Buy a Double Star .40 , Stalker .40 or a Brodak .40 . Save yourself time ,money ,frustration and get a modern, better running engine .

Brad

Chris -

     PM sent.


Best regards,

Harold Brewer

I argued this once but have now proven if a Fox .35 Stunt vibrates rea bad on start up, shut it down and rotate the prop 180 degrees.  This using 10-6 props that have not been balanced.

Sometimes the "old ways" were better.  OMG, I would purposely unbalance props a bit just to do this!  What is this younger bunch going to do when all of the "old tricks" are gone?

Ken

FWIW,  Larry Foster's Fox .35 instructions, including crank balancing.

Both dimensions related to the notch removal seem to be 0.200".  As you note, the numbers have some "digitization distortion."

I would say that the notch is likely to offset the intake window in the shaft. It is almost directly opposite. This would get you closer to a static crank balance. Which is a good starting point before you work on the dynamic balance. Larry apparently had access to a dynamic analyzer, and noted the locations he placed the accelerometers. That can be very helpful if done correctly, and you have the time to try lots of configurations.

If you don't know this already, Fox Stunt .35 cranks are fragile. Many of them break in normal usage. Anything that compromises the strength of the stock crank is to be avoided. Larry points out that overheating the crank during rework would be one such way to reduce the likely life. Running heavy props is another. Running with a lot of vibration is probably bad, too.

They are equal opportunity failures: they can happen to any Fox user, and they can happen anywhere. The most likely places, based on a bunch of broken ones I've seen, are:

--Crack starts at a corner of the intake aperture in the shaft. Shaft fails in flight, with the front end often spinning off into the weeds pulled by the prop.
--Crack starts at junction of shaft and crank disk, due to poor undercut geometry/surface finish, and marginal design strength.
--Cracks across the crank disk, separating the throw/pin from the rest of the disk
--The crank pin wears heavily, due to the rod walking off the pin, which occurs because the old-style backplate had far too much end clearance

I don't count the bent shafts that come from hitting the ground. Those are all on the pilot.

The Divot

Here is a picture of a crank I modified based on Larry's drawing. The hard to read dimension is 0.2".
The horizontal one does not realy work, and I seem to recall opening up my L&J motor to get a better idea how to do it.

I have it drawn in CAD (and programmed in CAM), just now quickly added a few dimensions.

Just picked up a NIB 40th anniversary motor at our club swap shop, will probably do the various Larry foster mods to it.

When is the Fox crank balanced?   Is it balanced outside the engine?

Buy a Double Star .40 , Stalker .40 or a Brodak .40 . Save yourself time ,money ,frustration and get a modern, better running engine .

Brad

Three choices, but the only one I would take is the Brodak 40.

With both DS and Brodak coming from Belarus, I would count on Stalker (Ukraine) for more reliable availability and quality. L

     The Stalker 40RE was, hands down, the weakest 35/40 I have ever seen anyone use, even weaker than a lot of Dixon-modified engines. A bone-stock Fox was A LOT more powerful. If it had been just one example, I would chalk it up to unit-to-unit variation, but one guy around here got 5-6 different Stalker 40 SE and REs, and they were *all* that way. He had to practically melt it to get barely acceptable in-flight speed with a 10-6, it ran OK with a 10-7 but it was correspondingly wimpy. The Double-Star/Brodak 40 started as a Fox clone, and it was more-or-less a match for a Fox.

   It shouldn't take a special effort to get an ABC 40 to fly a Nobler at acceptable speeds, but that appeared to be far beyond the Stalker 40. The Stalker 46 ran OK, about like a ST46 without the problems, and the 51 that Gordon Delaney had was pretty darn good. All the 61's that anyone around here had were completely hopeless as they came, with venturis about 50% too large to the point they wouldn't get through the takeoff roll.

    Brett

Way back when, I modified several stock Fox 35s, with a similar, but different approach.

One was carving the crankdisk. My idea was to try to get the shaft to static balance with the crankpin vertically above the shaft centerline. That would (should?) reduce the shaft's mass asymmetry. Most other dynamic - motion related - factors like rod swing are closely symmetrical and should cancel over a complete revolution.

Asymmetry in the crankshaft seemed to me to resemble a spinning washing machine load with clothes stacked on one side. But, the engine doesn't have a compliant mounting to let the shaft approach turning on its actual center of mass.

The crankdisk was cut so that the side with the shaft port was straight from a radius about the crankpin to the diameter as Larry's article shows - that is, the diameter at a right angle to the line from crankpin center to shaft center.

The other upper flank of the crank disk is undercut in effort to static balance by removing metal opposite the shaft port side. The goal - static balance with the crankpin vertically above shaft centerline - usually wouldn't come out perfectly, but 'close' worked with noticeable effect. Both flank surfaces were cut and finished with various Dremel grinding points. Surfaces were finished very smooth. It is a "do-it-until-it's-done" process, not a dimension-governed thing. There was enough variation in Fox 35 pieces that no two came out identical.
 
Running loads: - This method keeps metal inside the directions of power stroke loads, and is undercut on the compression stroke side.

The engines I did this to seemed (at least) to vibrate less, unless there was some other glaring goof somewhere in the OEM unit. Larry Foster did it more "right." He acquired many, many replacement parts from Fox, matched the best fitting stock pairs, made his mods and tested the results. More power than from a well-broken in Stock Fox 35. He does (did?) great work*. I sought merely to make up for mass production shortcuts, and add things that had worked for me - in Stock OEM Fox 35's of that era.

Larry sold many of his engines through RSM, but it's been years since Fox factory parts were available. I don’t know if any of Larry's are still available there.

Roots,

Looks like you are using a pre-Anniversary backplate. You might want to check the backplate-to-crankpin clearance. If there is too much, the rod will walk off and wear out the crankpin, along with some other undesirable things. Since you also took material off of the front and the back of the case, unless you measured things, you don't really know what the clearances are now.

The concern is compounded if you use an electric starter and end up pushing the crank into the backplate. Unlikely on a Fox, but you didn't say how much you took off the front of the housing.

After all that work on the crank, it would be a shame to have the parts wear out prematurely.

Let us know how it runs, and if it seems smoother....

The Divot

Roots,

I think .010" would be a pretty safe number for a Fox Stunt. That leaves room for an oil film. This would be when the crank is pushed all the way back and a prop installed.* You probably only need 2-3 thousandths--but since it is a Fox Stunt, it might be hard to ensure that happens. Note that you'll have to account for the backplate gasket. It looked from the pictures that you are using the Fox cork part? This will compress over time and reduce your clearance. Paper gaskets are more predictable, but generally thinner, so again you would have to pick one and should stick with it. If you are familiar with OS engines, they use what is often called "vegetable paper." Just a natural fiber paper that works well for gaskets. I've always assumed that the reason Fox used cork was that because it can compress much farther, it could take up larger non-uniformities in either of the surfaces of the parts. I've seen cast backplates with pretty large cold shuts which otherwise would have leaked. I've seen the same thing on some McCoy Redheads of the same era.

If you do find a so-called "stuffer" backplate**, be aware that these were made with more precision (for the anniversary cases) and much tighter fits than the old cast parts, which had enormous diametral clearance. I just resuscitated a number of old Fox Stunts that a club member had collected and run hard. In most cases, really hard. He came to me because he could not get the stuffer backplate out of the older Fox cases. The various different engine reworkers had tried to remove the internal mold flash and grind the ID a bit rounder, but in three engines, the backplates were really seized internally. They must have pressed the backplate in, heat shrunk it in, or maybe just forced it on with the screws. The metal had galled and...well, you get the idea. As you have an earlier case, be prepared to do some work if needed to get the barstock part in.

Another way to confirm clearance is to put down multiple layers of tape on the face of the backplate. Reassemble the engine and slowly seat the screws while dithering the crank back and forth (rotation). When you feel added drag, you're touching. Take some tape out and keep trying. Then measure the tape. This kind of in-situ measurement can either confirm parts measurements, or account for unknown gasket compression. You also don't need five hands to hold the case, push the crank all the way in, and hold a depth mike or calipers without fumbling the whole deal onto the concrete floor....

One other thought since you are into tweaking this engine: if you can't find a barstock backplate, and if you have a ton of end clearance, consider making an aluminum plate and bonding it onto your current backplate. If you got the parts thoroughly clean and used JB Weld it should work fine. If you make the plate diameter oversize, you can dress it after the bond cures. Something like 6061-T6 would work nicely.

The Divot


*--If you have a ton of shaft endplay with the prop on and the backplate off, you might want to put a precision shaft spacer between the drive hub and the case. Or a washer, if you have one thin enough.

**--The real value of the barstock backplate is in the much reduced end clearance, saving your rod and crank, and not in attempting to fill the huge internal volume of the Fox and improve base compression.

I wrote this on the Fox 35 hop up thread. I always use a starter on my Fox speed engine with no problems after this mod.
Al   
The Stock Fox 35 has plenty of power to fly a Nobler, you just have to use 58’ .015 lines. I like the APC 10.5x6 prop and 10 nitro 25 all castor fuel. Every Fox 35 back plate that I removed had mark from the Rod hitting. I even tried the stuffer backplate on a old engine and that got chewed up real bad and sending the aluminum though the engine killing the piston and liner. Now when I set up a Fox 35 I cut a piece of .015 spring steel to the shape of the backplate and JB weld on to it. Rough up the backplate and the mating side of the spring steel with 80 sand paper before gluing, then I polish the side that faces the rod. I tested this backplate mod in my Fox Speed engine that turns over 18000 rpm with over 50 flights on the engine with no signs of metal wear, even after swabbing the crankcase with a Qtip looking for it. I also heard the old folklore story about the Fox 35  adjustable backplate test, but from what I heard there was no improvement in performance.
Al

Did you account for the thickness of the gasket? That would increase the clearances that you have calculated.

You would appear to have about .020" shaft end play. Maybe not unexpected for a Fox Stunt, but if you are already tweaking this one...you might want to take the extra step and reduce it.  I measure a nearly new 50th Anniversary engine at .012" endplay. For another example, a similar vintage OS .35S has .011" endplay. The OS uses a .006" steel washer on the front to reduce the grinding of the front of the case when contacted by the prop drive washer. It isn't as clean an application on a Fox Stunt due to the splines on the crank. The Fox also is not aluminum on aluminum since the drive washer is steel, which is a better situation. And I suppose that the bronze bushing, although only a small portion of the contact area, counts for a better interface as well.

If you pursue the shaft spacers, you don't actually have to make them by hand, although if you can mill a crank you certainly could. McMaster sells shaft spacers to industry.

     https://www.mcmaster.com/shaft-spacers

In Al's solution, not sure if he is letting the crank rub the modified backplate or if he is getting good contact on the front of the case? A stuffer backplate and lots of shaft endplay may allow the crank pin to contact the backplate when using a starter and cause the kind of damage he describes. You shouldn't see it from just contact with a floating rod, unless something is crooked. Most engines use this same "rod retention" technique without resorting to hardening the backplate. But a lot of them are ball bearing engines and the crank is not floating around inside....  I believe I've seen some European engines with this steel wear plate as well. Just can't recall which ones....

Dave

FWIW, I checked a stuffer backplate and it is 0.380" deep.
This was one of the Teflon anodized ones, I never used a gasket with it just RTV silicone.
No rub marks on mine, but I never used an electric starter.

   Whether it rubs or not depends on how square the axis of the cylinder is with respect to the crankshaft. If it's far enough off, the conrod is pushed back into the backplate by normal running. That's what the "stuffer" backplate was intended to control, to not let the conrod wear prematurely. It has nothing to do with performance.

     Far enough off, and hard anodized or not, it will dig a divot in the backplate in a few flights. That's what happened to my first Larry Foster Fox. He told me that he checked many crankcases for square and sent them back if they were too far off.   

     For stunt, I wouldn't spend a lot of effort trying to get a stuffer backplate, it was intended to extend the conrod life for Foxberg Racing.

    Brett

Crankshaft
Al

McMaster Carr is about half of that. But since you need so little, this is the kind of thing that scrounging often pays off. I'd tell you where to look--but by definition, it's scrounging....

Dave

Ok, so here are some jump-start scrounging ideas....spring from a broken 1" wide tape measure; gear out of an old windup clock, lid off a tin can....