Piston is tight at TDC. what to do?

[frank mccune]

      Hello All:

      I have assembled a ST G20/.15 from spare parts and I find that the piston has a pinch at tdc.  What should I do to get a nice fit?

                                                                                                                        Tia,

                                                                                                                        Frank McCune

[bob whitney]

  u need to send it to me.  no seriously if u have the patience put about a 7/3 or 4 on it and run it on the bench at a rich 4 stroke until u can lean it out and u should have an engine that will last for a long time,  that has a steel piston and sleeve and needs a diff break in than an ABC

[RknRusty]

That pinch is normal for an ABC or ABN piston and liner. If it's new, try to resist the urge to cold flip it through the pinch because you don't want to wear it down and damage compression. The liner and piston will change shape with heat and have a prefect fit while running. The new ones can be difficult to crank with the tight pinch. If it's a new piston and liner, to aid in the first runs you can lower compression with more head shims for the first run, and/or heat it with a heat gun, then crank it while it's hot. Don't do a rich break-in like you would with a steel liner, turn it on up to full scream, it needs the heat right away. You should richen it briefly and turn it back up, and keep doing this until it will hold a full speed needle without sagging. Bolt it on a plane and go fly. It will get faster and faster as you use it more.
Rusty

EDIT: Okay another post while I was writing. Now we've given you conflicting information. Maybe Bob's right. I thought it was an ABN. If a magnet sticks to the liner it's steel. If not, it's ABN or ABC.

[Phil Krankowski]

Iron piston supposedly.  If it is too tight, using a heat gun on the head with the piston down might make it a little better for starting.  However the piston and the cylinder I expect won't be vastly different temperatures after running a bit, and if they are similar material, which I expect for the vintage, it will get really tight again once running.

Some pinch is good though. 

Phil

[RknRusty]

Yeah, if it turns out to be steel, iron, in any case if the magnet sticks to it, run it in wet like Bob suggested.
Rusty

[Brian Hampton]

I have an iron piston engine that had an incredibly tight pinch and was impossible to flick over. I considered lapping the liner but in the end decided to see what might happen if I could ever get it to start. Suffice to say that eventually I got the electric finger to get it past TDC and once it started it burbled away quite happily at 6000 revs and as rich as possible. For the first hour, every time the tank drained it went to peak and then stopped instantly with the piston jammed into TDC. At about the hour mark it finally stopped with a bounce back against compression but needed another 30 minutes or so before I could hand start it. I honestly thought that, at the very least, it was going to break the conrod on the first run (no differential expansion with iron/steel) but after running it in I stripped it for a look and everything was perfect. About a year later after a lot of detective work I found out that only 50 of this type of engine was ever made! If I'd known that from the beginning I wouldn't have even tried to run it .

[Chris Wilson]

Diesel or glow G 20/15?

[frank mccune]

     Hello All:

     The good news is that the ST is running very well! The bad news is it was my fault that the piston-cylinder fit was off.

     I had placed the engine in storage with the fasteners left a bit loose.  When I tightened the head screws, the inch abated and the engine would start one flip hot or cold!

    The engine is a converted Team Race G20/15D that still has the Tee Dee .049 venturi fitted to the case.  It is now a glow engine.   It is too small to permit the engine to be leaned out to a rich four stroke mode.  I ran it fast with a small prop just to see if it would run.  All of the parts are new but assembled to make a running engine.

     Anybody have a nva or just a "banjo"  for a  Tee Dee .09 or .15 engines?  Where may I purchase these items?

                                                                                              Thanks for the replies,

                                                                                              Frank McCune

[RknRusty]

Anybody have a nva or just a "banjo"  for a  Tee Dee .09 or .15 engines?  Where may I purchase these items?
Thanks for the replies,

Frank McCune

Frank, I'm glad that's all it turned out to be. I checked with http://coxengines.ca/ and they don't have any NVAs. You might have some luck if you ask over at www.coxengineforum.com  There is a guest forum where you don't have to be a member to post. You just have to tell the nagging Log-in popup not to show again.
Rusty

[Chris Wilson]

So, its a G20/15D that is converted to a glow.

Probably has more of a taper on the liner and therefore more pinch at TDC.

If its for sport use then why stick to a Cox venturi and needle valve assembly?

[frank mccune]

     Hello Chris:

     Good question! The engine has the TD .049 venturi and nva "glued" into the crankcase and I was eager to see if the engine ran.  I will attempt to remove the TD venture an add a real ST venturi and nva. for future use.  The engine is a real gem!  Perhaps the pinch from the Diesel p&c was the real deal!

                                                                                                              Stay well my friend,

                                                                                                              Frank