Drill bit size for M10 x 1.25 helicoil

[Craig-SM]

I have a M10 x 1.5 helicoil set. I need to fit a 10 x 1.25 helicoil so just checking the drill bit 10.4mm would still work with it being a finer thread.

 

[hotponyshoes]

10.25 is the recommended size.
Depends how critical the job is I suppose!

 

[Craig-SM]

10.25 is the recommended size.
Depends how critical the job is I suppose!

it’s for a gearbox crossmember mount that’s captive in the chassis with no access other than the hole in the nut so prefer not to cock it up. I'll get a drill bit ordered as better safe than turning the air blue with expletives

 

[hotponyshoes]

Just remember that the difference between a 10mm and a 10.25mm drill bit is only 0.1mm each side of the hole.
That's reamer tolerance territory and, even then, you would need specialised measuring tools to check it.

I'd be trying a 10mm drill bit and seeing how the tap feels when I thread it...

 

[tom2207]

run the drill you have and hold it against a flap disc , stationary of course ,,, then throw the mic on it and repeat , you will soon get it to the size you want.

 

[graffian]

Go to the WTI site, there is a catlogue of inserts with all the dimensions, if they are not in other tap makers catalogues.
Don't the taps have the drill size on them?

 

[MetalMonkey]

I'd be trying a 10mm drill bit and seeing how the tap feels when I thread it...

I'd second this - presumably you are hand drilling it, so it's going to end up slightly oversize anyway

 

[beeRS]

I'd be trying a 10mm drill bit and seeing how the tap feels when I thread it...

I’ll third that. When hand drilling the tolerances will be greater anyway.

 

[timgunn1962]

10.2mm is the usual tapping size for M12, if you have a metric drill and tap set knocking about.

 

[Craig-SM]

Cheers all. I’ve ordered a 10.2 but will try a 10 first and see if I can tap that first before opening it up any further.
Yes, it will be hand drilled so probably not as accurate. Probably being over cautious due its location and the ball ache it will be if I cock it up.

 

[grim_d]

I did some testing a few years ago in aluminium and found basically that a 50% reduction in recommended insert thread engagement was still more than enough to shear the bolts inserted into them.

Of course using the right size is preferred but there is some considerable room for error.