Couple of questions about glow engine cylinder lapping techniques

[digiex-chris]

I'm trying my hand at refurbishing a glow RC airplane engine. There's a fair sized scratch that seems to be letting out too much compression when the thing is warmed up and expanded to running size. It's an aluminum piston, with a nickel plated brass liner, non-ringed. It appears I'm going to have to go almost completely through the nickel plating to remove the scratch, so I expect it won't last too long after I do it, but I'm doing it more of a proof of skills before I go on to building my own engine. Maybe I'll even make a cast liner for this guy later. This guy is definitely not worth the trouble, I can buy a whole new engine for $65, but when has that ever stopped us from spending more money and way more time to fix it ourselves?

Anyway, how much of a reduction in life should I expect with the brass exposed? If I can get a gallon of fuel out of it I'll be ecstatic, but I wouldn't mind a benchmark to other people's experiences with brass cylinders. I'll be making a new piston of course. Any gotchas I should be aware of?

Second question, the more important one, is the making of the lap. I measured the cylinder with bore gauges, and found a taper of about 0.001" over the 1.5" length of the cylinder. I even went as far as chucking up the cylinder, and sweeping with a DTI to match my compound to the angle. Then I cut a taper on a piece of aluminum that barely fit into the bore. But when I added lapping compound, I couldn't fit it anymore! So I cleaned it off, and took another 0.001" pass off. Now I can fit the lap in the cylinder, but the taper is so long to get me to the size I need, that I can't cut that length with my compound.

There's gotta be a better way.

This must be why people make expanding laps. I was thinking of trying my hand at a barrel lap. Is there an easier design? How do you determine the starting un-expanded size of the lap to account for the lapping compound? it's a tapered bore. I'm planning on lapping it without pulling the lap all the way through, leaving the most expanded part at the bottom of the cylinder to get the taper. Is this the right idea?

And the piston: with the tapered cylinder, do you use a cylindrical piston? This original seems to be. Is there an improvement making it tapered to match the bore?

 

[mu38&Bg#]

If your compression seal is bad now, lapping the liner will not improve it in a ringless ABC or AAC RC glow engine design. If you intend to make a new piston please be a ware that the piston alloy used is not a common alloy, but a high silicon content alloy. Common alloys will bind when hot.

Greg

 

[digiex-chris]

I couldn't just make the cold clearance a little more? I'm sure that would make cold starting more difficult, but it can't be worse than I've got it now (starts great with a huge scratch in the cylinder). Any idea what a suitable alloy would be? I'm not scared of ordering some.

 

[moconnor]

Hello,

If you search this site, I am certain that you will find information on laps and lapping, it comes up often.

A great source for commercial laps is: http://www.acrolaps.com/index.htm

They are available from many sources like Travers Tool, Enco, MSC, etc. and individual laps are not very expensive.

The taper is not reproduced by making a tapered lap, it is a function of dwell time and over/ under stroke while lapping. In other words, it is the technique that you use. You would need to experiment a bit, especially if you have never done this before, but you would spend more time (dwell) at the bottom of the liner where the bore is larger. It doesn't take much to get a taper and you would still want to exchange ends or flip the part often to make sure you keep the lap parallel.

I don't believe that the small model piston skirts are tapered, but you may be able to map the diameters of your original piston. Check to see if it is round as well. In larger engines the higher heat of the combustion chamber requires the pistons to have some rather odd shapes in the static state to compensate for the expansion they encounter during operation. In your case, the key is the taper in the liner.

Good luck with your project. Hope this helps you.

Kind regards,
Mike

 

[mu38&Bg#]

I suppose you could, but I've never heard of anyone actually doing it. In addition the high silicon alloy is much more wear resistant that common alloys. Three feet of 1" round 4032 from McMaster is 150USD. It might work. When I have broken engines like this I just put them aside and watch the swaps or ebay for parts or complete engines with unrelated problems. If you want to do it as an experiment don't let me stop you. To be honest though, if you'll be doing this without re-plating the liner, I wouldn't bother. If it was a valuable engine for which parts weren't available, that's a different story.

Straight sided pistons are most common. You'll find some where the top ~2mm of the piston has a slight taper. The taper of the liner you measured is common, but depends on the the specific piston and liner alloys and operating temperature. Ringless engines are designed such that the liner expands more than the piston. This prevents seizure in the case of overheating. They are interference fit at TDC cold. I think most liners are ground in production these days, but the guys building and modifying their own are still lapping them.

 

[digiex-chris]

I think you've convinced me to scrap it. How's this sound as a better idea? Not for a cost benifit, but just success of a project. I'll grab some cast iron and make a CI/CI liner/piston. They appear to be dead simple and well documented. Indexing the ports might give me some trouble with the equipment I have, but that's something I have to solve before I start my own engine anyway.

 

[digiex-chris]

p.s. it's not valuable at all! It's a O.S. .46LA. They work good as a diesel conversion though. Like you said, it's an experiment. Not much flying gets done in the winter anyway.

 

[digiex-chris]

Hello,

If you search this site, I am certain that you will find information on laps and lapping, it comes up often.

A great source for commercial laps is: http://www.acrolaps.com/index.htm

They are available from many sources like Travers Tool, Enco, MSC, etc. and individual laps are not very expensive.

The taper is not reproduced by making a tapered lap, it is a function of dwell time and over/ under stroke while lapping. In other words, it is the technique that you use. You would need to experiment a bit, especially if you have never done this before, but you would spend more time (dwell) at the bottom of the liner where the bore is larger. It doesn't take much to get a taper and you would still want to exchange ends or flip the part often to make sure you keep the lap parallel.

I don't believe that the small model piston skirts are tapered, but you may be able to map the diameters of your original piston. Check to see if it is round as well. In larger engines the higher heat of the combustion chamber requires the pistons to have some rather odd shapes in the static state to compensate for the expansion they encounter during operation. In your case, the key is the taper in the liner.

Good luck with your project. Hope this helps you.

Kind regards,
Mike

Thanks, that helps plenty. So I'm less concerned with a perfect fitting lap? I can use a 0.030 smaller lap and just expand it to fit, then dwell more in the bottom end?

 

[mu38&Bg#]

That's a great idea. I actually have a few glow engines waiting for CI/steel piston and liners for diesel conversion, for "some day" projects.

 

[dalem9]

Micro-Mark Has plating kits that may work for you. You may be able to plate the strach to . Hope this will help . Dale www.micromark.com part #83085

 

[digiex-chris]

That's a great idea! I bet we can do it cheaper though. It SHOULD just be a conductive solution, some nickel, and some low voltage dc. Now I get to try to find some nickel.

I probably have to lap it back into shape after plating, right?

 

[dalem9]

Yes better do some experimenting first . I have the copy chrome kit and it works great .The nickle is in solution in the mix . Dale

 

[mu38&Bg#]

It will be pretty close after plating, plating of course adds material. The lapping will give oil something to hold on to. You should check to see if that's decorative nickel or engineering. Wear resistant nickel plate contains phosphorus. There are several grades. I have an electroless nickel kit from Caswell, but haven't tried it yet. Maybe on a paying project this month.

 

[moconnor]

Thanks, that helps plenty. So I'm less concerned with a perfect fitting lap? I can use a 0.030 smaller lap and just expand it to fit, then dwell more in the bottom end?

Correct. The barrel laps will expand 15%, so you have plenty of adjustment for wear. Just make sure that you clean the bore before you measure it with your small hole or bore gauge so as not to damage your measuring instruments with the lapping compound. And use solvent and then hot soapy water to clean the liner when you are finished to get all of the abrasive out of the bore. Actually, wash it several times. This is real important.

 

[Mosey]

p.s. it's not valuable at all! It's a O.S. .46LA. They work good as a diesel conversion though. Like you said, it's an experiment. Not much flying gets done in the winter anyway.

I have 3 OS 40 LA's, and I love them! They always start, even after sitting for a couple of years with last run gunk in them, and they run well. And they put out plenty of power. Can't say enough about them. when they wear out, just throw em in the bin and buy a new one.
You are the first guy I've seen on this forum when suggested to scrap a dodgy idea has actually listened to reason. Bravo!
Good luck on your fitting.

 

[steamer]

Doing something like this to improve your skills is a great idea!

Lessons learned! The best way is the try.

:bow:
Dave

 

[jpeter]

In those types of abc engines the piston fits really tight at the top. A new engine is hard to turn through the top dead center when cold. The idea ,I guess, is they fit right when warmed up. I know if they're loose when cold they're worn out. If the chrome has pealed such that brass is showing anywhere the engine is toast. Re-chroming sounds like a fun project. It might just work. Some makers use hard chrome and others use nickle plate. Never-the-less the fit has to be pretty tight.

Jim

 

[digiex-chris]

does the same count with a cast iron liner/piston? it won't expand nearly as much, should I still try to match the taper?

 

[jpeter]

I've made a few c/i piston and cylinder engines and they do run well. I bore and lap my cylinder straight as I can. Then I turn the piston so it can be pushed through with some reasonable hand pressure. Some sanding with 180 grit might be needed to sneak up on the size. After a few pushes the piston will loosen up but will still need some slight push to get through, but easy to push. Keep it oiled. You don't want to wear it out before you use it. Anything looser won't be tight enough. A bit of running and the piston will slide perfectly smoothly and have unbelievable seal.

My largest engine with c/i piston and ring has a 1-1/8 bore and 2 inch stroke. Back in the day all small model airplane engines were made this way. I don't in mine but model airplane engine guys used Castor oil in the fuel for these types of engines. The ABC type setups use 100% synthetic oils. I think Castor gums them up.

 

[DICKEYBIRD]

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!