Author Topic: Fox 35 with a pinch at the top of the stroke? (Read 2633 times)

frank mccune · « **on:** July 28, 2013, 07:27:30 PM »

Hi All:

I recently purchased a 65 year old Fox Stunt .35 that has a firm pinch at the top of the stroke. Just like those new fangled engines.

I pulled the head and backplate to see if anything was stuck or binding. I could not find anything wrong with the engine. It has a ton of compression and a great p&c fit! It will hold compression until one gets tired of holding it on compression. The cylinder was smooth as glass. There was no evidence of plier marks on the cylinder flange.

The combustion chamber was very carbon coated, a lot of castor based fuel must have gone through this engine! Perhaps it was another type of oil. I have NO thoughts of cleaning this engine or disturbing the p&c fit!

Now back to the original question: What is causing the pinch at the top? The only thing that comes to my mind is perhaps the cylinder was chromed and tapered to have the pinch at the top. This would explain the smooth cylinder walls and the vast amount of carbon build up in the engine. I have no idea as to how long this engine would run if it were chromed. The rod was not worn on the crankpin.

Any ideas?

Tia,

Frank

dennis lipsett · « **Reply #1 on:** July 28, 2013, 08:01:35 PM »

It could be simply varnished up and needs to be deglazed.

Dennis

frank mccune · « **Reply #2 on:** July 28, 2013, 08:33:26 PM »

Hi Dennis:

Thanks for the suggestion.

How does one deglaze a cylinder so small?

I am very nervous about working on the p&c as the fit seems to be great! Perhaps a tank of all synthetic fuel is in order. I have a quart of Sig's Lubricin that I think is a detergent that is supposed to remove crud from the engine.

Perhaps I should run the engine on a test stand and see how it responds after it is at operating temps. From what I see, all looks great!

In any case, I am going to take it slowly to avoid making a poor choice. I do have a new wrist pin, p&c, con rod and case for this engine in case things go bad. Lol

Stay well,

Frank

Andrew Tinsley · « **Reply #3 on:** July 29, 2013, 02:11:09 AM »

Frank,
I suspect that your good fits are due to varnish on the cylinder and carbon on the piston. If you start running it on synthetic fuel, you are likely to lose much of this varnish and maybe some carbon. There is a very good chance you will finish up with a clapped out engine. If as you suggest, it has a very large amount of fuel through it, then it sounds highly unlikely to have "super fits" and compression,
My advice would be to leave the engine alone and run it on all castor fuel. If you don't or you devarnish it, then expect to have a worn out engine at worst.
Most times you have binds in a Fox 35 it is due to incorrectly torqued screws. I am not a Fox expert, having only an unrun 50th anniversary model and a few slant plug 15s. Just looking at the Fox 35 it is VERY lightly built and I would expect it to distort if screws were over tightened. Wait for a Fox user of repute (Where are you Marvin Denny?). They can tell you what torque figures to use on the Fox bolts.

Regards,

Andrew Tinsley.

frank mccune · « **Reply #4 on:** July 29, 2013, 05:49:31 AM »

Hello Andrew:

Thanks for the reply.

The pinch can be felt when the cylinder head is removed.

I will contact Marvin for his advice before I run this engine.

Stay well,

Frank

Dennis Toth · « **Reply #5 on:** July 29, 2013, 09:03:23 AM »

Frank,
If you remove the plug and turn the engine over is it just a pinch near the top to the stroke? If yes it is most likely varnish build up as others have stated. To remove the varnish you can pull the head (which I think you have done) and take a small strip of Green Scotch crub pad and using you finger rub it around and up and down with a twisting motion in the upper half of the cylinder. Go slow, you will get a feel for it, after a few strokes clean it out with a paper towel and some solvent (fuel, thinner will work) and see if it still has a strong pinch. You want to leave it with just a touch of pinch, then run some 50/50 25% oil, 5% fuel through it rich for a couple tanks. That will clean it out and you should be good to go. The Fox likes high castor fuels I believe RMS has some 5% high castor fuel for the Fox that has 3% syn and 25% castor that will work fine.

When you put the head back on the crankcase use the Fox Philips head screws, the head just fits the relief in the head, you can try the socket heads but my experience has been that they bind against the head and cause binding. Use a cross triangle tightening start at either the front or back and do the triangle first then go to the second triangle, this pulls the head down even. Bring the screw to finger tight then go 1/8 turn on each as you pull it down. After each full round of tightening check that it still turns over smooth will the same amount of pinch at the top. If it binds hard, loosen the bolts, remove the head and run a # 30 drill through all the bolt holes, clean off the aluminum chips and start the tightening again. Some of the heads have the holes just a touch off and as they tighten it binds and distorts the head, drilling them out give it the extra room and usually cures the bind. Also check for smoothness as you put tighten the back plate, if it binds drill them out with a #29. Last thing is to use either straight maple motor mounts or if you use aluminum pads on you mounts put 1/64" curse pads under the mount lugs, again as you tighten down and as you put the muffler on check for binding with the plug out. The mount holes need to be free and not forced to fit as they will also twist the case and cause a bind.

Best, DennisT

RandySmith · « **Reply #6 on:** July 29, 2013, 09:04:46 AM »

Quote from: frank mccune on July 28, 2013, 07:27:30 PM

Hi All:

I recently purchased a 65 year old Fox Stunt .35 that has a firm pinch at the top of the stroke. Just like those new fangled engines.

I pulled the head and backplate to see if anything was stuck or binding. I could not find anything wrong with the engine. It has a ton of compression and a great p&c fit! It will hold compression until one gets tired of holding it on compression. The cylinder was smooth as glass. There was no evidence of plier marks on the cylinder flange.

The combustion chamber was very carbon coated, a lot of castor based fuel must have gone through this engine! Perhaps it was another type of oil. I have NO thoughts of cleaning this engine or disturbing the p&c fit!

Now back to the original question: What is causing the pinch at the top? The only thing that comes to my mind is perhaps the cylinder was chromed and tapered to have the pinch at the top. This would explain the smooth cylinder walls and the vast amount of carbon build up in the engine. I have no idea as to how long this engine would run if it were chromed. The rod was not worn on the crankpin.

Any ideas?

Tia,

Frank

Hi Frank

No one can tell you exactly what you have unless they can see it, it could be a retro mod P/S, or just a good fitting P/S, or one sealed with castor from use. It maybe would be best to just use the 35 and enjoy it.
You may want to read my Care and Feeding article pinned at the top of this forum, and a word about Lubricin, do not use it, it WILL fleck castor oil very badly and can end up clogging the spraybar filter and causing all kinds of grief in you castor fuels. Use it in synthetic oil fuels or let it stay on the shelf.
You could clean the head, but leave the rest of the FOX as is.

Randy

frank mccune · « **Reply #7 on:** July 29, 2013, 09:27:29 AM »

Hi Dennis and Randy:

Thank you so much for your replies!

I "made' new screws for the head as the orginal were badly damaged. They are 3/8"X 4x40 RH.

This engine has a 4 bolt head and a two screw backplate. The backplate screws look like they are 2-56 as they are quite small.

Oh yes, I can not remember why I bough the Lubricin but I never used it on advice from my friend who is a chemist.

I am old school and feed my Fox engines only 28% castor fuel. Perhaps I am living in the past! Lol As amatter of fact. I do not run any modern engines. All of mine are cast iron and steel! I may use a ST that is ABC but I think the Fox blend would be fine in that engine also.

Stay well my friends,

Frank

Jim Kraft · « **Reply #8 on:** July 29, 2013, 11:39:48 AM »

Frank, I'm with you. When I got back into control line 20 years ago I had an old Fox 35 from the 60's that I put on a Ringmaster and had a ball. Then I built a Banshee and got a few OS 40 FP stunt engines for it. Those ran OK after doing some work to get them there but just did not do anything for me. I just love the way old iron runs and sounds.

I have also found that Fox 35's have subtle differences. My friend Andrew Hathaway has one that he got off of ebay that was well broken in, and he put it on a Ringmaster. It has a 2-4 that rivals my McCoy 40 Red Heads and runs as good as any 2-4 engine I have ever seen. It is completely stock with nothing in the bypass, and does not burp. I think it is a late 60's or early 70's motor. I bought 2 of the later ones a few years ago with the full muffler ears. One had super compression with pinch at the top, and the other was nice but no pinch. I broke the one in with the pinch and flew it in a Nobler for 3 years and after several gallons of fuel, it still has pinch at the top. I do have a stuffer back plate and one of Marvin Denny's hemi heads on that one, and it is the most powerful Fox I have owned.

Dennis Toth · « **Reply #9 on:** July 30, 2013, 04:04:27 PM »

Frank,
You have one of the first Fox 35's produced by Duke Fox (there is a thread on Fox production motors over time on here that shows pictures and dates of the various different runs). The 4 bolt head, 2 bolt backplate is 1947 -48 vintage with the sand cast case. The chances of it being anything but iron/steel are - no.

So that brings us back to the pinch and the cause - varnish. If when you put the plug in and a few drops of fuel and it flips with a good pop, just run a half a dozen or so, 4oz tanks of 50/50 (read that as Sig) 5% through it rich and it will likely clean up then switch to normal castor fuel and go fly. BTW this motor works great in a Barnstomer for OTS.

Best, DennisT

frank mccune · « **Reply #10 on:** July 30, 2013, 05:28:20 PM »

Hi Dennis et.al.:

I have decided to run the engine on a test stand and see what the p&c fit is like when it is hot.

Is the pinch a bad thing?

I am afraid to remove any castor build up as it may give me an engine tht is very badly worn and not useable

Stay well,

Frank

Dennis Toth · « **Reply #11 on:** July 30, 2013, 06:02:53 PM »

Frank,
I feel that if when you go to turn over the engine with the plug out it feels like it is sticking at the top you might consider a few tanks of fuel with 50/50 oil just to loosen it up a bit. The pitch is not normally good for an iron piston setup as it puts extra load on the connecting rod. In an ABC set up once the motor is up to temperature the pinch is suppose to be minimal. A little pinch when cold us ok just not a lot.

I like your idea to test it hot, if it is not there hot don't worry about it.

Best, DennisT

Brett Buck · « **Reply #12 on:** July 31, 2013, 06:50:32 PM »

Quote from: RandySmith on July 29, 2013, 09:04:46 AM

No one can tell you exactly what you have unless they can see it, it could be a retro mod P/S, or just a good fitting P/S, or one sealed with castor from use. It maybe would be best to just use the 35 and enjoy it

Listen to Randy, just see if it will run as-is (on regular Fox fuel). If it doesn't work, THEN start trying to fix it. In particular, like the pinned fuel article says, you can lose the magic very easily if you starting being too clever by cleaning it up.

Brett

dennis lipsett · « **Reply #13 on:** July 31, 2013, 07:44:18 PM »

I have always maintained that if you get rid of the castor build up and the compression disappears then the P/C was already wore out and go get another one and alleviate your problems. If you run a few tanks of 50/50 as recommended it will be beneficial to the engine. I have seen Foxes so varnished up that you could hardly turn them over and you had to whack the prop to get them running. Try to tell some of those guys to deglaze the engine is like asking them to walk on water. I really do think that some Fox lovers have a masochistic streak in them.
The advice that has been given should be seriously considered about using a different fuel for a few runs.

Dennis

Brett Buck · « **Reply #14 on:** July 31, 2013, 08:05:46 PM »

Quote from: dennis lipsett on July 31, 2013, 07:44:18 PM

I have always maintained that if you get rid of the castor build up and the compression disappears then the P/C was already wore out and go get another one and alleviate your problems.

Oh, don't misunderstand, I wouldn't be a bit surprised if it's worn out but still OK only because of the varnish, so you are probably exactly right. Randy's fuel article mentions the same thing. But, it might be OK for a while if you leave it alone.

Brett

RknRusty · « **Reply #15 on:** August 01, 2013, 03:01:51 AM »

Quote from: Dennis Toth on July 29, 2013, 09:03:23 AM

...if you use aluminum pads on you mounts put 1/64" curse pads under the mount lugs, again as you tighten down and as you put the muffler on check for binding with the plug out. The mount holes need to be free and not forced to fit as they will also twist the case and cause a bind.

Best, DennisT

I'm not familiar with the term, "curse pads." What material would they be made of?
Rusty

Dennis Toth · « **Reply #16 on:** August 01, 2013, 07:39:05 AM »

Typo - should be "crush" pads. I use 1/64 plywood. The Fox 35 has a very slight relief angle to the bottom of the mount lug. When we only used maple mounts it would simply settle into the wood as the motor ran a couple runs.

If you use aluminum or composite pads on your mounts, that many fliers do to prevent the motor from changing position, you need something to let the motor lug push down to level as you tighten it to the mount. If it you pull it down on hard mounts it can distort the case and cause the motor to bind and give very bad runs.

Best, DennisT

RknRusty · « **Reply #17 on:** August 01, 2013, 09:38:23 AM »

Quote from: Dennis Toth on August 01, 2013, 07:39:05 AM

Typo - should be "crush" pads....

Oh... Doh! I get it now. Thanks, Dennis.

Jim Thomerson · « **Reply #18 on:** August 03, 2013, 10:39:11 AM »

A machinist friend leveled the bottoms of the mounting lugs on my Fox 35s. I think that is one mod they should all have. I have used the crush plates as mentioned. As I have said, the last several hundred runs on one of my 1976 Fox stunt 35s have been with Sig Champion 10% nitro 20% oil, half and half. It has excellent compression and starts and runs reliably. The whole point of the castor varnish is that there is not metal to metal contact and thus no wear. On the other hand, if you let your Cox 049 varnish up, it will not run well. I got hold of some bad diesel fuel and broke three engines from them varnishing up while running.

RknRusty · « **Reply #19 on:** August 04, 2013, 07:54:47 AM »

Quote from: Jim Thomerson on August 03, 2013, 10:39:11 AM

...The whole point of the castor varnish is that there is not metal to metal contact and thus no wear. On the other hand, if you let your Cox 049 varnish up, it will not run well...

I had been running my Tee Dee .051 on Glowplugboy 24/17 straight castor, spiked with enough castor to bring it up to 20%. After Mike Mcgraw so sadly died and his fuel was no longer available, I switched to Sig with 50/50 castor/synthetic. The TD has had about 10 tankfulls on that, and now the compression seems low. I'm even using just two head shims with those high compression clamps and drop-in plugs Bernie sells at Cox International. I wonder if the synthetic in the Sig washed all of my beneficial varnish out. My other Tee Dee .049 has a brand new #4 cylinder so It'll probably be just fine on the Sig.

On a side note, in mid-June, a guy named Steve posted on coxengineforum, saying he was a friend of Mike and had bought the GPB equipment, recipe and website, and was going to start it back up. But so far Mike's old website is still offline. It was good stuff for the Coxes, I miss it.
Rusty