Not the answer you are looking for, but back in the '60s, Fox used to sell a product called Lustrox, if I remember correctly.  It came as a powder in a little plastic bag and it was intended to be mixed with the fuel to help the break in process.  It was a form of lapping compound.  

Based on the theory that if a little bit is good, a lot is much better.  After all, the new Enya engines were advertised as being hand lapped.  I thought I could do as well. I totally ruined a couple of new Fox 35s by the excess use of Lustrox.  

Well, I came up with an even better idea (I thought).  I made a holding fixture for my engine lathe that would rotate the crankcase and cycle the rod and piston while the end of the crank was held in the tailstock chuck.  I put the Lustrox and some cutting oil to it and set it to run at 3,000 rpm.  Vibrations not withstanding (good thing the lathe was bolted down), my idea of lapping in a new engine without heat or combustion worked so well that engine #3 was ruined in much less time than the other 2.  

Lustrox was very profitable for Fox Motors.  It made me buy 3 engines.

Note to self:  There are no shortcuts.

No one competent has answered, so I'm going to.  

Mine was the only reply and I never claimed to be competent in lapping airplane engines. In fact my first sentence set the stage with a disclaimer.  Such an outrageous post as mine would surely @#$% someone off enough to jump in.  Looks like it worked.

I use rotten stone and wd 40 to lap bushed cranks I use a brake cylinder hone to fit abc p/c's and also use the hone to finish ringed cylinders so they hold oil and keep the ring from wearing out. Don't know that I have a clue but these things have served me well.

No need to do any lapping on almost any modern engine.  All the manufacturers, except of few of the Chinese suppliers, make motors that are pretty well built with good fits, nice squarely cut parts, wrist pin and crankpin well-hardened and ground, etc.  

Even the new Fox 35's are much better built than any but some of the 40th Anniversary ones.  Jim Thomerson's method of lapping with DuPont white rubbing compound can help speed up the break in.  I usually did it with the piston on the rod and lapped with the piston upside down.  The idea is to try and lap a little bit of taper into the sleeve.  The top edge of the piston will wear into shape with just a few runs.  It's very important on iron piston engines to keep the initial runs cool and slow with lots of lubrication, so the parts wear slowly and get warmed up and cooled down.

Lustrox= pumice, A big no no.
grit type stuff embeds in poors of the metal, the angular shape causes micro fractures that soften the material.
NO Compound!!!!of any sort.
Well there is one polish available for aluminum aircraft restoration for EAA members.. Good stuff, No angular material.

Modern engines of good quality should not need lapping. Fox is probably the only lapped type engines left an all other is ABC ANC or someC or another, or ringed.

In the past and sometimes recently I lapped the(old) STs, Foxes, what ever, under running hot water.
Piston upside down and never go to the top of travel in the sleeve.
For combat and Rat would taper/enlarge the bore mostly from exhaust port down and little to none above this.
Do not know how this would work out on Stunt and for R/C its not good for idle.

When things are bad off restoration from junk type work sometimes need a well worn sleeve to work a piston in until round again and sometimes the reverse for bad sleeve.

I think hand fitting is going to become a dead art.
We been replaced by CNC!

 Hi.

 Here is a problem that I have seen in many Retro Discovery engines. Usually the cylinders are of very good quality but very often the piston is not round, it has several "angles".
 Because the correct fit is determined by how deep up the piston goes in the sleeve with a known force, the engine with this problem feels very good when new. Problems come usually after about 200 flights, first I notice difficulties in start, and very soon after that a loss of power and then the engine jams completely.
 But in the beginning, engines with this problem give a wonderfull, strong 4-stroke run. I assume that's because of detonation caused by hotter-than-normal piston. Engines with this problem seem to run best with all-castor oil fuel, many synthetic oils cause unbearable detonation.
 I think that the reason of this problem is bad quality control, it's never an issue with engines tested by the maker or with the engines that come with ready-to-go models. I guess that the deformation originates from some tool vibration from raw-machining or grinding. There is simply not enough material left to lap the piston round and smooth.
 But to the original subject: This piston cannot be rectified, I would just end up with a loose fit. For this engine, I fitted a new piston, starting from an oversized one.
 There is also another thing visible in the picture, I find it in well fitted engines too, and it should be taken care of after running-in. In both ends of piston "wall", there are marks of high friction/surface temperature. In a good piston the primary sealing area (near the top edge of piston) should have a constant width, but as you can see, the sealing area is wider near the piston wall ends. When the engine runs, the wall is hotter than the rest of the piston, and it expands and compresses against cylinder walls. I think it's very important to correct this deformation after the initial running-in, about 30 flights. It's easily done with #1500 sandpaper and some oil. I also relief the ends of piston wall by about 1/100mm.
 Sometimes similar "shiny spots" can be found also elsewhere in the piston surface, usually near the wristpin ends. They should also be taken care of.
 Also, in the RD.60 engines, the cylinder/case fit is sometimes too tight. I allways make sure that the sleeve has a smooth sliding fit to case. I rectify it by lapping the outside surface of the sleeve. If it binds when pushed in the case, it won't be round and you end up having high friction/temperature spots in the piston/sleeve interface. not good.
 All these things cause bad and uneven thermal conduction and a normal user only may notice the detonation. And usually he tries to fix it by lowering the compression which may help a little, but actually it just messes up the thermodynamic balance of the engine.
 There is also several small things to do to improve the RD.60 bottom end (with bearings etc.) if someone is interested.
 But please, don't do anything unless you really know what you are doing. Most engines are just fine.

 Lauri     

 Hi Randy.

 You are right of course. They should be made right in the beginning. I find it amazing how well these engines run with details like that inside. It may even be so, that the "unintelligent" solutions improve the way they run.
 But, hopefully next year there is no more baffle!

What I find curious here is the David (in his original post) never really goes into the specifics of what he needs the information for - apart from the obvious and I find that the best reply was posted from Tim Westcott with the link to http://www.modelenginenews.org/.

It directly answers the question and one could easily assume that David is indeed intending to make his own engines at home or that he simply would like to know how to do something. And since his signature is "analog man trapped in a digital world," this would be my educated guess.

And Jim Thomerson has it right, its the fits that determine whether or not a design performs well or not.

Hand fitting a dead art? I hope not because companies like PAW, Oliver, MP Jet, Fora etc would then have to close their doors for good, and that would rob a dimension from our hobby.

Cheers.

A suggestion for your Fox 35 and Banshee, which no one else will agree with.  Try a couple of different 11 x 5 props. 

The reasons I remember from the day on lapping Foxes was a line from the Team Race guys. They found that if that just went to spinning the Foxes without fitting the p/c they would wear out the rest (rod and crank bushing) of the engine waiting for the p/c to come in. Sooooo out came the lapping compound. On another note you couldn't  beak in a Max III without seriously "shrinking" the piston. The list  supplied is what I use to solve problems. I would also include a very fine file to remove high spots. Silicon carbide sandpaper leaves grit behind. High spots on pistons can be VERY carefully filed.

Heat cycling the engine parts in the Wife's pottery Kiln before fitting would be good, for the engine, not the relationship..

But as Brett says, the next stage after hand lapping is machine lapping or running it in so don't try to shorten your run in time -

  Well, what I meant to say was something different - more like, the first step after hand-lapping is buying a new piston/liner! At least with any current stunt engine.

    For ancient slug-piston engines, what I had intended to say was that if you could get it to run at all, and particularly after it has some time on it, you don't want to hand-lap it. Needing to hand-fit engines (or disassemble/clean/lube) was something you sometimes *had* to do because of the poor design and manufacturing. Not something that you should *want to* do or is desirable.

     I guess I understand on some level the desire to use the old clunkers (you ought to see the radio I listened to the Giants game on yesterday) but Foxes and McCoys haven't been mainstream stunt engines for 30-40 years now. I just hate to see people steered towards this sort of thing, when you can get far better performance with far less effort for generally far less money (add up the cost of all the trick parts you can put in a Fox - could buy a PA or RO-Jett) .  Yet I still see people endlessly struggling with the endless mods, trick fixes, rework, and getting pretty much nowhere. Then, on the rare occasions they get it to work properly, still get generally very poor performance by current standards. And quite commonly, never know better.

     Brett