Author Topic: Fox 35 50th Aniv. motor (Read 8064 times)

Carl Cisneros · « **on:** December 22, 2014, 08:00:48 AM »

Folks;

I was given a Fox 35 50th Aniv. motor by one of the club members the other day.
It has very low time on it.

What needs to be done to it, if anything to make it a nice reliable motor.
This is my first Fox 35 stunt motor I have ever owned.
It will more than likely be going in to a profile type model.

Thanks much

John Paris · « **Reply #1 on:** December 22, 2014, 01:53:38 PM »

Carl,
If you are going to mount it on a profile then some people suggest stuffing the bypass with a piece of basswood or filling with JB Weld to keep the air/fuel mixture velocity high to help reduce the "burp" when you pull an outside manever. If it is upright or inverted mount then this is not necessary. I prefer 10 percent nitro and 25 percent all castor fuel. If the engine is relatively new the castor synthetic blends can be used without too much problem although I would add a little extra castor to be safe. I have found the Tower Hobbies plug to work very well for me but would suspect that any good plug would work well. I also like the APC 10x5 or 10x6 for this engine depending on the airplane. They do tend to vibrate a little bit (Randy Smith's High Zoot crank helps) but is manageable. Not a real high performance engine but a good solid performer when everything is matched up right.
John

Steve Helmick · « **Reply #2 on:** December 22, 2014, 02:03:18 PM »

I'd put on the optional Fox hard anodized barstock backplate and do "the popsicle stick in the bypass trick" to get rid of the infamous burp. I'd also put in a Randy Smith Fox .35 NV Assy., and would at least start with an RSM 10-6 wood prop. Wood props are easier on the crankshaft. If it's already broken in, you'd be good to go...if you have fuel with enough good oil. Maybe Randy would get another run of High Zoot cranks made, but I believe he said they were all gone. Stock Fox cranks break, but you should be able to get them from Fox. Plastic props are harder on the cranks, so wood is the way to go.

Steve

Dennis Toth · « **Reply #3 on:** December 22, 2014, 03:00:31 PM »

Carl,
Since you indicated you were planning to use it on a profile you should do a search on this forum for the "Fox Burp". Bret Buck has posted the path to the article that details the stick in the bypass that eliminates the "Burp" while allow you to run the engine in a solid 4-2-4 run. There are some who run the engine lean to eliminate the burp but that stress the engine and you have less room for the classic stunt run. I did one with JB Weld (cause I ground out the lip in the bottom of the case) and may have gone to far. I would do the "stick" as it is well proven.

I run a muffler and although I am still working on getting the motor where I want it, it seems that a smaller prop (~9x6 old Top Flite style) a hot plug that doesn't drop rpm once the battery is removed and fuel with about 27% total oil is a good starting point. One other thing is to mount it on either wood maple mounts or if you use hard pads put a 1/64" plywood pad under each mount lug to give some "crush" cushion to allow for the small relief angle of the bottom of the mount lugs.

With the Fox 35 you need to understand it's limitations. Light smallish ship 34 -36 oz with 48" or less wing span. They will pull a larger ship (back in the day they did 56" spans 52oz) but you will be pushing the motor hard, specially with a muffler. For the bigger ships there are better suited power units available.

Basically, you run a Fox because you like the sound of the lower rpm 4-2-4 run, the look in an OTS ship and just curiousity (some might say morbid curiosity). If you stay with in its limits and work with it, it's a fine motor.

Best, DennisT

Andrew Hathaway · « **Reply #4 on:** December 22, 2014, 04:24:45 PM »

If you have to ask, it's probably best just to skip the Fox learning experience and get something simpler like a modern OS.

I wouldn't really recommend the Fox, because unless you have a nostalgic attraction to it, you're probably not going to like it's personality. Also a low time, or new Fox 35 takes a long time to really run-in and start developing nice manners (for a Fox anyway). If you don't fly much, you could spend a miserable season flying-in a tight engine. Now assuming you've read my suggestion and still want to do it, I'd leave the engine alone as much as possible. You don't need fancy reworking, or custom parts to have a Fox run OK, and all the parts and skilled labor isn't going to make the engine that much better, it'll still just be a Fox.

My usual approach to a used Fox 35, is to start by freeing up the stuck piston from the liner by heating the case with a heat gun until the castor melts and flows. Next I pull the backplate which generally destroys the cork gasket, and hook behind the connecting rod with an Allen key or other tool with a 90* bend, and carefully pull the rod while watching the wrist pin. It should move freely, if it doesn't it's likely the castor oil has frozen the rod to the pin. If the engine is run with the pin frozen, it'll run like poop and probably damage the engine. If it's sticky, heat it up with the heatgun until it can be moved. If it's really bad, or if it's been run while stuck, I'd probably pull the head and piston/liner assembly and check to be sure the wrist pin retainer clips are fully seated in their grooves in the piston. If the stuck pin has rubbed against the keepers, either by running the engine or by tugging the rod trying to free it up, the keepers can pop out and they'll eat the liner when the engine is run. If the rod isn't stuck, or frees up easily without much trouble, I make a new backplate gasket and reassemble the engine. Lube the engine with after run oil, add a prop and flip it through a few times. It should be smooth, free of drag, with good compression, if any drag aside from compression is felt, a bit more oil or heat may help. If it flips smoothly, freely, with good compression, I move on to the needle valve assembly. Assuming it's got the old conventional style NVA with the fuel inlet on one side of the case and the needle on the other, I make sure the spray bar holes are at the front-back and not top-bottom, and that the needle is the full taper version, not the older style with a flat on one side. It's usually a good idea to put a piece of fuel tubing on the fuel inlet of the spray bar, and blow through it while screwing in the needle, make sure the needle can completely close off the airflow. If it's going on a profile where the needle will be sticking straight up, I cut off the top knob and grab the top 3/8" of the shaft and bend it over 90*. That will keep the needle out of harms way, where it shouldn't be damaged except in a bad crash, also it's easier to adjust. Finally I cut a piece of fuel tubing about 3/8" long, and slide it over the needle's threads, and screw the needle in so the tubing compresses between the lower knob and the spray bar.

From there it's just a matter of running the engine, a lot. If you can't handle the shaking, noise, long break in, and special fuel, you probably still won't be happy with it. But fortunately there are always OS engines.

Chris Wilson · « **Reply #5 on:** December 22, 2014, 05:44:57 PM »

Quote from: Carl Cisneros on December 22, 2014, 08:00:48 AM

What needs to be done to it, if anything to make it a nice reliable motor.

I think that this post has yet to be answered, what makes the Fox 35 'reliable?'
(Not what improves the run.)

At least 25% castor oil, a bit of nitro and good cooling.

P.S. In hindsight post 2 was pretty spot on.

Steve Helmick · « **Reply #6 on:** December 22, 2014, 08:06:46 PM »

Quote from: Chris Wilson on December 22, 2014, 05:44:57 PM

I think that this post has yet to be answered, what makes the Fox 35 'reliable?'
(Not what improves the run.)

At least 25% castor oil, a bit of nitro and good cooling.

P.S. In hindsight post 2 was pretty spot on.

Adding a good NV assy. will improve consistency. Adding the optional backplate will add to conrod lifespan. Using wood props will improve crankshaft durability to the point that it may last the life of the engine. Aren't those part of "reliable"?

Steve

Chris Wilson · « **Reply #7 on:** December 22, 2014, 11:45:04 PM »

But does it 'need' to be done Steve?

I have seen too many stock standard Fox 35's that have needed nothing other than a high castor content, good cooling and a touch of nitro.

Wooden props, I have a nice Brian Eather carbon 10X 5.5 prop and you are proposing that I use instead timber because it's more reliable?

It's just that I see the words Fox 35 followed by a trailer load of 'must have' modifications and most posts tend to focus on better performance not what give the venerable old beast reliability.

Cheers.

Dennis Toth · « **Reply #8 on:** December 23, 2014, 07:09:12 AM »

Chris,
I agree that there are lots of "stock" Fox 35 chugging around out there that don't know they aren't suppost to be able to run. I would say the only strongly recommended mod if it is going to be mounted on a profile is the "stick in the bypass", granted there are lots flying without this that again don't know they can't run on the profile. The "stick mod" is so easy why not?

For me I like the stuffer backplate because I have them (oddly enough the old 4 bolt Fox had the backplate that was a stuffer, seems they changed to the current one to allow faster assembly by being able to put it on without looking for the notch to point up). I also like to lap mine in. I have done this for all my engines. For the Fox I have used simple white DuPont rubbing compound just work slowly (takes about 30 mins once you get a feel for it). You do have to pull the sleeve and piston. I put a small dab of compound around the inside of the sleeve then put a wood dowel through the bottom of the con rod to use as a handle. Slowly work the piston up and down into the cylinder in a twisting motion. As it gets fit the point where the piston gets difficult to move up will slowly go further up into the top of the sleeve. As it gets to where the piston is about 85% up, remove the piston clean off the compound and test the fit. I fit it so the piston stops at about 95% of full stroke, leaving the last bit to run in. Once there clean off all the parts in hot soapy water, dry, oil and assemble. Doing this allow me to then run a tank on the ground rich then fly a couple tanks rich and its good to go.

Best, DennisT

Steve Helmick · « **Reply #9 on:** December 23, 2014, 02:37:15 PM »

The "stuffer" backplate fits closer to the rod and is hard anodized. The entire point of it is to keep the rod on the crankpin and stop fore/aft slop, which adds to wear. Stopping the wear of rod against backplate will also increase glowplug as well as P/L life, because neither particularly like aluminum shards.

Hard anodizing is really hard chit...ruby hard, not diamond hard, but ruby is pretty hard as gemstones go. This was a modification suggested to the Fox factory by our own "Big Iron"....Marvin Denny...a highly respected Fox specialist. Whether one wants to do any of this or not is, of course, up to the individual. It sounded like the OP was looking for suggestions, so why not give all the options?

Steve

Chris Wilson · « **Reply #10 on:** December 23, 2014, 04:29:42 PM »

Ok, I agree that a tighter tolerance between the back plate and the crank pin would be a good thing but how do you fully realise this when stock engines have various distances to start with?

You would have true up the crank case casting, know what clearances are valid and adjust accordingly.
This is something most engine tuners would reflexively do anyway.

Dennis Toth · « **Reply #11 on:** December 23, 2014, 05:38:55 PM »

Chris,
The backplate distance is pretty simple. You pull the head, piston & sleeve bolt a prop on the shaft and put the backplate on. Then you can look down the case, push the crankshaft back against the case and see what the clearance is. If you wanted to get it really close you could vary the gasket thickness. I don't think the clearance is that critical, the stuffer is just a little closer than the stock backplate and keeps the conrod from excess movement. You can probably get the stock unit closer by using a thin gasket, might not be as good as the stuffer but could help some for a stunt run.

Best, DennisT

Chris Wilson · « **Reply #12 on:** December 23, 2014, 06:11:52 PM »

The point that I am trying to make is that even with stuffer back plates installed you could end up with a GREATER clearance than on some stock engines that have no stuffer installed - so the engine as supplied can vary enough to negate any perceived benefits.

You will need to know the new and correct running clearance in order to go beyond just 'make it closer.'

Bolting a precision made part onto a not so precise, non critical machined casting may not give you the results sort after.

As you say, best to have all the options explored and you could even get the same result by using the standard back plate and a hardened steel shim to pack the crankcase like some of the PAW diesel combat fliers do. But you really should know the running clearance before hand.

Does anyone here know what the recommended clearances are supposed to be with a Fox 35 backplate?

I am guessing that 5 to 10 thou is a good aim point.

Steve Helmick · « **Reply #13 on:** December 23, 2014, 08:26:36 PM »

The point I'm trying to make is that the OP asked what one could do to make a Fox .35 Stunt reliable and a good runner (and oh, he's putting it on a profile), and it only makes sense to give him all the options. I don't really care if he does any of them, but I'm pretty sure that between Marvin and Fox Mfg. and the potential buyers of optional deluxe goodies, there should be enough common sense to get the thing working. But why not give the options of what is available and what others do? Not having to replace conrods very often should be a good thing, no? It's not like anybody makes aftermarket Fox .35Stunt conrods...yet. How long will they be available from Fox? You just never know, so seems like avoiding the need is better to me.

Steve

Mike Keville · « **Reply #14 on:** December 23, 2014, 09:05:40 PM »

You guys are a riot! I flew Fox .35s and .29s from 1952 to 1990 (?) with NO modifications (other than the addition of a ST spray bar/needle valve), and they performed very well....some on profiles, others on built-up fuselages.

Never used a "popsicle stick" insert or a "stuffer" back plate. The only thing the Fox .35 (or .29) required was a prolonged and careful break-in with plenty of castor.

The Fox often gets a bum rap from those who didn't grow up with it.

Fact is, it was a great little engine. Just look at the Nats results, 1950-1965.

Lyle Spiegel · « **Reply #15 on:** December 24, 2014, 10:55:15 AM »

I agree with Mike. My Fox 35's can run as well as my B40 or OS46LA's as setup by Byron B. My Fox's use ST NVA, hemi head, stuffer backplate. One has ceramic P&L, other factory original. Muffler is Aluminum tube from Scott Dinger. Fuel is 10% with 29% all castor. Plug is either Sonitronics Glowdevil R/C or Thunderbolt R/C from Hobbypeople. Grey THUNDERTIGRE 11-5 prop. Engines start easily, run like sewing machine with reliable 2- 4 all day. One in my Smoothie, one in Nobler just like it is supposed to be. Will build up another when I can locate new 50th Anniv case .

Andrew Hathaway · « **Reply #16 on:** December 24, 2014, 12:18:34 PM »

When I asked Marvin Denny why he developed the stuffer backplate, he told me it was to prolong the lifespan of his Fox 35s on Goldberg racers. Not that the parts were used in competition, but during practicing the engines would be worn out. Now on a stunt or sport plane, it certainly wouldn't hurt but two things come to mind... First is that we're not running the engines nearly as hard or long in a stunt or sport plane. Second is that most of us are never going to put as many runs on a Fox 35 as Marvin has. It can't hurt but it's definitely not something that "needs to be done... to make it a nice reliable motor".

I've never worn out a Fox 35 stunt rod, for that matter I've never seen a worn out rod in any of the used Fox 35s I've obtained over the years. I did rebuild a well used Fox 35 this fall, which exhibited quite a bit of slop between the rod and crank pin. I had at least 4 more rods, including some that were new in package and used parts that had been on piston/liner assemblies that were worn out. None of them fit any better on the crank pin than the sloppy rod I wanted to change. I grabbed a new in package Fox 35 stunt crank, and again tried the pile of new and used rods, the slop was the same as the engine with several hundred runs.

The point is, if the Fox is properly broke in, sealed, and adjusted right, it'll work right out of the box and be reliable for a long time. It's just not a modern engine, and no amount of tinkering or expensive aftermarket parts is going to make it run like a modern engine. Nothing "needs to be done" if you have realistic expectations.

Bill Little · « **Reply #17 on:** December 24, 2014, 01:01:38 PM »

My first "big" engine was a McCoy .19 followed next by a Fox .35 Stunt (just like the "big boys" were using) in 1963. I still have that engine and it still runs like it is supposed to. The first model it was on was a Green Box Nobler, of course! LOL!! Then a Midwest/Hemstraught PT-19 and then a Veco Mustang. It is now back in its original box with the other engines.

Fox .35 Stunt engines might not have been built with the greatest of QC for a while, never have been the strongest engines, but if run as suggested, they are very reliable. A 4-2 every time.

Now we have found that a "better" (in many minds) stunt run is a high rpm/low pitch prop set up. Not something the Fox was designed for. Never the less, it is ice (in my mind) to hear a Fox 4-2 run in an OTS or Classic model is really sweet when set right. And it can still get the job done if it is matched to the proper air frame.

BIG Bear
RNMM/AMM

RknRusty · « **Reply #18 on:** December 24, 2014, 04:18:25 PM »

Carl, I say do a few break-in runs, bolt it to a cool sport plane and fly the damn thing. If after all the tank position adjustments and prop trials, it stumbles turning inverted too bad for safety, the jam a stick in it. I was taught to use a Fox by guys in the Philly Fliers and everybody else hates the way they do it, but even down in SC it works great on my only, so far, Fox 35. Mine works best with either a 9x6 APC or a 10x5. Just to irritate everyone, I use Powermaster 10/22 fuel. If I cook it, I'll get another one off e-bay, so I'll understand if you err to caution and use some 25% oil in your anniversary edition.
Rusty

Carl Cisneros · « **Reply #19 on:** December 25, 2014, 07:06:34 PM »

Thanks ever so much guys.

Have a very Merry Christmas.

more than likely will just run it until the cows come home.

BUT, will keep the plugged bypass in mind if it does indeed decide to show its' ugly head.

Chris Wilson · « **Reply #20 on:** December 26, 2014, 01:20:51 AM »

http://www.clamf.aerosports.net.au/newsletters/ACLN/Issue97.pdf

Good Fox article here.

Dennis Toth · « **Reply #21 on:** December 26, 2014, 07:09:46 AM »

Chris,
Thanks for the link to the Fox article. Interesting information, what I found that peaked my interests was that he did the tests with a muffler. It seems that for a typical Fox size ship (for me ~38 - 48" span, 30 - 36 oz) running at ~9300 rpm neither the nitro level nor the spraybar diameter had significant impact. Also the fact that he got what he needed with the muffler was surprising. He seemed to be running it up close to the 9800 ish rpm range. If he does a follow on article with the larger tube type expansion muffler please post the link.

Thanks, DennisT

Chris Wilson · « **Reply #22 on:** December 26, 2014, 03:03:14 PM »

Thanks Dennis, what piqued my interest is was that it gave voice (again) to reason why rod wear was prevented by a stuffer back plate but cured by a stronger crankshaft.

That and the piston holes ( like those found in an Enya 45) balanced the piston better.

RknRusty · « **Reply #23 on:** December 26, 2014, 10:24:53 PM »

Yes, Chris, +1 on the thanks. I didn't fully understand what a stuffer backplate was for. Now I think I'll get one for mine.
Rusty

Carl Cisneros · « **Reply #24 on:** December 26, 2014, 11:07:44 PM »

where does one get a stuffer back plate assembly these days?

Thanks

Dennis Toth · « **Reply #25 on:** December 27, 2014, 06:46:19 AM »

You can order the stuffer from Fox on their web site (http://www.foxmanufacturing.com/), select Fox Engine Parts in the left column, scroll down it is listed as the "35 Hemi Rear Cover". They have the head also and the complete package.

Best, DennisT

Carl Cisneros · « **Reply #26 on:** December 27, 2014, 02:08:52 PM »

I thought they were no longer doing motors and such any more,,,,,,,,,,,,,,,,,

Dennis Toth · « **Reply #27 on:** December 28, 2014, 06:17:03 AM »

Fox is still in business, they have parts and complete engines (no ceramic 35's) but have other higher priority machine shop business so new engines are not being produced at this time. Once shop time clears another batch will be run.

Best, DennisT

Carl Cisneros · « **Reply #28 on:** December 28, 2014, 02:21:05 PM »

Dennis

I will give them a shout tomorrow and se if they have a stuffer backplate and gasket in stock.

Carl

Dennis Moritz · « **Reply #29 on:** January 02, 2015, 03:32:29 AM »

Philly Flyers run lots of stock Fox 35s. 5 or 10 nitro, 22 (50/50) castor/synthetic. Powermaster. Tight piston cylinders in new engines or new p/l sets are carefully lapped. Plenty of wins in Old Time and Classic. Ringmasters often the plane of choice. Other successful Fox 35 matings include Orientals and Dan Banjoks fab Galloping Comedian. A few years back, Mike Palko earned his way into Expert (on the local level) flying a Twister powered by a stock Fox 35. 100% castor oil fuel will varnish up the engine. Necessitating disassembly and careful scrubbing with a scotch bright.

Sent from my iPhone using Tapatalk

Robert Zambelli · « **Reply #30 on:** January 07, 2015, 10:14:51 AM »

Mike K said: "You guys are a riot! I flew Fox .35s and .29s from 1952 to 1990 (?) with NO modifications (other than the addition of a ST spray bar/needle valve), and they performed very well....some on profiles, others on built-up fuselages.
Never used a "popsicle stick" insert or a "stuffer" back plate. The only thing the Fox .35 (or .29) required was a prolonged and careful break-in with plenty of castor.
The Fox often gets a bum rap from those who didn't grow up with it.
Fact is, it was a great little engine. Just look at the Nats results, 1950-1965."

I couldn't agree more! I really like my Foxes (I have MANY) and they have never let me down.
I only make two changes - I eliminate the backplate gasket and get rid of the Fox NVA. Yes, I know that some of them work OK but I prefer to use the ENYA NVAs. I like the ratcheting type.
Regarding fuel, I've been using Powermaster 10/22 (50/50) or Brodak 10/23 for as long as I can remember in both very used and brand new engines - never a problem.

I have found some inconsistency in the break-in requirements. I broke in two brand new 35s and one of them was ready to go after three tanks and the other needed around three quarts. They are both perfect now.

I don't think the Fox 35 will ever be out of style.

Bob Z.

Brett Buck · « **Reply #31 on:** January 07, 2015, 01:48:58 PM »

Quote from: Chris Wilson on December 22, 2014, 11:45:04 PM

But does it 'need' to be done Steve?

I have seen too many stock standard Fox 35's that have needed nothing other than a high castor content, good cooling and a touch of nitro.

Wooden props, I have a nice Brian Eather carbon 10X 5.5 prop and you are proposing that I use instead timber because it's more reliable?

It's just that I see the words Fox 35 followed by a trailer load of 'must have' modifications and most posts tend to focus on better performance not what give the venerable old beast reliability.

Well, of course, the "better performance and durability" modifications are desired because the "venerable old beast" (well-put) is so incredibly outclassed by almost every other currently available engine. The 50th anniversary Fox runs about like a "Brand new for '49 Fox", both good and bad. The only real improvement is that you can now mount a muffler reliably and that wasn't around for even the 40th. It was a competitive engine for a long time, maybe 15 years, but was surpassed something like *50 years ago* . Everybody else got better, the Fox did not, and the supporters tend to be proud of that.

Even if you stick with 4-2 break mode, it's hopelessly outclassed. Compare it to an Aero-Tiger 36 - maybe the best 4-2 break engine ever. In fact I contend engines like the Aero-Tiger have made a mockery out of "Classic Stunt" because it raises the performance level astronomically above what was possible in 1969.

But leave 4-2 breaks behind and the Fox can't even match up with a discontinued RC sport 21 from the 90's, which incidentally listed for roughly 1/3 the cost - or approximately what the *aftermarket crankshaft alone* for the Fox cost. That's why people want to improve the performance - it's not competitive as it comes.

I actually agree with you otherwise. A lot of times, as long as you don't have a profile, all it really needs is the right fuel, and most of the performance modifications only change the performance, and don't necessarily improve it. Adding 5% castor to 22% fuel is almost as big a power improvement as adding 5% nitro. The durability modifications are a mixed bag as well - the stuffer backplate, which apparently helpful for Foxberg racing, doesn't seem to address any problems you have in stunt, the rod lasts long enough already. The only unambiguous durability improvement is the Randy crankshaft, which addresses a known issue with the stock unit, and also permits reliable performance with more nitro. I hear it also reduces the vibration, which is a durability improvement for the airplane. But I have individual bone-stock engines with 40-50 gallons of fuel through them with no perceptible performance drop-off. 40-50 gallons might be more than most people ever use in their entire lives, at 4 ounces a flight. So, run in the manner intended, it's more that adequately durable. I frequently didn't get through 5 gallons on ST46s without having to fix something.

I have a reputation as a "Fox-basher" but I contend it's a matter of objective evaluation rather than waxing poetic of the "good old days" since I have attempted to compete with everything from the bone-stock Fox to the best engine available today, and might have a better perspective.

Brett

Dennis Moritz · « **Reply #32 on:** January 07, 2015, 05:55:43 PM »

Dan Banjok's Galloping Comedian powered by a stock Fox35 came in second at the NATs in Classic. May have even won once. Usually places. He regularly wins Classic and Old Time in local meets with Ring-a-ding powered by a Fox 35. A profile last time I checked. Fuel was as stated. 22% castor/synthetic. Different Philly Flyers came in 1st or 2nd at Brodak Old Time many times. Mostly flying a Ringer powered by a stock Fox 35. We have tried the high zoot. Wasn't in the winning planes. Bobby Z and the rest of us, including folks at Huntersville, will validate placings. Dan won Classic with his Galloping Comedian, Fox 35, last year at Huntersville. Competition was all kinds of flapped Classic planes with expensive engines or 'lectric power. I remember at least one Aerotigre. No flaps on the thin airfoil Comedian. Come to think of it. That was Danny flying the Galloping Comedian on the Cover of Stunt News an issue or two back. Power. Stock Fox 35. Seems I have typed this information before. More than once.