Board logo

retap 3/8 to m10
MikeR - 14/7/22 at 08:32 AM

I've messed up. Bought a custom (lovely) gearnob and ordered 3/8 instead of m10.

As this wasn't cheap wanted to check before I bugger it up. Any reason why I'd struggle drilling and tapping to m10?
It's Ali with loads of thickness on the gearnob, so the question is about thread sizes. I think it's fine. Just want reassurance/ obvious hints and tips first.

(Measure twice before ordering isn't the hint I'm looking for)

Theshed - 14/7/22 at 09:15 AM

How annoying!

I think you will struggle to put a half decent thread in the gear knob. The outer diameter of the existing threads is 9.5mm or thereabouts. The pitch is 24 threads per inch so much finer than M10. Much of the existing thread will interfere with your M10 thread.

That said..... If you give it a go and use an epoxy to bond it on you will probably get away with it.

Alternatively make an extension.

Good luck

monkeyarms - 14/7/22 at 12:30 PM

Drill it out to 12mm, press fit some solid 12mm bar in. The drill & tap M10 ?

gremlin1234 - 14/7/22 at 12:41 PM

quote:
Originally posted by monkeyarms
Drill it out to 12mm, press fit some solid 12mm bar in. The drill & tap M10 ?


or m10 helicoil
https://www.screwfix.com/p/helicoil-eco-thread-repair-kit-m10-x-1-5mm-14-pieces/265fr

monkeyarms - 14/7/22 at 01:25 PM

Not very "lococost" is it

gremlin1234 - 14/7/22 at 02:32 PM

quote:
Originally posted by monkeyarms
Not very "lococost" is it

while the full helicoil kit I linked to, is not locost for a single job, the principle is sound.

I guess, the locost way would be drill out to 12mm, fill with epoxy, and glue it in place
if you will want to remove it later grease the threads to avoid them sticking

big_wasa - 14/7/22 at 03:21 PM

Cut an inch of the gearstick and re thread that.

coyoteboy - 14/7/22 at 03:57 PM

helicoil or keensert. You just need the tap and the insert, the tool can be locosted at home.

Or be super cheap, drill out the existing thread to M10 outer, fill the void with metal filled epoxy, drill it back out to M10 tapping diam and hope an epoxy thread is enough to handle your grip strength.

[Edited on 14/7/22 by coyoteboy]

indykid - 14/7/22 at 05:20 PM

Is it 3/8 UNC or UNF? Also how much internal thread is there? UNC is 16TPI, UNF is 24TPI

A retapped thread will be stronger than any suggestion of drilling out and forming an epoxy thread. If you're concerned, do both but don't drill it out first!

You should have no trouble tapping M10x1.5 without redrilling the pilot hole.

gremlin1234 - 14/7/22 at 07:05 PM

quote:
Originally posted by indykid
Is it 3/8 UNC or UNF? Also how much internal thread is there? UNC is 16TPI, UNF is 24TPI

A retapped thread will be stronger than any suggestion of drilling out and forming an epoxy thread. If you're concerned, do both but don't drill it out first!

You should have no trouble tapping M10x1.5 without redrilling the pilot hole.


I do agree!, try retapping first, and if that does not work, only then, use one of the alternatives mentioned above

edit sp

[Edited on 14/7/22 by gremlin1234]

Simon - 15/7/22 at 09:48 AM

I'd try running the tap in before drilling...

coyoteboy - 18/7/22 at 08:49 AM

Ugh kludge a dodgy thread through a bunch of other threads as preferable to a bonded thread as per high load concrete anchor bolts, are you guys bananas?

This is a gear stick knob, it just needs to be screwed on and not move, no-one's swinging off it, it's not taking high loads. Kludging threads through other threads just minimises the engaged thread area, maximises the chances of cross threading and stripping.

Meh, do as you wish.

indykid - 18/7/22 at 11:03 AM

quote:
Originally posted by coyoteboy
Ugh kludge a dodgy thread through a bunch of other threads as preferable to a bonded thread as per high load concrete anchor bolts, are you guys bananas?

This is a gear stick knob, it just needs to be screwed on and not move, no-one's swinging off it, it's not taking high loads. Kludging threads through other threads just minimises the engaged thread area, maximises the chances of cross threading and stripping.

Meh, do as you wish.


Yesterday I drilled 2off 21/64" holes (for worst case engagement) in some 6mm ally plate (alloy unknown but probably 6082) and tapped 3/8 UNC then retapped M10x1.5. With an M10 screw, I can't strip the thread applying sensible torque (probably ~20Nm). One was pretty much dead on for matching the thread start, forming almost full threads, the other was more like 180° out so worst case for short threads.

In an inch of thread, 1.5mm pitch (16.933TPI) will produce one more thread than 16TPI so in half an inch of thread, it will likely generate something approaching both cases regardless of how the thread starts. I'd expect a gearknob would have at least 3/4" of thread, so you're almost guaranteed to get full form threads at some point in the engaged length.

Concrete anchor epoxy relies on the surface roughness of the inside of the hole and a large bond area. That's also why the embedded length of studs are threaded or knurled - it's a cost on operation which implies it's necessary, otherwise, screwfix would be selling round shank studs. The specific tensile strength isn't massive, but the significant shear area and mechanical interlocking gives the joint its overall strength. It's not ideal for short smooth bores.

Also protecting the ability to remove the gearknob is pretty handy.

coyoteboy - 19/7/22 at 08:37 AM

quote:
Originally posted by indykid

Concrete anchor epoxy relies on the surface roughness of the inside of the hole and a large bond area. That's also why the embedded length of studs are threaded or knurled - it's a cost on operation which implies it's necessary, otherwise, screwfix would be selling round shank studs. The specific tensile strength isn't massive, but the significant shear area and mechanical interlocking gives the joint its overall strength. It's not ideal for short smooth bores.

Also protecting the ability to remove the gearknob is pretty handy.


As I said, do as you wish. If properly prepped, I guarantee you can hang yourself from a 20mm engagement of epoxy in a smooth steel bore. And if done properly, you can happily unscrew the knob afterward - light oil on the thread surface causes it to release immediately. There's chuff all need for surface roughness on the scale of rock surface roughness and knurled metalwork, it's not a mechanical bond unless your chemical bond has failed.

I've used it a lot for small medium-load fasteners quite a lot (6mm diam, 20mm engagement).

All of this is total overkill, it's a sodding gear knob, it will stick on with chewing gum and spit.

[Edited on 19/7/22 by coyoteboy]