Reviving a 1950s Fox Stunt .35

Many different opinions on this. I like a 10x6 for my Foxes. Even a light 11x4 works pretty good. I run 25% Castor. I believe Ken and Ron run a more casual mix with synthetics. Others will chime in with their opinions. I do not take my Foxes apart often. I do know that you should make a mark on the liner so you can line it up correctly.

I have two Foxes from this era. A standard Fox 35 Stunt which is covered in burnt castor. I also have a Fox 35 Two Speed. Both are swell runners.

RR

I thought I would add this. Hope Ken doesn't mind, I had told him I had a couple new Foxes in an email and this was his response:

If you have a new Fox.35, your in for a experience. Breaking in a Fox is not the easiest task. They can try your patience and I bypass a lot of the break in experience by lapping and polishing things prior to running. The 40th is a good engine, unfortunately the fits can be quite tight. The 50th however is a bit more friendlier due to looser fits. I've seen 50th anniversary engines fly after a few bench runs. I've had to run 1/2gallon through some of my 40th anniversary engines before they would go on the plane. They would be shaking all over the place. Ken

I also have an original early manual in PDF form. If you PM me your email I will send it to you as it has break in instructions.

Ron

22% oil (50/50 caster/syn) is great for a Thunder Tiger or a O.S. type engine, in fact its probably the best, but for a Fox .35 it's pretty light. I think most would agree at 25% all caster to 28-29% of some mix of caster/synthetic is best for cooling and lube.

http://www.powermasterfuels.com/index.php?option=com_content&view=article&id=6&Itemid=8

Thanks for your replies. Next time I post, tomorrow afternoon, I'll have more details about how the internals look. If it really does appear new after I clean it up, I'm in for a treat, albeit a tedious adventure.

I can pour off some of my Sig for it and add some extra goo and start breaking it in as if it's new. I'm not going to sweat the fact that I'm feeding it synthetic. Or, I do still have some 75/25 methanol/castor(zero nitro) I mixed for the MP Jet with drugstore castor that would probably work if y'all don't think the cheap gummy castor is a problem.

I have plenty of 9" props, up to 9x6, and I'll hit the LHS and get a 10 incher too. I'll follow the break-in procedure on silk-n-dope.com. Although it is for a different Fox, it sounds pretty safe. I'll probably be springing for more go juice before I finish.

Here's a chart for adding oil to Sig (20% oil) fuel:

Here is another opinion I can add. I have a lot of experience with the Fox be it racing, stunt and combat. TO achieve the power out of this engine, I spent many hours fiddling with them. I do run 22% oil in them and I run them hard. My fuel however is specific and although some fuels are designate a certain oil percentage, Powermaster GMA is what I use and nothing else. The older Fox Rusty that your describing in my opinion can be very good engines. However, they're fragile as egg shells. I wouldn't take the engine apart at all. Those head screws typically take me a 1/2 hr minimum to tighten correctly. The screws twist and distort the case which distorts the liner. They can try your patience and for the inexperienced, my suggestion is to run it. That particular engine WILL NOT tolerate a muffler. It's also loud as could be. Some older Fox's just have a loud cackling bark and the one without the casting web I found to be the loudest.

Even if it's sludged up to the point that it won't turn, just run it. Understand that getting the piston liner out of a Fox .35 isn't a easy task. Some will fall out in your hand while others make you wonder how in the heck they got it on in the first place. It's a little tricky as you need to rotate, twist and apply pressure without damaging the large end of the rod . What should be done with this engine is this, purchase a rebuild kit because as soon as you open the backplate the cork gasket is ruined. By now over the years, the gasket has probably already been squished and deteriorated. Check the rod that it moves freely on the wrist pin from front to back. It won't be free so turn the engine upside down and pour penetrating oil into the underside of the piston and let soak for a day. Return to see if it's moving. If not put a torch to it, heat it enough that the oil you had in there just begins to smoke and sneak a allen wrench behind the rod and pry it off the crank web. It will be quite sluggish as it moves. Keep pouring oil onto the rod and move it back and forth until it's completely free. If you choose not to do this, the wrist pin is turning with the rod and eventually will saw the hole of the piston into a egg shape. Your engine is probably so old that it doesn't have circlips but rather brass end pads on the wrist pin.


I wouldn't run this engine on a 50/50 blend of synthetic but rather full castor oil. I certainly wouldn't run 29% as 25% is more than adequate and will still give you substantial protection and power. A hot plug is in order K&B 1 L, Thunderbolt r/c long, Sig R/c Long etc. Keep a piece of fuel tubing over the stock Fox needle if your using one. For the Yak, my suggestion is to fly it fast. Prop choices in the 9x6 range, 9x7 depending on how the engine is turning up and not bogging in the turn. Master Airscrew makes the 9.5x 6 which seems to be my magic prop Fox. A 10x6 in my opinion doesn't do it . I look at it like this, big plane, big prop. Ken

I'm glad I read this before I went any further. So far my only offense is having removed the head. The screws were not particularly tight, just stiff and gummy, and the head was not at all stuck and came off easily. It has a relief with a bevel on the outside edge which looks a little nicked on the very top of the ridge, however there are no apparent scars on the top of the piston. Maybe the head was dropped onto a rough floor. The one head gasket is not round but looks serviceable.

I'll take a picture of it and post it later. About 8 hours ago I submerged the whole crankcase in a jar of denatured alcohol. I just ran out to the shop and dried it and wet it down with Liquid Wrench. I'll dry it off when I get back here early this afternoon.

I'm rushing off to the doc now, so I'll be back later.
Thanks,
Rusty

I took the heat gun and freed it up quite easily. The head gasket looks like it should seal just fine, and with the glowplug installed it has strong compression. I cannot tell if the wristpin is stuck to the con rod or not, but it's sitting upside down now with the piston full to the brim with Liquid Wrench. The rod doesn't want to come free of the crank pin and I haven't applied much force yet. If I can manage to get a good view of it rotating around on the wristpin, I'll leave it alone.

When I first started heating the crankcase with the head off and turned the prop, the liner rose up. I turned it back down and bolted the head on. Ken mentioned that the head is difficult to seat properly. I held it in place and rotated it back and forth and it felt fine so I carefully cross-tightened the bolts going around about five times to the final torque. Like I said, it seems to hold good compression.

I'm going out to play with it now. The cork backplate seal looks intact, so it the wristpin looks free, I'm going to give it a quick rip with the Sig 10% and a blop of castor for good luck. The glow plug looks good. I can't make out the markings but it lights up fine and looks like an OS except that it's anodized black.

Rusty, the proper way to check for a bind in the Fox is to have the piston sleeve completely free and dry of oil residue. Turning the engine over dry will reveal a bind if one is present. If present, this is where it can become challenging. The Fox can be a funny engine at times. If not broken in, or parts not fitting well can make fixing that bind problematic. This bind will show it's ugliness when the engine is running and it starts overheating going overlean and sounds like it's ready to seize. New Fox's will do this all the time when run. I used to run them all the time until this kind of wore itself in. This just resulted in a poor running engine. Running a tight engine will ruin the rod on both ends. I curtail this by doing my black magic voodoo on them and lapping and polishing many parts prior to running them. Older engines can benefit from certain fix's as well, but typically in my opinion most of the older ones I like to think were better quality. The Fox has a non bushed rod, I drill and provide oil holes on the rod ends as well. Does it make a difference? probably not, it's just what I do. Many events that I fly in don't allow the modifications that I do to them.

A rebuild kit includes all new socket head screws, a new gasket, and a head gasket. Although I've used the rear backplate gaskets over, I highly discourage reusing it. If you even have the slightest leak in the Fox backplate the engine will not run and it will quit at the drop of a pin. This is where I substitute the gasket for a terrific Permatex product which I suggested before which is Permatex Anaerobic gasket sealer. It's formulated to be used with alcohol and it's service removeable. In addition unlike certain silicone it dries in the absence of air. Certain silicone cures and forms acetic acid which is quite corrosive to certain metals. Rtv silicones don't form acetic acid but some of them are soluble in alcohol. Seeing that you have the engine apart to some degree, I would lap the back of the case on a piece of glass with sandpaper. They're are many casting flaws on a Fox case and this will clean it up quite nicely. I use some 400 grit paper and oil. Do not straighten the Fox backplate ears, they will break.

The stuffer backplate is far superior as it does many things, it holds the rod on properly on the crank pin and keeps the rod from working off, it's also a machined part vs a casted one. It's also hard anodized to provide a harder wearing surface. The stock backplate can get pretty chewed up especially if you hit the ground.

In addition to lapping the back of the case, I do this to the top of the piston liner as well as this can boost compression a bit and also make it flat. Once again, removing a liner from a Fox doesn't always result in it going back in the same identical position. This is why I suggest not to take them apart unless your familiar on how to do so. The Fox liner can go in backwards which many people do resulting in a engine that will run but horribly. Ken

Ken, I re-read your first post. I had missed the part about letting the piston sit full of penetrating oil overnight. I didn't give it enough time. I'll fill it back up with Liquid Wrench and let it sit until tomorrow. BTW, LW is superior to PB Blaster. I don't like the new stuff at all.

Is the rod supposed to slip off the crank pin just like a Cox? I guess the reason it won't come is because it's stuck at the other end. I already had decided to flatten the back of the case. I have a 3/8" glass pane I use for this.

Thanks for the tips. I will look into getting a rebuild kit.

Indeed the large end of the connecting rod slips off of the crank pin. The top typically gets glue to the wrist pin. Heat will get it moving and you probably won't have to wait overnight. I've had some though I had to let sit all week. Heat though makes it really move quickly. Once moving, I liberally pour oil it onto the wrist pin and keep working it back and forth.

In my opinion and I form a lot of them, the version your describing to me has always worked quite well . The downside is that they're loud. One question, is this a single hole spraybar or a newer two hole spraybar in the engine. Can you provide us a picture? . I'm looking forward to seeing the build. Ken

Two holes in the spraybar and an odd flat sided needle. I'll get a picture and post it tomorrow afternoon. I've closed up the shop for the night and if I walk back out there for the most miniscule reason I'll be in there till midnight. I think there's a chronosynclastic infundibulum in the doorway.

Oh, and i took another look at the glow plug. It has "OK" stamped on it. It's clean and glows brightly.

Rusty, you have indeed a Fox spraybar and needle. Although, it's not the original that came with that engine. The needle with the flat side does indeed look unusual. It works, not so well, but I prefer them over the newest tapered version. The needle valves snap off right where they enter the spraybar. I was telling Ron this. If you get stuck in a pinch, and you can find a 3-48 screw that's long enough, just grind a flat on the side or make a taper. It doesn't have to come to a point. Spraybar alignment is critical. When you look down the venturi, you shouldn't be able to see the holes. Try to rotate the spraybar so that if any of the holes are showing, your just seeing the tops of both of them. Another thing I like to do is deburr the holes in the spraybar with a #11 blade just chamfering the hole.

Fox needle valves aren't the best, they certainly take some fiddling. I find it funny at times as your screwing the needle in and it begins to richen. Air leaks are most problematic and need any treatment you can offer to them. But, they do work. The glow plug although it may be glowing is seriously old and should be destined for a backup. Why make more problems trying to use an old plug. Run it on the bench, for flying I would certainly change it.

The Fox's like to be quite wet when starting. They can give you quite a kick at times so backflipping the prop can also be of great assistance to a loaded engine. On the spraybar, where the fuel line hooks to is somewhat small. The fuel line likes to remove itself from there at times when you certainly would rather not have it do that. A small cable tie or wrapped copper wire can be most helpful and it can also seal any fuel and air leaks present. Ken

Looks like an old Fox.....

That has to be the original needle. Very possible it was crashed and replaced. Even if that was the case it should have been replaced with the same needle, unless it was a crossover year and he was able to buy a different needle for it. The owner may have been mistaken on the timeframe he bought it. Maybe Ken can tell the age of it from the picture.

Ron

cribbs74 wrote:Looks like an old Fox.....

That has to be the original needle. Very possible it was crashed and replaced. Even if that was the case it should have been replaced with the same needle, unless it was a crossover year and he was able to buy a different needle for it. The owner may have been mistaken on the timeframe he bought it. Maybe Ken can tell the age of it from the picture.

Ron

After I described it, Ken said it might have a newer generation needle. Seems odd to have that flat spot on the needle(not shown), doesn't it.

RknRusty wrote:

cribbs74 wrote:Looks like an old Fox.....

That has to be the original needle. Very possible it was crashed and replaced. Even if that was the case it should have been replaced with the same needle, unless it was a crossover year and he was able to buy a different needle for it. The owner may have been mistaken on the timeframe he bought it. Maybe Ken can tell the age of it from the picture.

Ron

After I described it, Ken said it might have a newer generation needle. Seems odd to have that flat spot on the needle(not shown), doesn't it.

I don't know enough about Fox engines to even wager a guess as to what needles go with what years. All I know is there is a taper needle and a spoon type needle. I have a taper on my '52 Fox. Well I did before I snapped it off.....

I am going to replace mine with different needles anyway. After you mess with that thing for a while you may want to as well.

Ron

I just bought a gasket and screw set for $3.89 on the Bay. The picture shows a head gasket but the description didn't. Same with both sellers of that item. If my head gasket doesn't seal I'll make one from a beer can. It looks about like that. It makes good compression while hand flipping though. It's still soaking to free the wrist pin. I only gave it a quick check today and didn't have time to work on it.

http://www.ebay.com/itm/400450219412?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649

Putting the last of linkages and leadouts on the Kitty Cat platter. I'm tired of messing with it now. The fun light has definitely flicked off. I made an awesome little Bee for it. It runs strong. It better, the damn thing is as heavy as my Baby Streak. I hope it flies.

I finally got the wrist pin free from the con rod.I went against Ken's advice and disassembled the engine. I just can't not know what's in there. I know which side of the liner has the exhaust slit, it's about 2mm wider than the intake port. Besides I marked it.

I got all the green crud out and cleaned and oiled the parts and reassembled with the new gaskets and screws. About 6 passes cross tightening and I think the head is on flush. The piston is pretty tight in the dry liner, so I'll see if, like Ken says, it's going to be a problem. There's an even drag all the way up and only gets harder to move near TDC. After oiling, it moves fine, again just tight at TDC. It has a helluva lot of compression just flipping it over, but it feels good.

Here's what the guts look like:




There is a chip out of the piston skirt. I have no idea if it was already like that or if i did it but it doesn't appear to affect the travel. There are some streaks on the side of the liner, probably from the wrist pin and maybe from the chip, but I can not feel any of them with my pinky. I have not done a leakdown test. What's the point, I'm going to run it anyway, and the compression is impressive when flipping the prop.

I haven't bought any fuel for it as funds are limited, but I'll add 5% more oil to a half pint of my Sig 15% just for a test run. That'll make it 75% castor/25% synthetic and a total of 25% oil. I'll buy some proper fuel when I get ready to use the engine. I still have the K&B plug in it. I have an OS#8 but I don't know if it fits. I'll get some better plugs before I put it into service too.

I was planning to crank it tonight, but now I think I'll just wait until tomorrow.

Rusty, as long as the chip isn't a raised burr, I wouldn't worry about it. The skirt has little affect on engine run assuming it's not the area that blocks off the bypass. If you have pinch near TDC also a good thing. All Fox's are going to leak down so trying to get an accurate interpretation from bubbles being present isn't going to help you much there unless it has no compression due to being clapped out. I'm sure it will run just fine. One other suggestion, clip that round knob off of the needle and bend that needle into a J shape. If you don't I can assure you the ground will do it for you. It's easier to needle with one finger opposed to your thumb and forefinger holding the knob placing the ball of your thumb into the back of the prop.Ken

The chip is 90 degrees away from the bypass port. I have a good feeling about this one, it looks good. I can see no wear on the crank or the bushings. I'll let y'all know how it runs tomorrow night.

My one real concern would be the circlips holding the wrist pin in. I try not to remove them unless ABSOLUTELY necessary as they will remove themselves. Of course this happens when it's running and with 100% certainty I can tell you the damage is irreversible. I try and keep the leg of the circlip vertical and not horizontal. I also check, recheck, and check again that they're engaged in the groove. Just some FYI, place the piston rod assembly inside a sandwich bag when installing them. Otherwise, they take flight and game is over. Seeing that you had the engine that far apart, I personally would've drilled oil holes in the rod ends. No worry, I just do it for my personal satisfaction and I really don't think it gives any real advantage. It's a Fox and parts were machined with axe's and hatchets. Aside from that I like them. Ken

There were no circlips. I disassembled it in a pan with 3" sides and magnets under the clean towel lining it. I pulled the piston out and the pin fell out. Nothing to retain it. I figured the hollow brass tips were to protect the liner from inevitable contact.