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Author Topic: JB Weld on fox 35 mounting holes  (Read 891 times)

Offline mccoy40

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JB Weld on fox 35 mounting holes
« on: January 09, 2022, 11:33:12 AM »
Hi all,

I did a search on JB weld in the forum and found a lot of good info out there but nothing particular to this one subject.

I have a fox 35 stunt that has had the mounting holes enlarged, and overly enlarged to about a size 10 bolt.  I would like to put jb wed or a combo of JB weld and Brass tubing to get the whole diameter down to a 4-40 size. I am just wondering if JB weld would be strong enough for this application.

My other thought was if it would not work then I will make a mounting clamp arrangement similar to what Windy Urtnowski used for his engines.   S?P

All advice is appreciated.

 H^^ 
Joseph Meyer
Philadelphia, PA

Offline john e. holliday

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Re: JB Weld on fox 35 mounting holes
« Reply #1 on: January 09, 2022, 12:41:21 PM »
My self I just use brass tubing to fill the holes. D>K
John E. "DOC" Holliday
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Shawnee, KANSAS  66203
AMA 23530  Have fun as I have and I am still breaking a record.

Offline Robert Zambelli

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Re: JB Weld on fox 35 mounting holes
« Reply #2 on: January 09, 2022, 01:38:40 PM »
McCoy, I've done it a few times - works just fine.

Bob Z.

Online Paul Smith

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Re: JB Weld on fox 35 mounting holes
« Reply #3 on: January 09, 2022, 08:08:31 PM »
Hi all,

I did a search on JB weld in the forum and found a lot of good info out there but nothing particular to this one subject.

I have a fox 35 stunt that has had the mounting holes enlarged, and overly enlarged to about a size 10 bolt.  I would like to put jb wed or a combo of JB weld and Brass tubing to get the whole diameter down to a 4-40 size. I am just wondering if JB weld would be strong enough for this application.

My other thought was if it would not work then I will make a mounting clamp arrangement similar to what Windy Urtnowski used for his engines.   S?P

All advice is appreciated.

 H^^

In the application you described, the JB won't be in any particular stress.  The tubing inserts will effectively downsize the holes, even without the glue.
Paul Smith

Offline Dave Hull

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Re: JB Weld on fox 35 mounting holes
« Reply #4 on: January 10, 2022, 12:03:26 AM »
Wow.  A #10 screw is nominally .190" diameter. That doesn't leave much meat on the lugs near the edges--probably about .065" based on a quick measurement here. A standard OD for a flat washer is .320" diameter, and that will go just about tangent to the outside of the lugs. But the problem is in "coning" the washer. They are pretty thin and you can see from my scaled sketch that without the tubing, you will be bearing on the edges of your metal-filled epoxy. That would work for a while, but it wouldn't surprise me if the screws kept losing preload as the epoxy creeps underneath it. Doubling up the washers would help; a thicker single washer would be better. I had a pair of steel pieces that were like a long rectangular "washer" that had both holes in it. That worked well. It's not the same as an engine clamp, which, as you mentioned would be another choice. Sleeving it with tubing such as the K&S 5/32" x .014" wall in brass would sure help if you can dress the tubing flush with both sides of the lug.

In the sketch, I penciled in the tubing and filler bond on the one side to see what that would look like.

If you look at the load path for the clamping force, it develops under the screwhead, is transferred thru the thickness of the washer and then--if the tubing is dressed flush with the top of the lug down into the engine mount. For a wood mount, the contact stress (compression) of just the end of the tube would be very high if there was no bond effectiveness or load transfer from the tubing thru the bond into the lug. So the bond joint is in shear, with an approximate area of .096 in^2. If tightened to about 5 in-lb of torque, resulting in approximately 240 lb of preload, the shear stress on the bond is around 2.5ksi. The JB Weld should be good for that as long as it doesn't get too hot. So should you use a sleeve? Yes! Should you clean everything really well and bond it in? Yes!

One more comment regarding the JB Weld material. I have found that it is more long-lived in a repair around fuel and oil if the material is left with the "skin" on it. If you cut or file the stuff after it cures, it seems to open up pathways thru the filler material letting the fuel attack it more aggressively. I have done plenty of venturi repairs, sleeves, etc and they hold up better with the skin left on.

Dave

Offline Peter in Fairfax, VA

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Re: JB Weld on fox 35 mounting holes
« Reply #5 on: January 10, 2022, 07:35:42 AM »
The JB Weld is not a bad idea, though the "long rectangular spanning washer serving as a clamp" sound even better.

Based on my experience with the Brodak motor mount system, which has been good, I've learned that pretty much any motor mounting method relies on clamping, not pinning.  If engine mounting bolts are so loose that the lugs contact the threads of the bolts, the engine is likely loose.

Offline mccoy40

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Re: JB Weld on fox 35 mounting holes
« Reply #6 on: January 10, 2022, 08:19:38 AM »
Thanks All!

Yep it is a number ten screw with wiggle room on the number ten in the hole.

I will definitely use the flat washer concept as well on the repair.   :)!

Thanks for the info on the JB Weld.   <=  I didn't want to start the repair, finish it, and then mount it and find the engine flying off in mid flight.

Another Question, why would anyone drill out the mounting holes to this extent? What possible benefit could you gain from doing something like this?   D>K
Joseph Meyer
Philadelphia, PA

Offline Dan McEntee

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Re: JB Weld on fox 35 mounting holes
« Reply #7 on: January 10, 2022, 09:55:36 AM »
Thanks All!

Yep it is a number ten screw with wiggle room on the number ten in the hole.

I will definitely use the flat washer concept as well on the repair.   :)!

Thanks for the info on the JB Weld.   <=  I didn't want to start the repair, finish it, and then mount it and find the engine flying off in mid flight.

Another Question, why would anyone drill out the mounting holes to this extent? What possible benefit could you gain from doing something like this?   D>K

         It allows them to mount the engine with 4-40s, and if they get the holed for the bolts out of position, then cam wiggle the engine around to get it straight. Some people have problems with being as precise as you need to be to get the mounting holes correct. Depending on the size of the engine, some don't think that 4-40 screw is string enough to hold the ST.46 or what ever they are mounting, and then it becomes a combination of needing bigger holes for bigger bolts, and the bigger engine holes to help with alignment.
    Type at you later,
    Dan McEntee
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Online Dave Rigotti

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Re: JB Weld on fox 35 mounting holes
« Reply #8 on: January 10, 2022, 10:01:44 AM »
OR..... They used lag/wood screws instead of bolts?
Dave Rigotti
AMA 66859
Chesterland, Ohio

Offline Dan McEntee

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Re: JB Weld on fox 35 mounting holes
« Reply #9 on: January 10, 2022, 12:33:41 PM »
OR..... They used lag/wood screws instead of bolts?

  YES!! I have seen that before also!

    Dan McEntee
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Online Ken Culbertson

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Re: JB Weld on fox 35 mounting holes
« Reply #10 on: January 10, 2022, 12:38:40 PM »
OR..... They used lag/wood screws instead of bolts?
In my IC days I drilled out the holes in everything to take a 6-32 then used 4-40's for precisely the reason Dan stated, to be able to move the motor around a small amount.  I used Hex head bolts, double washers with a lock washer between them.  I never had any problems.  Maybe I was just lucky.  I also tightened (not over tightened) the bolts before every flying session as part of my bench preflight the night before.  I do the same check with electric.

Ken
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