Fox .35 Modifications

What I see with regularity is two places where a Fox .35 stunt shaft breaks. The first and most common is the cutout on the shaft. It either cracks or fully departs. I think this is generally assisted by backfires. This is also Fox metallurgy and poor machining practices. The cutout needs to have a radius in the corners to prevent stress risers. Some I know will use 15% nitro and this will also shorten the lifespan of the shaft. The other area where the shaft breaks is where the crankpin is on the web. Seeing how the crank is ground, this is weakening that area even further due to the heat generated by grinding and the lack of material surrounding it.

Myself and many of my club members use the Fox .35 more than any other engine available to us. There's no need to try and balance the shaft. There's far too many other issues with the engine to do so. If one build the motor crutch properly, this is all that's needed to insure consistent smooth runs. The larger and bigger problem is the runout of the shaft from Fox even when it's brand new. Randy Smith made the Hi-Zoot shaft which was really the finest shaft you could have for the Fox .35 stunt. It wasn't hand hammered with stone tools and ground to size on the concrete. Randy's shaft was balanced, it also had no runout issues and it was properly hardened. The cutout in the shaft was also done precisely with the proper radius in the corners. It had a price tag which was a $60. Not too many were interested in purchasing and a limited amount were made.

What is the motor crutch? Sorry for my lack of experience Ken.

Never mind your talking engine Mount so trouble have the least vibration

Never mind your talking engine Mount so trouble have the least vibration

Ken, does anyone run the Fox 35 Speed event over there? I saw Henry Nelson fly two old F2C wings this weekend here. I didn't ask about the engines, but people were saying in typical Henry Nelson fashion, the only Fox part left was the case. They did go, but he was working out fueling issues on launch.

The biggest problem faced with any speed event is the aging members who participate. Not only is the control line arena losing attendance due to age, there's just not enough interest overall. Henry is also correct in terms of the only thing left is the case. Brodak just dropped Fox speed from the events. I'm pretty certain Henry has participated in that event at Brodak's. The Jersey crowd still utilizes Foxberg which means box stock .35 but they also have a event that is more or less anything goes in terms of mods with the Fox .35. In Philly , we have non official events using the Fox .35. We tend to utilize Brodak rules mainly because no one has to think them out and the judges are the same judges who judge Brodak speed events.

Question,Is this a fox 35?

Looks like the 1962 fox rocket thanks?
Am I correct?

In 1961 the Fox Rocket had it's case changed and every .35 from this point on was different. Bill Mohrbacher put this together some years back. http://foxmodelmotors.com/engines/36x/history.htm

          I would say this is the most accurate finding you will find on the internet. In addition, the case you have with no markings in my opinion is earlier than 62. I have the same engine which is the first Combat Special and I put a date on it as 57. The only reason I say this is because of the plain mill finished head. In 58, the head was black. Who knows as to why there was always mystery to the unmarked displacement. Some engines had a raised area on the casting while others had their displacement marked. Many times engines came with nothing in the raised portion which represented a .29 displacement.

        Some of these engines came with a steel restrictor in the venturi. I believe I see one in your engine. This offered a bit more more power but this usually required backplate pressure which is why the backplate has the pressure nipple.

Thanks Ken greatly appreciated
I see it has the lugged front case as the rocket with the red head. Definitely in the ball park

I keep forgetting the 2 slots in the liner which one goes towards I got the higher or lower one Fox 35 ? Thanks

Onelife wrote:I keep forgetting the 2 slots in the liner which one goes towards I got the higher or lower one Fox 35 ? Thanks  

In all cross scavenge engines, the side that has the higher slot in the liner is the exhaust. The opposite lower sided slot is for the intake bypass port.

A little trivia, I purchased a used in almost new condition K&B .35 Stallion earlier version (thinner streamlined web brace in exhaust port) that oddly had SPI (sub-port induction - air gap in some engines when piston at top dead center with exposed piston skirt). A previous owner had disassembled the engine and had the cylinder liner installed 180 degrees to proper.

https://www.coxengineforum.com/t12115p50-kb-35-stallion-acquisition#217960

After cleaning, I reassembled it correctly, the "SPI" disappeared. I bought it for a song at a parts engine price. No wonder why this 1960's Stallion looked almost like new inside. I gather that the previous owner never got it to run right, why I got a "new" engine.

Thank you. That SPI I also bought a engine on eBay with it but I’m sure it was modified to have it. Hard to tell from pictures but when pistion is at compression there is no piston showing. What exactly does it do ? Thanks again. The engine is a Fox 35 , case anyway head is the blue magic. I did not try to bench test yet I’m not sure if I want to Mount it on my Buster or keep it as a collection piece

I like to know what the previous owner wanted to Accomplish

Onelife wrote:I like to know what the previous owner wanted to  Accomplish

That's definitely a @Ken Cook question, he's intimately knowledgeable on these Foxes.

I find it interesting that this engine has an improvised venturi extension to help force air into it, also reducing the exhaust opening with that restrictor, which seems counter as a power tweak. (I think it may degrade the purpose of the SPI, but could be wrong.)

Another good place to ask is on the Stunt Hangar forum https://stunthanger.com/smf/index.php

Joining is free.

So the SPI is supposed to improve Proformance? Thanks.
I’m a member @SH

Myself I would like to have a Fox anodized I think those are really sharp looking. Like in a red or gold

Onelife wrote:So the SPI is supposed to improve Proformance?

SPI adds a puff of fresh air into the crankcase, to augment the fuel charge. It is a way to increase horsepower on a 2 cycle engine.

The earlier Cox engines used it on nearly all engines. It's use faded because it doesn't benefit when mufflers are used. Mufflers cause spent, inert exhuast gases to be re-introduced back into the crankcase. This dillutes the fuel charge, diminishing power. Back in the early 1970's, I experimented with a Tatone muffler for my Babe Bee on my Sterling Beginner Eindecker. It flew like it lacked a "third" of its horsepower. After a few dismal flights, I removed it.

Thus, the Cox engineers (Dale Kirn?) developed a non-SPI cylinder piston set to increase power through bypass modifications and eliminating exposed piston skirt, so power would be acceptable when muffled. The QZ Bee (Quiet Zone) was the result, followed by the QRC (non-throttled muffled Bee for R/C use), R/C Bee, Dragon Fly, Ranger product engine, and Sure Starts.

Production muffled R/C versions of the Tee Dee with R/C carburetor (during decline of Cox, Santa Ana, California before Estes?) used the non-SPI cylinder/pistons too.

Thanks for the info. That fox 35 is a weird one I’m 1-2 Leary on bench testing it

Onelife wrote:Thanks for the info.   That fox 35 is a weird one I’m 1-2  Leary on bench testing it  

I would get rid of that pin that blocks the exhaust and try to run it. Kind of doubt that it will run with what looks like a big chunk out of the bottom of the piston. There would be no base compression at all, hard to see what is going on there from the pic. though. Maybe the guy thought it was a way to port it. That is usually done on the intake. Anyway, what is the worst that could happen, it is a shelf queen no matter what? May as well use it if you can, it is not going to be worth millions if it is not run and it is not NIB. Usually the guys try to detune the Foxes for a steadier run and less backfarting, maybe that is what the pin is for in the exhaust. I would neck down the venturi before blocking the exhaust if that was the case.

Yes when the piston is up all the way there is no piston showing the arm what I can see of it does not even look like a fox part. I didn’t want to tear it apart until I see if she runs. it is hard to see from pictures. I will fire it up tomorrow. Thanks

And I heard guys talking about changing the timing. How is that done ? Or is that what everything done to this motor is doing

The timing can be changed by grinding or milling the ports, or shimming the liner but if you go too far it is ruined. That may have happened. It is usually for higher power band or for a tuned pipe. Not what you want probably.

That’s definitely not my pay grade lol. I’m not after speed in just curious what all that has been done to the 35,s thanks
What about holding the wrist bar in. I have one of each the one with the clips and one
That looks hollow with brass on
The ends. The one
With brass are they supposed to move freely? Or should it be mushroomed at the ends
And the
Clips how do you remove them ? Razor blade then pry it off ?

What @aspeed is alluding to is for example, modifying the cylinder windows cut for the exhaust side and intake bypass side to obtain "valve opening" and duration by modifying its edge with respect to the piston in degrees of crankshaft rotation, to attempt improving engine running characteristics.

I don't know specifically and someone like @Ken Cook or possibly Mike @sosam117 might elaborate more. There were others who had "dibbs" on this, but unfortunately they no longer post because they have passed on. (Examples, @GWILLIEFOX - Bill Mohrbacher, @RknRusty , @jsesere , etc. Hover the mouse over their name followed by "@" or avatar by their post and you will see next to "Rank:" the words "Rest in Peace".)

For example, in the engine test article, Sceptre Flight Engine Test: Fox .35 Stunt by Peter Chinn, Dec. 1967 Aeromodeller states,

Peter Chinn wrote:The liner contains one exhaust port and one transfer port, of moderate area, the transfer occupying only about 100 degrees of the cylinder circumference. The ports are timed to remain open for 132 degrees (exhaust) and 118 degrees (transfer).

Then there is also timing involved in emitting the fuel air charge into the crankcase where it can be further processed by the crankcase, the venturi valve time by the opening in the hollow crankshaft. How faces of it are cut control opening and duration.

IMO, most modellers would prefer the manufacturer to figure this all out, as they can ruin a greater number of cylinders and crankshafts than an individual at customer level could, in arriving at the most optimal "valve" settings. Basically wait until the manufacturer came out with their special say, stunt or racing engine.

Manufacturers aren't perfect, as they also need to make money especially for the majority buyer, the sport modeller, so, I gather that various individuals and specialists have attempted their "honing", which in some cases did result in improvements.

I myself am the sport flyer when I do it and sport collector, I just let the others do their experiments and limit myself to plug/head changes, head gasket shimming, fuel changes, prop changes, and etc.

For me, life is too short to ruin a perfectly good engine.