Author Topic: Steel washer behind Fox 35 drive washer (Read 17215 times)

kevin king · « **on:** July 07, 2024, 08:59:31 PM »

I recently hit the ground nose first with a fox 35 powered Bi Slob. After another flight i noticed play in the crank shaft. Is there a s up supposed to be a thin steel washer behind the drive washer? I think I may have shredded it off! Where do i get another one if thats what happened?

Dan McEntee · « **Reply #1 on:** July 07, 2024, 10:09:40 PM »

Nothing behind the prop drive on a Fox .35. You could try one from an OS .40/.46 and see how it fits. If you can measure the clearance between the back of the prop drive and the case, you might be able to make one from shim stock. Just leave about .005 clearance for oil.
Type at you later,
Dan McEntee

kevin king · « **Reply #2 on:** July 07, 2024, 10:48:58 PM »

Thank you Dan!

Ken Culbertson · « **Reply #3 on:** July 07, 2024, 11:23:45 PM »

I don't remember ever seeing one. This parts breakdown seems to confirm it.

https://sceptreflight.com/Model%20Engine%20Tests/Fox%2035%20Stunt%20(2).html

You might have bent the connecting rod slightly.

Ken

Dan McEntee · « **Reply #4 on:** July 08, 2024, 03:16:31 PM »

Quote from: kevin king on July 07, 2024, 08:59:31 PM

I recently hit the ground nose first with a fox 35 powered Bi Slob. After another flight i noticed play in the crank shaft. Is there a s up supposed to be a thin steel washer behind the drive washer? I think I may have shredded it off! Where do i get another one if thats what happened?

How did the engine run on the second flight? If you hit on grass, and the next run was OK, probably no issues. I don't think you could "compress" the case any, if you happened to hit exactly square. I ruined a OS MAX-S .35 case once by hitting the pavement at a contest inverted at about a 30 degree angle. Pulled the engine from the fuselage to check it out and found that I could not rotate the crank through a complete revolution after I put another prop on it. I pulled the back cover off the engine and saw that the rod was half off the crank pin. I pushed it back on, tried to rotate it again and the rod worked it's way back off the pin again. I could not see anything broken or cracked on the engine after looking it over closely. It wasn't until I help it next to another OS .35 that I could see that the case was tweaked and the crank was about 10 degrees or so out of square with the center line of the cylinder. No broken or cracked aluminum anywhere!! The case was tweaked up, and that put the bore of the cylinder out of round, and that in turn distorted the cylinder liner. I think the piston managed to stay round, and somehow the crankshaft didn't get bent!! I think I was able to use the head, back plate, prop drive, prop washer and nut, and back plate on another engine that I restored to running condition. I saved the case just as an oddity!! A Fox .35 case is probably thinner than the OS and I would guess it is possible to distort the case by pushing the main bearing housing straight back into the main case, if it landed at exactly the right way at exactly the right speed!! You could never do it in a million years on purpose!! You could also have had just a hint of some flashing washed over the bushing that was taking up some slop, and it got broken off in the impact. I have worked on engines like that and filed off excess material that should not have been there.
Type at you later,
Dan McEntee

Dave_Trible · « **Reply #5 on:** July 08, 2024, 03:55:17 PM »

There isn't a washer there. It is also not needed. The Fox has a very large amount of shaft endplay as is and is not an issue. Once started the thrust pulls the shaft fully forward for the full run, the backplate is avoided and the everything else is lined up as designed......no worries. It was always there however you may have not noticed it until you were looking for damage from your unplanned landing approach. Part of the reasoning in the 'stuffer' aftermarket backplate was to take up some of this extra internal space. Not sure it achieved anything- a Fox is still a Fox, disdained or beloved.

Dave

BillLee · « **Reply #6 on:** July 09, 2024, 07:09:23 AM »

As Dave said: a Fox 35 has a lot of end play in the shaft.

Question: is the rear-ward movement controlled by the clearance of the rod/crankpin - to - backplate? Or is the back of the drive washer on the front contacting the front of the bearing in the case?

Without going out to the shop and looking at an engine, I'm pretty sure I know the answer, but would like comments.

Dan McEntee · « **Reply #7 on:** July 09, 2024, 09:04:59 AM »

Quote from: BillLee on July 09, 2024, 07:09:23 AM

As Dave said: a Fox 35 has a lot of end play in the shaft.

Question: is the rear-ward movement controlled by the clearance of the rod/crankpin - to - backplate? Or is the back of the drive washer on the front contacting the front of the bearing in the case?

Without going out to the shop and looking at an engine, I'm pretty sure I know the answer, but would like comments.

I have seen this end play vary from engine to engine. If you take the back plate off a Fox.35 and look at the big flat surface, you will almost always see swirl marks, most likely from the face of the rod crank pin journal. This is supposed to be one of the reasons the stuffer back plate was developed with the Teflon surface in order to keep the rod more in position on the crank pin. The drop drive hub fit to the crank shaft varies from engine to engine. Some can be difficult to get seated. I think the earlier prop drive hub dimension have varied as the engine progressed in development through the years. Crank shafts have also certainly changed. Mix any of these parts when repairing or rebuilding an engine could show up a lot more end play. And as I mentioned before, aluminum flashing from the casting process can wash over the end of the bushing. i have had to file this off when fitting a different prop drive hub or replacing one that was missing. i have never really been worried about it, know like Dave said, when running the prop thrust tends to pull the crank forward. While running, i don't think there is any issue, it's when the engine takes a good hard knock on the end of the crank that things come into question. If the clearance between a stock back plate and the end of the crank pin were minimal, I think a good hard knock from a lawn dart type impact would leave a definite mark on the back plate. I don't think I have even seen that in any of the engines I have been through.
Type at you later,
Dan McEntee

Dave_Trible · « **Reply #8 on:** July 09, 2024, 10:19:37 AM »

The worst case of endplay occurred when I was ten years old and planted my Firecat with a Fox full tilt vertically into the concrete circle at Swope Park in 1965. The crank was IN the fuel tank. Dad sent it off to Duke for repair(?). Duke sent back a whole new engine for the cost of mailing.....

Dave

Ken Culbertson · « **Reply #9 on:** July 09, 2024, 11:27:18 AM »

Quote from: Dave_Trible on July 09, 2024, 10:19:37 AM

Duke sent back a whole new engine for the cost of mailing.....

Dave

Duke did that a lot. 1964 Nats my Fox was giving me fits. Duke saw me having trouble (I was a Senior then) and asked if he could see the motor for a few minutes. He returned with a perfect running motor that I suspect was not mine. I ran that Fox for 10 years.

Ken

Steve Helmick · « **Reply #10 on:** July 11, 2024, 10:29:13 AM »

The "stuffer" backplates were said to be hard anodized. My recollection from aerospace QC of finish specifications by certified vendors, hard anodize sometimes also had a secondary coating, some sort of low friction stuff required by our customer, Mr. B. What it was, I'm not sure, but maybe Teflon impregnation. Can I say that here?

Steve

Gerald Arana · « **Reply #11 on:** July 11, 2024, 11:41:39 AM »

Quote from: Steve Helmick on July 11, 2024, 10:29:13 AM

The "stuffer" backplates were said to be hard anodized. My recollection from aerospace QC of finish specifications by certified vendors, hard anodize sometimes also had a secondary coating, some sort of low friction stuff required by our customer, Mr. B. What it was, I'm not sure, but maybe Teflon impregnation. Can I say that here? Steve

Sure Steve, as long as you aren't do the impregnating!

Cheers, Jerry

Dan McEntee · « **Reply #12 on:** July 11, 2024, 11:51:46 AM »

Quote from: Steve Helmick on July 11, 2024, 10:29:13 AM

The "stuffer" backplates were said to be hard anodized. My recollection from aerospace QC of finish specifications by certified vendors, hard anodize sometimes also had a secondary coating, some sort of low friction stuff required by our customer, Mr. B. What it was, I'm not sure, but maybe Teflon impregnation. Can I say that here? Steve

Marvin Denny's originals were coated with a black surface that Marvin told me was Teflon, and he specified using a playing card as a source of proper gasket material, so that's what I use. Fox factory stuffers are a dark gray all over, so they must be anodized, but the main surface doesn't look any different. In true Fox fashion, I'm sure they went this route because it was cheaper!! I think most of what I have in service are all Denny back plates, mainly by chance and because I like the idea of the Teflon.

Type at you later,
Dan McEntee

Steve Helmick · « **Reply #13 on:** July 12, 2024, 10:27:10 AM »

Quote from: Gerald Arana on July 11, 2024, 11:41:39 AM

Sure Steve, as long as you aren't do the impregnating!

Cheers, Jerry

I'm no longer qualified, Jerry!

I do remember getting some parts back from the certifiable finishing company that were supposed to be hard anodized AND given the slick coating stuff. The "certs" said it was done. They looked to be hard anodized, but not given the slick stuff. I used a VOM to check for anodizing, because it won't conduct electrons. Thirty years later, I wonder if maybe I should have had a phone conversation with the finishing company. Never heard a peep from the customer on that.

Regarding the Fox "stuffer" backplate, their sole function was to keep the conrod on the crankpin and prevent walking fore & aft, extending the conrod life. Bare aluminum would wear to some extent, and that aluminum would not be good for the piston/cyl. fit, and will also eat glowplugs quickly. The hard anodize plus slippery coating stuff would fix that for decades of 4-2-4 runs. Speaking of runs...

Steve

Peter in Fairfax, VA · « **Reply #14 on:** July 12, 2024, 11:09:11 AM »

And an electric starter can create backplate swirls, for better or for worse.

BillLee · « **Reply #15 on:** July 13, 2024, 08:18:11 AM »

Here's a little set of experiments that might be educational.

1. Take your Fox 35 apart. Piston/rod/liner out of the case, crank left in..

2. With a prop bolted onto the front, observe the end-play in the shaft. Note where the various surfaces are in contact when the shaft is moved forward and backward.

3. Bolt the backplate with the usual gasket back onto the case, no P/L, standard tightness.

4. Again, observe the end play in the shaft.

(To be anal, you might want to use a micrometer or depth gauge to actually measure the values.)

Observations:
.... if the end play under these two conditions is the same, there is NO contact between the crankpin and the backplate.
.... If you have swirl marks on the backplate from previous running, they are caused by the rod moving back off the crankpin. This is very common, unless the engine is made to push the rod forward on the pin when running or the engine has been damaged.

Dennis Toth · « **Reply #16 on:** July 13, 2024, 09:42:36 AM »

For the FOX 35S seems like a 3D printed backplate from high quality nylon would work here?

Best, DennisT

Dave_Trible · « **Reply #17 on:** July 13, 2024, 10:54:19 AM »

Quote from: BillLee on July 13, 2024, 08:18:11 AM

Here's a little set of experiments that might be educational.

1. Take your Fox 35 apart. Piston/rod/liner out of the case, crank left in..

2. With a prop bolted onto the front, observe the end-play in the shaft. Note where the various surfaces are in contact when the shaft is moved forward and backward.

3. Bolt the backplate with the usual gasket back onto the case, no P/L, standard tightness.

4. Again, observe the end play in the shaft.

(To be anal, you might want to use a micrometer or depth gauge to actually measure the values.)

Observations:
.... if the end play under these two conditions is the same, there is NO contact between the crankpin and the backplate.
.... If you have swirl marks on the backplate from previous running, they are caused by the rod moving back off the crankpin. This is very common, unless the engine is made to push the rod forward on the pin when running or the engine has been damaged.

In my discussions with Andre Ming who worked at Fox he explained that the engines were designed with (and here I forgot the terminology) about 1 degree of 'offset'-my word, so that the rod would always work forward on the crankpin and not backward toward the backplate. I'm sure this was important given the shaft endplay. I can see where any foreign matter that got into the engine COULD get picked up at that point and scrape the backplate a bit until it washed away. I have had an issue picking up sandy grit from my flying field and causing scratching inside a couple of my RO Jetts which Dub had to polish out upon rebuild. I'm not sure (with the Fox) about using an electric starter -whether tolerances allow the crank to grind on the backplate. It might depending on vintage and fits. I've learned NOT to use an electric starter in general but for sure on anything without ball bearings to take the shove back.

Dave

Dan McEntee · « **Reply #18 on:** July 13, 2024, 01:37:18 PM »

Quote from: Dave_Trible on July 13, 2024, 10:54:19 AM

In my discussions with Andre Ming who worked at Fox he explained that the engines were designed with (and here I forgot the terminology) about 1 degree of 'offset'-my word, so that the rod would always work forward on the crankpin and not backward toward the backplate. I'm sure this was important given the shaft endplay. I can see where any foreign matter that got into the engine COULD get picked up at that point and scrape the backplate a bit until it washed away. I have had an issue picking up sandy grit from my flying field and causing scratching inside a couple of my RO Jetts which Dub had to polish out upon rebuild. I'm not sure (with the Fox) about using an electric starter -whether tolerances allow the crank to grind on the backplate. It might depending on vintage and fits. I've learned NOT to use an electric starter in general but for sure on anything without ball bearings to take the shove back.

Dave

I have read other stuff about this "angle" in the crank/cylinder alignment, but I can't remember the fancy name either!! given the quality control at Fox through the later years, I would wonder if they could really hit that one degree angle with consistency. I think any swirling you might see is from the rod, given the way that Foxes run, on and off the throttle so to speak with the 4-2 break, and other surging in the 4 stroke mode. That would let the rod wonder a bit. In thinking about it, a light spring on the wrist pin would keep it in place, as long as you can move the rod back far enough to get it off and on when working on it. It's pretty tight as it is, so not sure that would work. One way to check crank pin clearance would be just to put a spot of masking tape on the face of the back plate, and then do the crank/prop sweep check on it. If it doesn't grab the tape, you know you have clearance Clarence !! A better indication of how much clearance you have could be found by putting a thick dab of clay or something similar on the crank pin, wax up the face of the back plate, then install the back plate and work it fore and aft without rotating it. Take the back plate off and see how much you squished the clay. Hotrodders and engine builders often us some stuff called Plasti-Gauge to check the clearance in rod bearings and crank shaft main bearings. Assemble the pieces with the material in place, torque everything to spec, and then remove it and measure with a caliper of micrometer. On a Fox .35 back plate I don't think you could get it to crush correctly.
Type at you later,
Dan McEntee

Brett Buck · « **Reply #19 on:** July 13, 2024, 09:43:11 PM »

Quote from: Dave_Trible on July 13, 2024, 10:54:19 AM

In my discussions with Andre Ming who worked at Fox he explained that the engines were designed with (and here I forgot the terminology) about 1 degree of 'offset'-my word, so that the rod would always work forward on the crankpin and not backward toward the backplate. I'm sure this was important given the shaft endplay.

I am sure Andre is saying what he knows, and maybe that was the intent, but, I can tell you for sure that sometimes, this doesn't happen, and it gets pushed off the back sometimes. This was the point of the Fox stuffer backplate invented by Marvin Denny, to be a close fit to the back of the crank to prevent the rod being pushed too far to the back in Foxberg racing. The surface was hard-anodized to make it a reasonable wear surface.

Even with that I have seen plenty of them where this is not enough, and the pressure apparently exceeds the yield strength of the anodizing and wears it away, then very quickly digging into the underlying aluminum.

Brett

Dave Hull · « **Reply #20 on:** July 13, 2024, 10:04:37 PM »

Here's perhaps a contender for "Most Chewed Fox Stunt Backplate." I forget what I measured the ground-down periphery at, but you could have done it with a 6" scale---no calipers needed!

The exploded Fox piston is just there photobombing. Not sure whether it came out of the same engine or not.

Dan McEntee · « **Reply #21 on:** July 14, 2024, 10:07:59 AM »

Quote from: Dave Hull on July 13, 2024, 10:04:37 PM

Here's perhaps a contender for "Most Chewed Fox Stunt Backplate." I forget what I measured the ground-down periphery at, but you could have done it with a 6" scale---no calipers needed!

The exploded Fox piston is just there photobombing. Not sure whether it came out of the same engine or not.

That back plate doesn't look like a stock, diecast back plate either!! You gotta wonder about some people sometimes!! That should have made enough metal to seize the engine!!

Type at you later,
Dan McEntee

Dave Hull · « **Reply #22 on:** July 14, 2024, 07:14:17 PM »

It is a stock (older) diecast backplate. If you look at the picture closely, you can see the rough mold parting line along the edge of the flange. I'm sure that it happened over a long period of operation. I received it along with other "junk" parts in case I could salvage enough pieces to build one good runner. (No steel washer behind the prop drive hub.) Here's how that ended up.

Brett Buck · « **Reply #23 on:** July 14, 2024, 08:15:14 PM »

Quote from: Dan McEntee on July 14, 2024, 10:07:59 AM

That back plate doesn't look like a stock, diecast back plate either!! You gotta wonder about some people sometimes!! That should have made enough metal to seize the engine!!

Type at you later,
Dan McEntee

You can see the mold parting lines, so I think it is the stocker. It's been ground away flat to the point it removed the radius at the "bottom" of the casting.

Brett

BillLee · « **Reply #24 on:** July 15, 2024, 10:01:17 AM »

Quote from: Dave Hull on July 13, 2024, 10:04:37 PM

Here's perhaps a contender for "Most Chewed Fox Stunt Backplate." I forget what I measured the ground-down periphery at, but you could have done it with a 6" scale---no calipers needed!

The exploded Fox piston is just there photobombing. Not sure whether it came out of the same engine or not.

Dave, do you think it was the rod or the crankpin that did the damage?

Lauri Malila · « **Reply #25 on:** July 15, 2024, 10:14:06 AM »

From what it looks like, it must be the rear face of conrod big end. Just the shaft pin would make a circular groove. L

Motorman · « **Reply #26 on:** July 15, 2024, 12:45:21 PM »

I have factory cast backplates with alloy steel plates held on with a flat head 6-32 nut/bolt in the center and assembled with JB weld. Haven't worn one out yet.

MM

Dave Hull · « **Reply #27 on:** July 15, 2024, 02:22:44 PM »

Bill,

Definitely the rod was making contact and doing the chewing. The slight tri-lobular shape carved into it was the result of the non-circular face of the big end of a stock Fox Stunt rod which only rotates thru a limited angle relative to the case. The rod in the picture (supplied as "loose parts" to me) was too clean to have been the culprit in this "backplate buffet."

The reports I got said that the rebuilt engine ran fine in a club member's reprisal of an old-time design by his childhood flying partner. Good times!

While I have not done the steel wearplate on any Fox engines (that I can recall; instead, just on one racing engine), Walter's suggestion makes perfectly good sense to me. The clearance in a Fox Stunt is extreme and does the rod no favors. On several engines I have assembled the bottom end to gauge the crankpin clearance. Then I lap the back of the case, and if it doesn't have any custom factory dings, I switch to a thin paper gasket (OS used to call them "vegetable paper," I like vellum) instead of the cork horror show and install a machined (not cast) backplate. Next, if you break the edge on the rod boss, it will act less like a scraper (think oil control rings) and help keep the castor barrier film intact. With these tweaks things seem to hold up better.

Dave

Dan McEntee · « **Reply #28 on:** July 15, 2024, 02:44:42 PM »

Quote from: Dave Hull on July 14, 2024, 07:14:17 PM

It is a stock (older) diecast backplate. If you look at the picture closely, you can see the rough mold parting line along the edge of the flange. I'm sure that it happened over a long period of operation. I received it along with other "junk" parts in case I could salvage enough pieces to build one good runner. (No steel washer behind the prop drive hub.) Here's how that ended up.

I gotta get new glasses!! I leaned in more towards the screen and could finally see the line. At normal distance, everything seems much more clean and square like it was all machined. One thing I thought of that I forgot to mention that may cause some one some problems, is the way that the prop drive keys onto the crank shaft. I have had to work the hubs over a bit if replacing them to get them to fit the square lugs correctly. In all of these I have had in my hands, fits range from good to terrible. Some one not knowing what they are doing could put the drive hub on, then prop, washer and nut, and if the hub is cockeyed a bit, it won't go on all the way and then there is all sorts of end play. In my early days of messing with Fox.35sI remember having some that were difficult to get seated, and they would only go on one way, and that is they way I thought they all were. This is another place where production tolerances and quality control can figure into whether you have a good time or not!!
Type at you later,
Dan McEntee

M Spencer · « **Reply #29 on:** July 15, 2024, 09:46:15 PM »

thye PROP DRIVER washer , scrolled , looks like around 58 there was a flat steel one with rounded edges radiused aft edge .
About 80 The .25 baffle had a STEEL one , a bit like a bigger die cast alloy sucker , a little bigger . but STEEL . BLACK they were .

The Alloy ones are like early Jap Motorcycles .

May suffer under duress or severe use , if capeable of it .

Al Ferraro · « **Reply #30 on:** July 16, 2024, 09:47:25 AM »

Quote from: Dave Hull on July 13, 2024, 10:04:37 PM

Here's perhaps a contender for "Most Chewed Fox Stunt Backplate." I forget what I measured the ground-down periphery at, but you could have done it with a 6" scale---no calipers needed!

The exploded Fox piston is just there photobombing. Not sure whether it came out of the same engine or not.

Is that the McSpeed piston mod?
Al

Dave Hull · « **Reply #31 on:** July 16, 2024, 09:23:00 PM »

Dunno if McSpeed was involved in that "reworked" piston. Puzzled me a lot when I first saw it. How do you explode the skirt? I could maybe see that happen in a model boat that did a huge nose dive and swallowed a slug of water thru the vespucci and broomstrawed the works on a downstroke...maybe? I woulda thought the crankweb would shear first. But then the rod would have been gone. And, there were no other related parts from the dirty deed, so we are left to guess. Post your best, most complicated theory.

If you're doing Unlimited Fox Speed, you might want to try lightening the piston like that. I can save you some work, though. That exact piston, with custom rework already started is resident out (waaaay out) on LongGuyland. After that, just get handy with the JB and make a triple-wart Dooley case. Then.....

McDivot

Motorman · « **Reply #32 on:** July 18, 2024, 07:53:04 PM »

Looks like the piston was hitting the top of the back plate. If you do want to make a steel washer to take up the slop in the shaft just watch you don't get it locked between the shaft and the drive washer when changing props.

Brett Buck · « **Reply #33 on:** July 18, 2024, 10:35:42 PM »

Quote from: Motorman on July 18, 2024, 07:53:04 PM

Looks like the piston was hitting the top of the back plate. If you do want to make a steel washer to take up the slop in the shaft just watch you don't get it locked between the shaft and the drive washer when changing props.

That was my guess, too, but if it was stock, it has plenty of clearance.

Brett

Dave Hull · « **Reply #34 on:** July 19, 2024, 12:37:22 AM »

And since there is so much clearance the backplates don't have a recess or notch so there is no way to install it wrong resulting in contact. So, let's try this idea: someone was running it hard and the crankpin (or disk) let go and the momentum of the piston on the downstroke crashed into the backplate?