Fox 35 crankshaft end play

[Marty Hammersmith]

I bought a couple of used Fox 35 engines from ebay. The first has arrived. It is very clean with nice compression. How much crankshaft end play should these engines have?

[Dave_Trible]

They can have what seems a lot.  Don't worry about it.  Thats the way they came from the factory-it's not wear.  Just be sure to always run 28-29% ALL CASTOR oil fuel,  especially in those that already have some run time on them.  As long as it has good compression and doesn't look burned it should be good. 


Dave

[Steve Lotz]

Lateral play is what you don't want.

[Brett Buck]

Lateral play is what you don't want.

  It will tolerate a fair bit of lateral play, 1/16" at the prop tips is fine.  And it needs to be fairly loose to do a good transition back and forth.

     Brett

[Motorman]

A good end play on a plain bushing engine would be about .010"- .015". Good luck getting that on a Fox. Never tried it but you could turn down the front of the case and press a brass cap on it to take up the space.

MM

[wwwarbird]

 Gotta love those Fox tolerances. 

[Dennis Toth]

I have been adding an OS thrust washer from the 40 FP behind the drive washer on my Foxes and takes up  much of the loose.

Best, DennisT

[Dave_Trible]

Once started thrust pulls the crankshaft pulls forward against the internal boss of the bronze bushing eliminating the endplay altogether until the engine stops running.  Shimming doesn't do anything and the endplay is meaningless.   If it were ball bearing then you'd need to worry.  Some of the combat engines were needle bearing.  The shaft could move back and forth the over those.

Dave

[Paul Smith]

The packed backplate with the Teflon face eliminated most end play.

By the way, in case you haven't noticed, end play is death with an electric starter.

[Marty Hammersmith]

The packed backplate with the Teflon face eliminated most end play.

By the way, in case you haven't noticed, end play is death with an electric starter.

I dont use an electric starter. Only a Hobbico chicken stick.

[Andre Ming]

First off, I would not use an electric starter on a Fox 35 or other similar engines. Doing so can cause the end of the rod and/or crank pin to rub against the rear cover with great force which can produce burrs. Metal shavings in your Lil' Doozie wonder engine is not a good thing.

As for the crank slop and how it effects engine running:

I vaguely recall from my years at Fox Mfg that the engines had an ever-so-slight over-square between the bore-to-crank center lines (side view). In other words, instead of it being a perfect 90 degrees, it would be something like 90.01. (Figure used for illustration only.)

That way, the act of running the engine caused an ever so slight forward movement on the connecting rod, which gently pushed it forward against the machined area for same on the rear of the crank disc/counterweight.

With a nigh 90 degree machining, the connecting rod would work back and forth on the crank pin, which increases vibration and wear on the connecting rod.

Less than 90, and the connecting rod would move to the end of the crank pin and in some cases rub against the rear cover in some circumstances, which is also a totally undesirable situation.

All of this machining on the case was done in what was called "Second Operations". That is where the raw cylinder casting started through the processes of "Second Operations", and at the end of the line was ready for use in the Assembly Dept. The "Second Operations" department was critical to the goal  of producing a dependable engine with a very good service life.

I also vaguely remember Jay Davis (the Second Operations Manger at the time) bringing me the first samples of new cases that had just been put through all the processes for me to assemble for testing.  To do this, I would hone the nose bearings on the the case, or cases if more than one, and then using assembly line parts, assemble an engine. Once assembled I was off to run the engine(s) in an abusive manner (cut-down prop w/high nitro to produce more RPM than the engine was designed for), as well as over-propped (to produce heat.). If the engine(s) passed muster on the test stand, I would then disassemble, and inspect for issues. If all looked good, the line was fired up and cases started passing through Second Operations.


Andre

[C.T. Schaefer]

Great input Andre!

[Motorman]

instead of it being a perfect 90 degrees, it would be something like 90.01.
Andre

More like +/- 5 degrees from what I've seen and I've never seen a Fox 35 that didn't rub the back plate.

 I was always curious to know how the cases were held for machining the cylinder hole because, the mounting lugs where not square with the engine or parallel with each other. I suspect they were clamped to a block, warping the case then nicely machined then unclamped and case springs back to original shape warping the machined areas. Probably why the piston locks up if you tighten the head bolts.

It just amazes me that no one cared to machine the bottom of the motor mounts flat inside of 60 years. 

MM

[Dennis Toth]

The Fox20/35's are not designed with a Desaxe cylinder offset. In the Engine Review by Peter Chinn (https://sceptreflight.com/Model%20Engine%20Tests/Fox%2035%20Stunt%20%282%29.html) he measures all the timing and they are symmetrical indicating no offset (he also states this). Almost all other Fox engines use the Desaxe offset. Because of this 35's could use a reverse crankshaft, which Fox did offer as an optional part.

The mount lugs I agree with MM are not the most accurate. They work ok with maple wood mounts but don't work well with hard metal mount pads. To use them with a hard metal mount pad I make up a 1/32" plywood crush shim to put between the metal and the case mount lug. I assemble the motor, ply shim and hard mount pad/with maple crutch and tighten the motor to the pad letting it form fit to the mount lug. I let it sit a day or so retighten every so often then mark which ply/hard mount pad goes to what side of the engine and mount in the ship with epoxy. You can also do it in the ship just keep track of which ply/al pad goes to what side of the engine.

This works well and allows a solid engine mount without distorting the case as you tighten the mount bolts.

Best,    DennisT

[Al Ferraro]

  I do my back plate mod on every Fox 35 that I own and tune. Using a Fox 35 stunt stock back plate, I JB weld a piece of .015 spring steal to it, deburr and polish. I have used a starter on my FOX Speed engine hundreds of times with no ill effects and it turns over 18000 RPM when running. I have tried the Fox stuffer back plate in the past and it would get chewed up sending black death oil out the exhaust.
Al

[Andre Ming]

DT:

I wasn't talking about the Desaxe offset.


Al:

Good modification. As you have learned, to not do it that way will result in the "Black Death" when using electric starters.


All:

Frankly, it's amazing the things (Fox engines) worked, and yet the overwhelming majority worked well. Some of you got lemons and thus have hated Fox engines ever since, and never miss an opportunity to disparage them. Others have had good service out of them and like them a bunch. Those of us that enjoy Fox engines tend to keep our mouths shut to avoid the disparaging comments/rants. (Or feeling the need to validate our choices.)

I know there are better made, higher quality, higher performing, engines out there, but I still like the familiarity of working with my old Fox engines. I also enjoy their nostalgic nature.


Andre

[Dave_Trible]

It seems to me having been a user and an observer of others using various Fox engines over the years (decades) is that when they were treated as originally intended you got very good service from them.  They were almost the ONLY stunt engine to use for 40 years if you were serious.  Where we left the reservation with the Fox .35 was demanding more and more from them;  the airplanes grew in size and weight,  folks started dumping more nitro and less oil into them,  there started to be 'racing' events for the Fox (a no-no) and guys kept trying to make them run more like ball bearing higher rpm motors for which they were never intended.  When I attended the last VSC I had a gent explain to someone else I was talking to why my Fox ran so well.   He was saying how I had a 'stuffer' backplate, and a list of other crap (sorry).  I had to stop him and say that in fact it is an original 1958 model engine straight out of the box with no modifications whatever.  Not sure he believed.  I then explained it ran on 29% all castor fuel,  a light wood 10-5 prop and set to smoothly four cycle-not "wet two scream" as the current language goes.  Also the airplane is in the low forty ounce range flown on 57' lines and adjusted to come on-off cycle at 10 and 2.  When used properly I'd still say it's very hard to beat even today in an airplane under 45 ounces.  Being impressive against our current .65-.76 or electric behemoths is another matter of human impression in terms of competing.
I also had a measure of combat experience in the 60s and 70s.  Though I found myself using the ST G21 .35 for the most part like everyone else was-follow the leader- the Fox .36Xs (NOT BB!) I had would flat outrun them......I think they were just a little harder to start hot.
You got from Fox in relation to how you treated them and what you should expect from a mass produced, fairly low cost engine.  Fortunately thanks to the power of Ebay and 60 years of manufacture there will be enough supply of the old Fox to last many more years.

Dave

Marty we drifted your thread again-thats what happens around here...

[Brett Buck]

It seems to me having been a user and an observer of others using various Fox engines over the years (decades) is that when they were treated as originally intended you got very good service from them.  They were almost the ONLY stunt engine to use for 40 years if you were serious.

  It's more like about 15 years - early 50's to early-mid 60s, when the top experts like Lew McFarland and Bob Gialdini started switching to engines like the Veco 45 and later other larger engines like the ST46. Gieseke soldiered on against the tide, while most of the similarly competitive pilots ran 40's and 46s and the like, but even he gave up in the late 70's-early 80s. Lots of others kept on with Foxes long after the experts moved on.

     Brett