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Author Topic: lapping a Fox 35 pist/cyl with Brasso  (Read 1554 times)

Offline Ara Dedekian

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lapping a Fox 35 pist/cyl with Brasso
« on: September 25, 2022, 08:34:09 PM »

      You never know what kind of life an estate engine led. Singled this estate Fox 35 out of my junk pile because of the stuffer head and backplate. It was as black as the prop and cleaned up nicely with a crock pot and LA's Totally Awesome but some odd things were going on. A 1/16 hole was drilled through the venturi sides and the head was drilled and tapped for 6/32 head bolts. Then, it took two hands to pull it through TDC.

     I took it on as a challenge. Stripped down, nothing was bent or worn, the shaft turned freely. The binding was between the piston and cylinder. It wasn't distorted, there was a perfectly shiny ring around the piston, but not where it was supposed to be, it was 1/3 down from the top of the piston. It would run on a prime but would stop bound up, so I decided to lap it.

     The only material I had that resembled lapping compound, besides toothpaste, was Brasso. Assuming a coarse grit I gave it a 10-15 sec lap on the drill press by chucking the shaft and more hand lapping by turning the prop for about 10-12 strokes. The venturi holes were plugged with toothpicks and cyano.

     Just pure luck, but I now have a stuffer/Fox 35 that starts on a couple of flips and the best part, will permanently hold compression. I'll run it a bit more and debut it at the Fly-A-Thon.

     But why bung up the hemi head for 6/32 head bolts, a poor man's helicoil job? Why the holes in the venturi walls, to effectively increase the diameter?

Ara

     

     

Offline George Fruhling

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Re: lapping a Fox 35 pist/cyl with Brasso
« Reply #1 on: September 26, 2022, 11:38:19 AM »
Holes in the venturi may be a misguided attempt at better fuel atomization.

Offline Steve_Pollock

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Re: lapping a Fox 35 pist/cyl with Brasso
« Reply #2 on: September 26, 2022, 01:58:14 PM »
The holes in the venturi could also be for a flood-off if the engine was used for FF.  At one time Fox .29s were used for class B FF, and a .35 for class C, allowing a flyer to use one plane for both classes.  Or they could be used to hold on a filter.

Offline kenneth cook

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Re: lapping a Fox 35 pist/cyl with Brasso
« Reply #3 on: September 26, 2022, 02:28:12 PM »
           That looks like a Tatone Peace Pipe muffler. The engine might not be happy with that muffler.

Offline Reptoid

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Re: lapping a Fox 35 pist/cyl with Brasso
« Reply #4 on: September 26, 2022, 03:31:33 PM »
    The correct way to "Lap" a piston is not be turning the assembled engine over. The piston and liner should be removed from the engine and the piston lapped by rotating it while moving it in and out of the cylinder (Like using a hone) Running it up and down without rotating it will create high and low spots and cause it to go out of round. It may work OK and loosen it up enough to be free but it's not the best (correct) way do do it.
Regards,
       Don
       AMA # 3882

Online Lauri Malila

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Re: lapping a Fox 35 pist/cyl with Brasso
« Reply #5 on: September 26, 2022, 11:27:56 PM »
    The correct way to "Lap" a piston is not be turning the assembled engine over. The piston and liner should be removed from the engine and the piston lapped by rotating it while moving it in and out of the cylinder (Like using a hone) Running it up and down without rotating it will create high and low spots and cause it to go out of round. It may work OK and loosen it up enough to be free but it's not the best (correct) way do do it.

The correct way to lap the piston is to lap it separately from cylinder, with a external lapping tool.
If cylinder needs attention, it should be honed with an internal honing tool. Both tools are easy to make with even simple machine tools if you know what you are doing.
If you try to "lap" by rubbing the piston & cylinder together with some abrasive, it's the cylinder that gets eaten by the abrasive, not the piston. And as an extra, you have a big possibility to contaminate the piston with embedded abrasive.
If the piston is just tight because of carbon buildup, often just a mild abrasive, like paint polishing compound (Farécla G4 or G10 are good) does the job without damaging metal surfaces. L

Offline Reptoid

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Re: lapping a Fox 35 pist/cyl with Brasso
« Reply #6 on: September 27, 2022, 02:32:51 PM »
The correct way to lap the piston is to lap it separately from cylinder, with a external lapping tool.
If cylinder needs attention, it should be honed with an internal honing tool. Both tools are easy to make with even simple machine tools if you know what you are doing.
If you try to "lap" by rubbing the piston & cylinder together with some abrasive, it's the cylinder that gets eaten by the abrasive, not the piston. And as an extra, you have a big possibility to contaminate the piston with embedded abrasive.
If the piston is just tight because of carbon buildup, often just a mild abrasive, like paint polishing compound (Farécla G4 or G10 are good) does the job without damaging metal surfaces. L
Technically correct but most people don't have a piston lap for a multitude of different size pistons, or access to machinery to make a lap. In addition, the average person would destroy the fit of a lapped pist/liner in about 10 seconds if they tried to "hone" the cylinder. In the case of a fox 35 stunt engine the liner and piston are both made of iron and unless you're extremely lucky, the piston has no taper and the liner little to none. My point was mainly that running it up and down by turning the engine over will likely produce an out of round result and not improve the lack of taper condition.
Regards,
       Don
       AMA # 3882

Offline Ara Dedekian

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Re: lapping a Fox 35 pist/cyl with Brasso
« Reply #7 on: October 03, 2022, 10:09:10 PM »

       I tore the engine down after more bench running and noticed a chamfer on the front end of the crankshaft and a squared off counterweight. Both are different when compared to a stock Fox crank, shown at the bottom. All dimensions on both are the same.

       Does anyone recognize the crank or have any info on it?

       Ara
« Last Edit: October 04, 2022, 04:44:48 AM by Ara Dedekian »

Offline Dan McEntee

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Re: lapping a Fox 35 pist/cyl with Brasso
« Reply #8 on: October 04, 2022, 05:05:38 PM »
     If you take 12 random old Fox.35s apart I think you will find 12 different crankshafts!! I think the one on the bottom in the photo is meant for a .29, mainly because I have had a few of those apart in recent history and they looked like that. Less counter weight due to smaller piston, I'm guessing.
  Type at you later,
  Dan McEntee
AMA 28784
EAA  1038824
AMA 480405 (American Motorcyclist Association)

Offline Ara Dedekian

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Re: lapping a Fox 35 pist/cyl with Brasso
« Reply #9 on: October 09, 2022, 12:48:05 PM »
     If you take 12 random old Fox.35s apart I think you will find 12 different crankshafts!!
     
     Dan

     I was holding out thinking someone would tell me I have one of those fancy Zoot cranks I've been hearing about.

     I went to my two new-in-the-box 40th Anniversary Foxes and pulled the rear covers, both cranks were the same as the stock crank at the bottom of the picture. I also have two Fox 35 reverse cranks whose balance webs show the same as the bottom picture, as did the crank on my Fox 29.

     So the lap job worked. Mystery crankshaft, Fox burp, wobbly Fox needle and iffy profile tank aside, the engine did OK in the air.

     Ara


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