Author Topic: FOX.35 Crankshaft (Read 4778 times)

Steve Tomlinson · « **on:** October 07, 2020, 12:30:18 PM »

I already Checked MECOA and scoured the web:
Any body have a Fox.35 Stunt Crankshaft?
The shaft in an old one I picked up is good, but I think it was cut down, not allowing ant threads to protrude past the nut with a .500 thick prop hub.

In fact I had a prop fly off before I diagnosed the issue, got 5 stitches in my hand as a result!
Ok , but dont want to go "there" again!

Let me know.

john e. holliday · « **Reply #1 on:** October 07, 2020, 12:36:57 PM »

Have you tried one of the prop nuts that extend into the prop nut. They are used for extensions on some spinners.

By the way if you don't I have one for you to try, just PM address to me.

Tony Drago · « **Reply #2 on:** October 07, 2020, 12:41:43 PM »

I don't think you will ever see a Fox 35 crank from Mecoa any time soon.

Steve Tomlinson · « **Reply #3 on:** October 07, 2020, 01:24:15 PM »

John E Holiday: yes i did still only gets a couple of threads oof engagement. I thought I might try a simple flat machine washer under the nut in front of the prop [provided washer has some " thickness" to achieve more engagement. I think the previous owner faced off the threads maybe because of crash damage.... I can provide measurements if needed

Steve Tomlinson · « **Reply #4 on:** October 07, 2020, 01:25:31 PM »

Correct Tony D thats why I posted this.

Andre Ming · « **Reply #5 on:** October 07, 2020, 02:24:39 PM »

If it's desperate, then look for a 35 Stunt on the Bay and gut it for parts. All the parts will interchange (assuming it's a 6-bolt head/3-bolt rear).

Here's a link to one on the Bay now:

https://www.ebay.com/itm/Fox-35-Control-line-motor-vintage-classic-motor-Excellent-condition/324322687395?hash=item4b832379a3:g:StMAAOSwy2NffN9T

Andre

Jim Kraft · « **Reply #6 on:** October 08, 2020, 10:48:17 AM »

I have an early sixties Fox Stunt 35 that I bought new, and it has much shorter crank threads than the later ones I have.

bobsrc · « **Reply #7 on:** October 08, 2020, 11:10:20 AM »

What you need is a 1/4-28 well nut and washer.

john e. holliday · « **Reply #8 on:** October 08, 2020, 01:18:51 PM »

If you have threads visible in front of the prop, the nut and washer is what you need that I have. I don't use spinners much any more. You do have to ream the prop for it though.

Steve Tomlinson · « **Reply #9 on:** October 08, 2020, 03:31:07 PM »

All respondents
For reference:
Presently, when I use a prop with hub thickness of .480-.500, there is not enough thread engagement of the nut [ no threads visible beyond nut face]. I consider a minimum of two exposed threads " Safe"

Some PRESENT dimensions:
From flat face of crank to end of thread
.959 ( no drive washer), aft of drive lugs

From face of knurled prop drive washer to end of thread
.830

Thickness of prop nut
.238

Thickness of prop [front] washer
.150

Maximum prop hub thickness with two threads exposed, measured between the drive washer and prop washer
~.357

All measurements taken with Starrett dial vernier
Bill Lee responded and after we compared measurement of mine and a new crank, we concluded that mine was ground down [ shortened]
I have a NEW CRANK ON THE WAY, present one is UNSAFE I HAVE SIX STITCHES IN MY HAND TO PROVE IT

Brett Buck · « **Reply #10 on:** October 08, 2020, 05:38:38 PM »

Quote from: Steve Tomlinson on October 08, 2020, 03:31:07 PM

All measurements taken with Starrett dial vernier
Bill Lee responded and after we compared measurement of mine and a new crank, we concluded that mine was ground down [ shortened]
I have a NEW CRANK ON THE WAY, present one is UNSAFE I HAVE SIX STITCHES IN MY HAND TO PROVE IT

That's a good solution, if you can find one for cheap.

However, as noted above, you can use the one you have safely if you use a nut that projects through the washer and part of the hub. The nut and washer from a shaft extension, for example, would engage all threads, and you probably have to cut it off or it would bottom out. The downside is that you have to open up the hole in the prop to 5/16" to clear it.

The only problem is that it might be harder to find a prop extension now, than it is to find a new crankshaft!

Brett

James Lee · « **Reply #11 on:** October 08, 2020, 06:24:24 PM »

Lee Machine Shop has prop extensions...
Jim

Brett Buck · « **Reply #12 on:** October 08, 2020, 06:48:22 PM »

Quote from: James Lee on October 08, 2020, 06:24:24 PM

Lee Machine Shop has prop extensions...
Jim

Excellent. I will send you an email later about a related topic.

Brett

Dave_Trible · « **Reply #13 on:** October 09, 2020, 06:21:31 AM »

If you still need a crank I think I can find one in my junk box.

Dave

John Given · « **Reply #14 on:** October 10, 2020, 07:21:03 AM »

Once the nut is fully engaged no other stud length is needed. Any “visible” threads are simply added weight. The visible threads may have a “feel good effect”, but they offer nothing in functionality. Proper assembly and torque are all that are required. If the nut is coming loose it indicates a problem other than insufficient thread engagement.

pmackenzie · « **Reply #15 on:** October 10, 2020, 08:14:33 AM »

FWIW, standard practice in full scale is to have one to three threads showing. No more, no less.

https://www.experimentalaircraft.info/articles/aircraft-building-1.php

Steve Tomlinson · « **Reply #16 on:** October 10, 2020, 10:42:12 AM »

All: the nut fell short by over one thread for full engagement. Its not a " feel good" problem, good engineering practice calls for ALL THREADS of a nut to be engaged with the mating threads AND at least 1-2 threads protruding after full torque. I have NONE of these things and as a result I have 6 stitches and a 3 1/2 hour visit to the ER to prove it. My fault I knew better. Word to the wise. Not " feel good"

John Given · « **Reply #17 on:** October 10, 2020, 11:54:47 AM »

The ugly truth about prop nuts is that they rarely follow the accepted best practice of 1d engagement. That is the length of thread engagement should be equal to or greater than the diameter of the fastener. A .25” diameter shaft should have a .25” deep nut. Most prop nuts are nowhere near that. This puts greater stress on the system.

Dan McEntee · « **Reply #18 on:** October 10, 2020, 03:05:42 PM »

Get your replacement crank shaft and see what you have then. All the greta Fox experts are gond now it seems, but they could tell you what early versions used a fairly think prop drive washer that still engaged those four lugs on the crank shaft. I'm pretty sure they were a bit thinner than later model engines and that may get you a few more threads. different brand props have different hub thicknesses also Master Airscrew wood props used to have the thickest hubs. Look around your shop or prop[ box and see what else you have that may work.
Type at you later,
Dan McEntee

De Hill · « **Reply #19 on:** October 10, 2020, 03:49:40 PM »

A thin plain prop washer and an O&R drive washer might help you. The O&R drive washer is much thinner than the Fox d/w.

BillLee · « **Reply #20 on:** October 10, 2020, 04:58:38 PM »

A new replacement crank will be in Monday's mail to Steve.

Regards,

Bill Lee

Well, Tuesday since Monday is a holiday! :-)

pat king · « **Reply #21 on:** October 11, 2020, 08:36:31 AM »

In SAE a finished hex nut is .8D thick. The materials of the male part and the nut being equal the male fastener will fail first in a tensile test. The practice in tapped holes is to have about 1.5D engagement to allow for poor threads. 1.5D engagement of a Grade 8 bolt in mild steel will pull the bolt apart in a tensile test. that is with a 75% thread in the tapped part. I agree that I would not be comfortable with a situation where the male part did not at least come flush with the nut. In hardened tool steel parts with small threads (1/4" and less) I have often used 30 to 35% threads. That will pull a 180,000 PSI bolt in two with 1D engagement. Trying to tap a small hole in some tool steels at 75% thread is asking to break a tap. It can be done, but makes no sense.

Pat