aluminum pistons and chrome plated brass sleeves with tungsten inserts in the crankshaft counterweights to balance the engine. These engines use caster oil and synthetic oil because of the high temperatures they must endure. Caster oil holds up better at high temperatures and helps carry away the heat out the exhaust.

My hobby is taking old (50-70 year old) model engines and rebuilding them to get them to run or for show for collectors. I have over 800 engines in stock and supply parts for a lot of old engines. I am asked many times to put a ringed piston in a old lapped engine. DOES NOT WORK. Making engines run at these speeds take careful design and fitting.

For example, Cox made over 75 million engines before they went out of business. Their little .049 Babe Bee sold for $3.95 in the 50's and had tolerances of .000010" on the cylinder and sleeve. Larger engines have lower tolerances but are still tighter than most hobbyist can make. Technique is crucial.

I am a member of MECA ( Model Engine Collectors Association ) that has a web site at http://www.modelenginecollectors.org/ for those who are interested. If any one is interested, I have a talk I gave at the national meeting in Muncie on how I rebuild and old engine. I am going to put it up on my blog but it is way down my list of things to do.

 

[jpeter]

Dedaddy007 is right, if you're gonna use rings you need to bridge the ports and pin the rings so the split can't rotate into the port area.

 

[digiex-chris]

ah, now I understand why some designs are a series of holes instead of a milled port. I'm pretty glad I asked!

I imagine a lapped tapered bore is less than ideal for a ring too, in that it causes the ring to open up slightly at the bottom and compress tighter at the top. Could this lead to cracking the ring?

 

[jpeter]

Somebody back me up: The idea of the taper is that the cylinder is hotter, thus expands more, at the top. When up to temperature the bore is pretty straight. I know from experience that if you put the taper in and get it backwards and your engine will hardly run at all, if at all.

 

[mu38&Bg#]

Yes, that's the basic idea. If you take it a bit further you can say that the clearance is smallest when the cylinder pressure is greatest. Whether or not this is still interference at running temperatures depends on the material and how tight the fit was to begin with.

 

[digiex-chris]

Well, here we are a couple of weeks later. I've got a chunk of G40 looking like a liner without ports. I havn't lapped the inside yet, since I've got it on a really close fitting mandrel for finishing the outside and milling the ports. How much of a gap in the port can I have before I need to bridge them to keep the ring out? The exhaust port is pretty wide, but the intake ports are 2 smaller ones. How do I decide?

actually I'm gonna start a new thread for this, this is probably going to be good info that should be easier to search