Gents; I got a used Fox Stunt engine from E-Bay for 16 dollars. With the exception of a few missing items, like the needle valve assembly and prop washer and nut, it appears to be in good condition. There is a problem, however, that might be insurmountable. The piston is very tight in the sleeve. I understand there is a "pinch" at the top of the cylinder bore but this set up is so tight you need a wrench to turn it over with the glow plug out. I bet the engine will run very hot and not turn much RPM, hence the reason it was sold so cheaply. My question is can I do anything about this problem like putting emery cloth on a drill and running it into the steel sleeve to gain a little clearance for the piston? Thanks and have a great New Year!

After I noticed the tough piston in sleeve movement, I took the engine apart and got everything exceptionally clean....I'm anal when it comes to clean engines. Anyway, after putting everything back together, the piston still is very hard to move in it's sleeve. I'm convinced there is an improper fit between the piston and sleeve since I've got several other motors like the old McCoy .35 (including a NIB McCoy .35) and K&B Torpedo that do not have nearly as tight piston and sleeve arrangement. I guess if you have to have a problem with a model airplane motor, it's better to have a tight piston fit than a loose one but where can I go from here? 

Yes sir, I checked the crankshaft fit and it spins like a charm with a little light oil on it. Also the rod fit on the crankshaft and piston pin are just right. However, the piston is just about unmovable (I can't hardly move it in the sleeve with my thumb). How do I "lap in the piston". I'm assuming you are talking about using valve grinding compound and somehow turning the piston into the sleeve. I have lapped valves in automobile cylinder heads, using a suction cup on the valve and twirling the valve. But, for the life of me, I can't see how I'll be able to do that with the model airplane engine piston and sleeve arrangement. javascript:void(0);
mad-wife

I bought four Fox 35's in 1977 and lapped them all.  Fox later replaced the piston and liner in one I fried, and noted that it was tight.  Darn right it was.  I took it apart and found a metal chip under the liner lip.  Removed that and it was still tight.  So I lapped it.  I use DuPont white rubbing compound which is water base.  I take the head off and apply a little compound around the top of the piston at the bottom of the liner.  Put a prop on.  Hold the engine so the liner cannot move and wiggle the prop back and forth. When it gets tight, back off and wiggle again.  Gradually work up the liner with it feeling free as you go.  Stop about 1/8 inch from top dead center and let that area break in normal. 

The liner is, I think, slightly warped, as the lapping pattern on the liner and piston are weird.  I've run the engine a couple of times, a little richer than usual. It starts right up and runs like it will be OK.

I don't think tooth paste is a good lapping compound for iron/steel.  It contains talc, which only has a hardness of 2. I don't know what the DuPont rubbing compound has in it but it will cut, fairly gently, I think. 

I don't think tooth paste is a good lapping compound for iron/steel.  It contains talc, which only has a hardness of 2. I don't know what the DuPont rubbing compound has in it but it will cut, fairly gently, I think. 

  Pearl Drops was not like regular toothpaste. They called it a "tooth polish" at the time.  It had a warning message about not using it regularly. It worked eventually, and it was definitely removing metal (turned gray in a few passes),  although it was slow. If I had to do it now, I would certainly use something more aggressive, starting with a brake cylinder hone. But there wasn't much chance of overdoing it with Pearl Drops. This was *a long* time ago - '72 or '73.

    Recall that Fox used to sell garnet lapping compound - and this is why. The problem with that was that it was extremely difficult to remove completely.

      Brett

Dallam, Lots of good advice here and I will just add a thought. I have had a couple Fox .35s that had the sleeves distorted in the exhaust port area, most likely from attemted removal with a good castor seal holding the sleeve tightly in the crankcase.
 A fine stoned spring loaded break cylinder hone was then carfully used to restore the liner, the hone will show any tight spots with just a little usage.

   I agree, its worth running something down there just to show the high spots, if any. At least you will know what you are dealing with. The unfortunate part is that if the liner really is distorted, it's probably because of pressure from the case or from a muffler lug, and just getting a new liner won't necessarily fix it.

       Brett

   HMMM  ED HMMM  Educational Disparity---  go back to school.  After I was out of high school for 17 years (and I didn't study then) I went to college to get my education--- so I know ALL about ED.

  Bigiron
  Added by edit---  There are some who are seriously affected by another form of ED---- "Earning deficiency''  Cannot be "Cured"  but sometimes the symptoms can be eased by  1. winning the lottery,   2 having a rich relative die and leave  a will to the person, 3  Marrying a rich widow,  4  working several jobs and lots of overtime

  Bigiron

Wow, I'm reading some outstanding and knowledgeable comments here. I've decided to use my brake hone with plenty of oil to see if I can get a better fit and will report back to you gents.

  Just try it for a little bit, and see if the wear pattern is uneven. It's still not clear if it's really just too tight or if there is some sort of distortion.

Oh yes, my wife noticed that I was impressed with your answers and wanted me to ask any of you if you knew anything about "ED" and how to cure it. Just what in the tar nations is she talking about?!   

I have absolutely no idea what she is talking about.

    Brett
   

I'll be sure to go easy with the hone and let you know how things turned out tomorrow.

     One general note (and I'm sure Randy and other engine manufacturers will agree) - Brake cylinder hones, hand-lapping, etc, is *not* something to be done in any less than extreme circumstances  - like, the engine is grotesquely too tight, and replacement parts are easily available in the (likely) event that the original parts are ruined by this technique. 

     In almost all cases with well-constructed modern engines like the PA, Jett, OS, etc, all that these techniques will do is ruin the liner instantly. Some of have had good results in similar "last resort" and special cases,  but under no circumstances should this be taken lightly. The guys who build these motors have $50000 honing machines to get them just right, anything the end user does to them *will not help*.

     Brett

Dallam, Your Fox .35 is NOT a tapered cylinder abc, abn or any other funney sort of set up (UNLESS SOME ONE HAS CHANGED IT FROM THE IRON PISTON/STEEL SLEEVE)
so the procedure with the hone can be very effective in restoring a distorted cylinder, the key is to go at it slowly, with the hone inserted in the cylinder, just a short blip on the trigger will tell if there are any high spots or other distortions. I have done a few with this method and have had excellent results. The most common thing that I have seen with the Fox S .35 is where some one has inserted some thing into the exhaust port in order to remove the sleeve and the thin sleeve is distorted. often you will find that just a very small area at the top edge of the exhaust port is causing the problem. Keep us informed...........:-) regards, Phil Bare

Hmm...do I or don't I hone the cylinder? Ok, Phil talked me into it. I'm going hone the cylinder very lightly. Lets just see what I can do with my 5 dollar Auto Zone hone.  I'm sure, however, properly honing will be a real challenge with a whiskey or two in me.  Not to mention  my wife yelling at me to stop playing with those stupid toy motors and go to the store a get her some wine spritzers and some cigarettes then get in the house and watch Jerry Springer with her! I cringe to think what the old Fox cylinder might look like after I get done with it. Funny thing, I'd rather be honing on my Fox cylinder than watching my wife (after six wine spritzers) laughing then shaking her fists at the quests on the Jerry Springer show! 

   I merely added my caution because I know modelers. One mention of brake cylinder hones and half the guys reading will want to go try it this afternoon on their brand new PA75! I felt obliged to mention that it's a very rare and special case that can benefit from this sort of thing, and the chances are high in most cases that you are going to do more harm than good.

      It's OK in this instance because there's no way it's going to work the way it is, and the worst case is that you will ruin the liner and have to get another one - which is also the only alternative if you *don't* do it.
   
    By the way, this process can be just as potentially damaging to straight liners as to tapered  liners. Anything you do to change the dimensions of an otherwise properly-fit cylinder damages it.

        Brett

Dallam,

The only way I would lap a Fox 35 cyl/piston is by hand. I use a medium paste of Lava hand soap (a pumice grit) which breaks down as it cuts, and does not embed in the softer piece to keep cutting the harder piece.

I lap with a dummy conrod (piece of 3/8" sq motor mount drilled for the piston pin) using a sort of figure 8 twist and slide motion from the bottom of the sleeve up. The grit can be replenished, or thinned with another drop of water if it starts getting too dry, or too tight. And it washes out completely with water, before reassembly.

I hold the sleeve in my hand, or trap it wrong way out on the block, using the head screws to hold it. And the hand pressure is switched about 30° after every five or ten strokes.

STAY AWAY FROM THAT TOP 1/8" OF PISTON TRAVEL!

Someone mentioned a possible 'bent' sleeve... I had one engine that acted like that, but it seemed more that the bore in the casting wasn't 'true'! Lapping the sleeve to the casting so it was a smooth push fit loosened things up nicely. Rare. Only had one do that, but the light lapping really helped it.

Also, on reassembly, start as was mentioned, with the head bolts loose. (Make sure he ports are aligned correctly! High one is exhaust.) If the piston moves smoothly and shaft turns freely, it is up to you to NOT torque in a bind or bend. If the front bolt is at 12 0'clock, I draw them in in a sequence that chases around the six of them like this:
12- 4- 8 o'clock, then across the head to 2- 6- and 10 o'clock. Then across the head to 8- 12- 4- then across to 10, etc.

After each THREE in a sequence have been drawn down a bit, recheck that sleeve and shaft are still as free as when you started. If you find a bind starting, back off and sneak in on it again. Takes a bit longer than on engines with sturdier heads and castings... not a lot longer, just some.

And if none of this helps, you may be able to contact Fox Mfg. They used to charge no more than half the current price of the engine and often just shipped the customer a brand new one, instead of going through the expensive hands-on analysis/repair/rebuild procedure. That my still be in effect.

I used 4/0 steel wool and lacquer thinner on a Fox 29 sleeve.  It just kept running slower and getting harder to turn over.  I have run it several times since, and it appears to be back to normal.  And it still has good compression hot or cold.  Maybe I was just lucky. 

Dallam, there is no built in pinch in a stadard Fox Stunt .35 so if the piston gets harder to move near the top of the stroke then more then likely, it is just carboned up and an over night stay in the crock pot would put it right....Regards, Phil Bare

Phil

Most every piston sleeve combo FOX ships out has the fit tighter at the top of the sleeve, most people would call this pinch. I just went thru 10 of them while building FOX  35s they all were much tighter at the top than the bottom or middle.  I have not seen a piston/sleeve combo from FOX that was not this way.

Regards

Randy

Hi Phil

No I am talking about the FOX piston and sleeve, it has a taper so the piston gets very tight at the top of the sleeve, all of them that I have seen are that way, they all have a skight taper...loose at the bottom very tight toward the top of the sleeve. The last piston / sleeve sets I saw that were perfectly straight were the OS 35S, you can push them right out of the top of the sleeve with no effort at all.

Regards
Randy

Wow, I'm reading some outstanding and knowledgeable comments here. I've decided to use my brake hone with plenty of oil to see if I can get a better fit and will report back to you gents. Oh yes, my wife noticed that I was impressed with your answers and wanted me to ask any of you if you knew anything about "ED" and how to cure it. Just what in the tar nations is she talking about?!   Thanks again and happy New Year's!

Try Viagra! 

2 things you can do. 1.-Tighten down all the head screws in a cross pattern like you have been. Put a prop on the shaft, and with the plug out, loosen one screw at a time about a 1/16 of a turn. Keep doing this until you get the least amount of drag on the piston. You may find that tightening one a little more will help also.

#2.- Remove the sleeve from the case, and put a little rubbing compound just on the under side of the lip of the sleeve, put it back in the case and turn it back and forth to lap it to the case.

It is not the head that is making it bind. It is the sleeve to case fit generally at the top of the case.

When I first met the misses, she was a six foot tall goddess and looked very much like Susan Anton. Now, 23 years later, she looks like Philis Diller and all she does all day is watch Jerry Springer, drink wine spritzers with a cigarette hanging out of her mouth with a one inch ash hanging off the end. For some reason Viagra doesn't work.

Well in that case... add a large bottle of tequila to the mix.

Thanks Lou. I wasn't as scientific or as thorough as you in truing up the sleeve but the suggestions from other gents like you (along the sleeve truing lines), were on the mark. One thing I've learned is that the Fox Stunt manufacturing tolerances are still not comparable to tolerances that Japanese company's like OS and the like employ. Unfortunately, for we Americans, not only do the Japanese make better model airplane motors but they make better auto's too and the American public has noticed this fact which has put Toyota ahead of GM. I simply could not believe the advertisement about the base Toyota Tundra truck having more horsepower and torque than the Chevy or the Ford trucks. They have beaten us at our own game...shame on us! OK, I'll get off my soapbox and get back to my old Fox stunt and oh yes, I took your advice and I took the Viagra instead of my wife... she's got a smile on her face I'm going to have surgically removed! Cheers