Advice for an odd case of stripped holes

Seaturtle · Jan 28, 2020

Solved: Senior machinist set it right, it was 5/16 24. The welder was way off guessing 1/4 24 and I was closer but still incorrect with M8x1.
TLDR: crossthreaded hole on a french airplane wheel. Metric bolt reads 7.6mm (0.299) with 24 tpi. Best core diameter read we got was .276 in. He's planning to use a 1/4 24 tap to try to right it. Is this correct?

I was asked by my welder coworker to look at a cross-threaded part of an airplane wheel. The bolt that fit the measurement was 24 in pitch, 0.309 in diameter or 7.6mm since it's a French part. The print he has shows no proper information on the dimensions of the hole or the bolt's size/tpi.

We ran into issues looking for a tap/drill chart that would include a 7.6/0.299 measurement tap so instead of risking a 8x1 tap with an estimated 26 tpi, .315 metric tap, he's going to go with the path of least resistance and use a 1/4 24 tap and clear out the core diameter at least.

My question is, will this be enough to clear the cross-threaded hole? I thought the thread diameter is the part that fixes a cross-thread. Are there any other options we may have forgot to consider?

jz79 · Jan 28, 2020

if it is mertric, then it won't be 24tpi but 1mm pitch, M8x1.00 is the fine pitch M8 thread, that is the tap you need to use, 24tpi = 1.06mm pitch, on 1D worth of threads that is already error of half the pitch, what sort of "repair" would that be then?

Scottl · Jan 28, 2020

When I see a phrase like "cross-threaded part of an airplane wheel" it raises alarm flags. If this is to go on an aircraft that actually flies rather than a museum display I would advise using a lot of caution.

First of all I don't think using a 1/4-24 tap on a cross-threaded hole of that size will create proper full thread. I think you'll either have to go larger or use an insert.

That 0.309 diameter 24 pitch bolt almost sounds like a worn 5/16-24 so I wonder if somebody already substituted a non-metric part in the past. I would have a careful conversation with the wheel's owner before doing anything and if the part is intended to fly you may need to consult the FAA.

EDIT: I'm changing my opinion. After measuring an assortment of 5/16-24 bolts I got a range of 0.305 to 0.309 so I'm going to say the bolt IS a 5/16-24. The 0.309 measurement was on some high grade Allen bolts while the smaller ones were plated consumer grade ones.

You MIGHT be able to use a 5/16-24 thread forming tap but I'd still consult with the owner first.

Seaturtle · Jan 28, 2020

jz79 said:
if it is mertric, then it won't be 24tpi but 1mm pitch, M8x1.00 is the fine pitch M8 thread, that is the tap you need to use,

I think this is the right idea, though I'll still ask a superior's opinion. I'll tell the welder to order an M8x1 as well. Thankfully this is not an end-of-the-day job.

EDIT: So i double checked and the senior machinist agreed it's a 1/4 24 tap. I'll happily side with their view.

Mtndew · Jan 28, 2020

Seaturtle said:
I think this is the right idea, though I'll still ask a superior's opinion. I'll tell the welder to order an M8x1 as well. Thankfully this is not an end-of-the-day job.

EDIT: So i double checked and the senior machinist agreed it's a 1/4 24 tap. I'll happily side with their view.

1/4-24 is a pretty uncommon thread size FYI.

Rob F. · Jan 28, 2020

Wouldn't m6x1 be really close to 1/4-24?

DDoug · Jan 28, 2020

More important....where to get AN approved hardware for such odd sizing ?

Rudd · Jan 28, 2020

And who is willing to put their A&P stamp on the repair....
Nuts.

Dan from Oakland · Jan 28, 2020

Some of your dimensions are very close to ISO 4570 Tire valve screw threads. Before you go much farther, check this link out:

MARYLAND METRICS -- THREAD DATA CHARTS (26)

310 Guy · Jan 28, 2020

FAA certified A&P mechanic with Inspection Authorization here...
My head just exploded. You're going to fix a metric bolt/hole on a French (metric) airplane with an inch-sized tap/drill? Please... don't.

I know of no polite way to say this, but your description of metric thread pitch as "tpi" tells me that you are in over your head. Please find someone who is familiar with metric dimensions AND airplanes to complete this repair.
Bill

guest · Jan 28, 2020

Um, if the minor is .276, a 1/4-24 tap isn't gonna touch anything anyway.

I'm of the opinion that it's a 5/16-24. Inch dimensions are the standard for aircraft, even French ones...

janvanruth · Jan 28, 2020

7.6 mm for a m8x1 mm bolt?
i doubt it very much...

guest · Jan 28, 2020

Mtndew said:
1/4-24 is a pretty uncommon thread size FYI.

It would be a special order everything.

And it would make no sense- on an airplane they would use a 1/4-28 if they wanted a 1/4" fastener.

tdmidget · Jan 28, 2020

Unless this airplane has only1 wheel held on by that one screw (possible if it is indeed French, I suppose) then see what the others are. In any case when I worked on aircraft parts you had damn well better have the router/work order, the latest revision of the drawing, the repair procedure, and any material certs at the machine. Sounds like you need to fall back and regroup.

john.k · Jan 28, 2020

Harley Davidson used 1/4 x 24 ,...is the bolt in a "chopper"?

DDoug · Jan 28, 2020

310 Guy said:
FAA certified A&P mechanic with Inspection Authorization here...
My head just exploded. You're going to fix a metric bolt/hole on a French (metric) airplane with an inch-sized tap/drill? Please... don't.

I know of no polite way to say this, but your description of metric thread pitch as "tpi" tells me that you are in over your head. Please find someone who is familiar with metric dimensions AND airplanes to complete this repair.
Bill

Naw....Next post will be "It's for a homebuilt, so we can just use grade 8 bolts, their just as strong as you know."
No worries Man.

Hardplates · Jan 28, 2020

If it were a boat you could use flex seal.

It's not an amphibious plane is it?

DDoug · Jan 28, 2020

Hardplates said:
If it were a boat you could use flex seal.

It's not an amphibious plane is it?

Without the tires, me thinks that's all it is.

Or this:
carnival plane - general for sale - by owner

Hardplates · Jan 28, 2020

digger doug said:
Without the tires, me thinks that's all it is.

Or this:
carnival plane - general for sale - by owner

That's a real bobby dazzler, better snatch it up before I come buy it or maybe trade him my blunderbuss.

steve45 · Jan 28, 2020

What kind of airplane? Who manufactured the wheel?

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