Hi All:

       I have been having vibration problems with several Fox .35 Stunt engines,  These engines have never been crashed and have used only for racing.  I noticed the vibration was quite severe when I installed them on a CG Shoestring. The vibrations were felt in the handle and my friend who was watching said that he could hear the plane vibrate while in flight.

      I mentioned this to a friend who works on engines and he suggested that the cranks were manufactured less than straight.  Twenty thousands of an inch run out had been observed in some Fox crankshafts.  This makes sense to me as I have three Fox .35 Stunts that shake very badly.  Another thing that came to mind that Randy Smith sells a Zoot crankshaft that is designed, I believe, to replace the bad Fox cranks.  There may be something to the bad crankshaft story to entice Randy to provide aftermarket cranks.   I just learned that Randy DOES NOT HAVE ANY ZOOT CRANKS FOR SALE!

      Has anybody else noticed bad  Crankshafts in the Fox.35 Stunt?  Has anybody used a Zoot crank from Randy and what were the results?  Can these cranks be straightened, if so, how.

       Several people have suggested that I get away from Fox .35 Stunt and stop beating my head against a wall.  Perhaps I will bring out my OS Max .35 Max I and Max III  three engines from the junk drawer. Lol I have them, so why not use them? How bad can they be?

        Thoughts/comments?

                                                                                                                  All of the best,

                                                                                                                  Frank McCune

p.s. Tomorrow I will fly an OS FP .40 powered stunt plane.  Perhaps this will make me change old habits. Lol

Frank,
One thing you should check is if there is a bind in the engine. Put a few drops of fuel in the venture. Flip the engine so the fuel loosens it up, take the plug out but leave the muffler on. Slowly rotate the engine through a full rotation (best is if you can also take the backplate off so there is no base compression holding it back). Feel the shaft as you rotate through the up and down stroke. A bind can develop because of varnish, case distortion from muffler bolts or head bolts. If you feel the bind, remove the muffler and test again. If still binding go around the head an loosen the bolts one at a time 1/8 turn, test after each bolt is loosened. If you find one that cause the bind to go away, mark the hole, check if the bolt is rubbing the side of the hole in the head. If so remove the head and run a drill one size larger then the existing hole (I do this to all the holes in my Fox heads). If it is not rubbing take the bolt and put it in a drill and file down the head (if you have replaced the factory bolts with socket heads you will need to file them all down to match the factory bolt diameters or they will rub on the fines and seat and distort the case causing binding) If still binding pull the piston/cylinder and de-varnish. For this I have used a small piece of Scotch Brite green pad with a little light oil. Run the pad up/down the bore with a twisting motion, also clean off any varnish spots on the piston. Go slow and test so the piston slides up the cylinder smoothly with no bind. While you have the P/C out check the shaft rotation for rough spot or binds, not normally an issue but I guess if the shaft is bent you would feel something as you rotate it. If bent you might need to replace the shaft and case (likely not worth it).

Best,   DennisT