Lapping in a Fox 35

[Alex Givan]

  The Fox 35 LA 25 debate had a couple of guys saying that a Fox my need to be lapped in. I gather that this is mainly the piston and liner and to some extent the crank bearing race. How is this done, and how do you know when you have it just right? What is the best method?  I feel sure many ways will work, but what is best and how is it done?

[Garf]

I have had great success using #7 white polishing compound. I'll post the procedure when I have more time. It will be a long post.

[Phil Spillman]

Lapping in Foxes isn't too new nor difficult as long as you watch what you're doing and don't get too aggressive. I use DENTURE toothpaste to lap the sleeve to the case, the head to he sleeve and the back plate to the case. This form of abrasive is quite mild and doesn't take off much metal. This is the main concern. A Little water with a little paste worked well for me. Stop when you are inclined to go just a little more! Once all surfaces are mated, lapped, smoothly together your need for gaskets, except for head spacing, will be next to nill. I also think that some lapping of the crank shaft would be helpful to help to shorten the break in period. I am interested to hear what the other man has to say about his lapping experience. In all things especially proper and complete clean up post lap is essential to avoid ruining your engine. And beware these kinds of treatments will in all likelyhood void your warranty!

Phil Spillman 

[Garf]

I begin by washing the piston & cyllinder in a solvent to remove all traces of oil. I then feel the clearance between the piston & cyllinder. It should slide in up to the top of the exhaust port then feel rough & tight from there. Do not force it at this point. I take the piston with the rod installed, and insert a small screwdriver in the lower end of the rod. Then I apply a small amount of white polishing compound on the side of the piston at the top. I also apply some at the lower part of the cyllinder. I begin lapping with a spiral motion working up very slowly. You need to lap, then clean all traces of compound and oil from the P&C, then check the clearances with a dry P&C. You are trying for a drop fit below the exhaust port, a slight bind from the exhaust port to 1/4" from the top, and leave the last 1/4" untouched. My last one has a mirror finish on the upper side of the piston. CLEAN THE P&C THOUROUGHLY. Lube the assy. with heavy oil and assemble the engine. The first run must be as rich as is possible to flush the last traces of compound out of the engine. Good luck.

[Alex Givan]

  I think I will give this a shot. #7 polishing compound? what is that and were can I find it? Is that close to Simicrome as that is great stuff and my do the job as you have described?

[kenneth cook]

      Alex, I've been doing a little of this black magic practice myself lately.  I can add a few tips though. First, I prefer to use a 3/16 piece of dowel approx 2 inches long through the rod. This not only fits tight but it does a few things as well. It doesn't allow you to go too high in the bore because it bottoms on the liner itself. It will snap if you try and over do it and it doesn't harm the bearing surface. I've found that many of the liners I tried have been tight as the piston is entering or just exiting the exhaust port. In these cases I've turned the piston upside down and really worked on just this area as so that no bind whatsoever is hindering the pistons performance. This kind of bind puts a lot of stress on the conrod and can oval out the wrist pin hole as well. Some of the liners already have a bit of taper in them. I've found this to be true with some of my 40th anniv. models. These also had a slight higher pinch at the top of the sleeve in which I had to dissassemble and re lap more than once. Remember, its always better if your not sure to do a little rather than over do it. I pay special attention on the top of the sleeve watching the carbon mark thats left from initial runs. You really don't want to remove this at the top. You also don't want it too tight up there either. After thoroughly cleaning the engine I reassemble and turn it over without the plug as to try and feel any bind. While the engine is apart a good crank polishing should be in order here as well. I chuck it in the drill press and give it the old oil and 2000 grit treatment followed by polish. Remember your trying to avoid any binding whatsoever. Any binding anywhere just leads to heat and performance suffering. Don't just move the piston up and down, move it as your spiraling it. It is critical though to rid the parts of whatever polish or compound your using. I've used all of the above products as mentioned by others. I've even used Brasso as a finer lapping compund. Also, after you've lapped the piston its a good idea to check the circlips to see that they haven't dislodged out of their groove. Generally, after assembly the piston falls on its own weight. You can visually see the highs and lows that occur on the piston during the process. I haven't successfully been able to do one of these engines in the first try. I've only done this on a few engines that I had that seemed to constantly sag . These engines are terrific runners now. I owe a lot to the fellows at the club I belong to that taught me how to do this. Ken Cook

[Marvin Denny]

  Most Fox stunt 25s really do not need "lapping in"  If you are in a real hurry to get into the air or do not have a good place or way to break the engine in by running it, then lapping will get you into the air quicker.  BUT-- be prepared to RUIN several piston/cylinder assemblies  before you get the technique down  good.
  The 50 th anniversary  already has a good taper, but is set up looser than previous series  of stunt 35 and  (in my opinion) already too loose, and any "lapping in" will only make it too loose.
  I NEVER lap in anything on my Foxes other than the forward face of the crankshaft web to the rear end of the shaft bushing.  And I only do that on one if it is blowing a lot of fuel out the front.

  Bigiron

[Alex Givan]

  Hey guys- Phil, Garf, Kenneth,Marvin thanks. I look forward to doing this and seeing the results that I can get. Any other methods out and about let me know. Lapped in and polished maybe a hemi head and stuffer back plate some day. It's like having a stock V-8 and doing some head work, hot cam,headers, and having the engine really come alive. I think that an OS is like a Honda or Toyota great and good to go right out of the box but no soul. Seeing a Fox, McCoy, ST,or an old K@B pop and sputter yet pull an aircraft around the circle with no problem is a real attraction to me. I know that engines can be bigger, better, stronger, faster but for right now this kind of stuff is half the reason that I love this hobby.

[Jim Kraft]

Alex; You are a kindred brother. I never tire of the fascination of the old engines. I can't explain it. I just know for me they are just way to much fun. And Foxes, even though they may be new, are still old tech. I still remember the first time I fired one up on a Sterling P51  profile many years ago. I still get that same feeling today.

[tom hampshire]

     Promised myself I'd never stick my neck out...  But here's some suggestions.  Marvin is right as to the newer foxes.  The breakin takes longer, but you come away with a thermally seasoned p/c set.  It is always more stable dimensionally after 30-50 heating/cooling cycles.  I don't think there's anything wrong with lapping simply to shorten breakin time, but for the best runs, breakin is also necessary.  You don't have to put the engine at risk by flying it too soon.  A dozen or so short (1 min) runs on the bench will get most of the thermal part of breakin done.
     BUT if you use an old Fox - 80's vintage - either with just the screw bosses above and below the muffler, or with the tiny breakaway screw bosses at either end of the muffler - you pretty much have to lap them in.  The tolerance spread is huge, so if you get a tight one, the main and rod fits will wear out before it ever breaks in the piston and sleeve.  Some of this vintage were guilty of absolutely terrible fits, which must be corrected before running.  It's cut and try with the old ones, without muffler attach points, some are ok to run, some not.  Feel the fit and see.
     Second, I always lap the piston upside down, with the skirt facing toward the head.  This really helps to get the taper in place quickly.  Try rottenstone as a lapping agent, it breaks down quickly, and makes it harder to go too far.  A very useful bit of protection for the first few that you try.  Lastly, ask around and find somebody who has done a few.  Then buy lunch and get him to give you a hands on lesson.  This will give you a fast and safe way to learn the al important fit/feel of the piston.  Remember, none of us has ever been inside one of these things while its running, so nobody knows it all.  Perhaps the most central lesson is that a fairly large number of approaches can be made to work very well.  Tom H.