The kitfox "Jesus Bolt"

[jrevens]

I made new blocks when I wasn’t happy with the fit I had gotten on the originals, and I did indeed use Delrin. I believe that’s a great material for this application. There is really not a lot of force on that bearing and it is a relatively large contact surface. As far as outdoor exposure is concerned, the issue is long-term UV exposure. That’s a complete non-issue with floorboards in place. Your aluminum parts do look very nice, and I would recommend perhaps some bonded dry lubricant on contact surfaces. I just wouldn’t want builders to have unwarranted concerns about strength of that part. As I believe has been stated previously, I don’t think that there has ever been a reported catastrophic failure of that bearing from any of the thousands of Kitfox aircraft that have been built.

[AvDES LLC]

At the end of the day that’s the point of this. The increase in strength is merely a justification to ensure I’m not replacing a part with a weaker one. Second, the point of owning the plane is to make it “mine”. Small parts that have high “flash value” are a great means to attract eyeballs to an airplane at an airshow / fly-in. Again, I agree with the previous statements from those that have stated they have never seen a failure etc etc. - I haven’t either, but if the plane is completely stripped down for rebuild, why not take advantage of that situation to create custom finishing parts.

[txmatt]

This is (was) the bushing in my Model 4. The plastic was very crumbly, though there was enough of it left so that the control column wouldn't pop out.


bushing.jpg

[AvDES LLC]

Well looks like we’ve all witnessed our first failure. Glad you made it back safe from that - I bet that was a pucker factor 5 moment.

Aluminum ones are gone or I’d offer you a freebee - I am running a few out of carbon fiber with a small inlaid groove for a delrin insert so it too will be self lubricating ( even though range of motion on this thing shouldn’t really need large amounts of looob).

whenever I finish those up and if they function as intended I’ll happily send you one for the fee of a bit of feedback if you at all need one still.

[jrevens]

This is (was) the bushing in my Model 4. The plastic was very crumbly, though there was enough of it left so that the control column wouldn't pop out.

It appears that your assembly may have been exposed rather than covered like most... is that correct? Delrin doesn't normally "age" and get brittle, except with long-term exposure to sunlight. Then again, it might not even be Delrin (just a possibility?). Glad you caught it... be kind of hard to miss. There is no real upward force on that bearing, unless you pull on the stick.

[txmatt]

Well looks like we’ve all witnessed our first failure. Glad you made it back safe from that - I bet that was a pucker factor 5 moment.

Aluminum ones are gone or I’d offer you a freebee.

Thank you very much for the kind offer. I made my replacement out of UHMW polyethylene.

IMG_0226.jpg

[txmatt]

It appears that your assembly may have been exposed rather than covered like most... is that correct?.

The manual I have just calls for polyethylene, which is what I assume the original was made out of and is what I used for the replacement. I did some googling but didn't find much info, I assume because these aren't typically a problem. This thread and all the talk of Delrin has me wondering though... Is that what is currently being used? Perhaps there was a change along the way.

I'm not sure what you mean by covered? Perhaps that's a post-4 thing? This assembly sits on top of the floor board in my Model 4. As far as I know the plane was always hangared, so it wasn't exposed in that sense.

[jiott]

The new Delrin bearing block has several advantages over UHMW polyethylene:

- It is a simple rectangular shape, not machined down in the bolt boss areas. This gives substantially more wall thickness in the areas where yours broke.
- Delrin is 3 times stronger in tensile strength; 10,000 psi vs. 3100 psi.
- Delrin is over 20 times stiffer; 420 vs. 110,000 flexural modulus.

Its one of the things that has definitely been improved over the early Kitfox models.

[jrevens]

I'm not sure what you mean by covered? Perhaps that's a post-4 thing? This assembly sits on top of the floor board in my Model 4. As far as I know the plane was always hangared, so it wasn't exposed in that sense.

It is under the rear floorboards on the newer series.

[n85ae]

Interesting it cracked like that, probably as was suggested it was in the open and exposed to UV too long
and became brittle. The real Jesus bolt is the one holding the lift struts on, but I've never heard of one of
those shearing ...

Jeff