SuperTigre G15 engine resurection!

[scattermaster]

Hey guys,
Is anyone familiar with the old SuperTigre G15 engines?
I have an old control line version of this motor that was apparently reworked by George Aldrich. (GMA Custom engraved on the head) I want to see if it'll work for 75mph combat.
It's been sitting for 40 some years. I cleaned it up and it felt pretty good but I thought I'd replace the bearings and the piston and sleeve.
Bearings, done. Good.
Piston and sleeve? Well I found a new set on Ebay and installed them and guess what, no compression.
I can see it leaking in the exhaust port.
So, what can I do?
One of my pylon racing buddies, Fast Freddy French, suggested heating up the piston to make it larger.
Has anyone heard of this?
I think my other options are to 1- squeeze down the sleeve, or 2. make a sleeve, or 3. give up on this tired old horse!
What do you think?
thx,
Jim

 

[josodl1953]

Did you try interchanging the old vs the new piston/liner?
In my C/L days we made a row of punchmarks just under the piston crown to enlarge the diameter.
After lapping withe very fine lapping compound the compression was back again. Worked even for diesels.

Jos

 

[scattermaster]

Jos,
I probably will check out the fit, old liner/new piston and new liner/old piston. Not so inclined to go to town with a center punch on the sides of that piston?
I learned a little bit about lapping tools yesterday. Maybe I can make a piston to fit the new liner? It might be above my pay grade but then that's how I learn.
Still another option is to squeeze the top of the liner and then lap...
As of yet, I haven't done any measuring. I need to do that.
Jim

 

[josodl1953]

Sure you can make your own piston if you have a lathe. Be sure to use cast iron .Back in the seventies I experimented with an aluminium piston in my G15 but as it revved up the piston seized because of the rate of expansion. Liners for ringless pistons are always a bit conical towards TDC. It is best to test fit the piston when lapping bit by bit. Measuring is difficult because the difference between a good and a lousy fit is often less than 0,01 mm.

Jos

 

[f2cf1g]

Can I ask what was wrong with the original piston/liner set? Only running will wear it out, not sitting in a box. I doubt that it's clapped out sufficient to stop it running or pulling a combat model round at 75mph. The process of enlarging a cast iron piston is often called "cherry bombing" and it does work.

 

[scattermaster]

My engine was leaking compression around the side of the piston. You could see it in the exhaust port. Really bad.
So, expanding the piston is a possibility then?
Nice!

 

[scattermaster]

"Cherry bombing"
Does that mean I need to get it red hot? (Scary)
I have material to make one but I'm more than a little intimidated by the prospect. It's a tiny thing.
Not sure how I'd machine the wrist pin keeper grooves. Really tiny.
Incedentally, a buddy loaned me another G15. It's compression is solid. Doesn't leak at all, so I know it can be done.

 

[WOB]

This info will provide a method to make a proper piston/cylinder fit from scratch. Be sure to use a stress relieved gray cast iron blank before final machining. http://adriansmodelaeroengines.com/catalog/main.php?cat_id=178 It involves making a cylinder hone, but the process is simple and works well if you have the patience.

WOB

 

[josodl1953]

Concerning the wrist pin retainer grooves : you may omit these providing the hole in the piston does not come across any scavenging ports. The only thing you have to do is fit some aluminium buttons in the wrist pin to prevent the pin scoring the cylinder wall. I did this on my 50-year old ST 35 and it works fine.
About drilling and reaming the pin hole: see post # 101 of my thread about the Edwards Radial 5

Jos

 

[ALEX1952]

Ditto josodl1953 post except common practice in model engines is a PTFE button very low friction etc. and thermally stable, just a bit of a pig to machine and can have health issues just google for sensible precautions.

 

[ALEX1952]

Forgot to say drill and ream gudgeon pin hole (UK speak for wrist pin' don't know the origin) then fit pin to hole.