reducing thread size

[ghostlymoron]

Is it possible to reduce a 8mm threaded hole to take a 6mm bolt? Helicoil or something?

 

[DinghyMan]

it depends... If its 8mm fine, 8mm x 1mm pitch, you could as the 6mm inserts would be 6mm x 1mm pitch, and would fit - going to double check when I go down to the workshop later today as I think I have both sizes of helicoil - but more likely it will be 8mm x 1.5mm pitch so wouldn't work as you would be trying to put a 1mm pitch inserts into an 1.5mm threaded hole

I'm guessing you can't do it the other way around and use a 8mm bolt?

 

[Skylark]

There are a number of ways to reduce M8 to M6 but they usually require access to workshop equipment. Why do you want to do it? Will M6 be strong enough? What's the material and application? Can you take the part in question to a workshop?

Fit an M8 into the thread, cut and file smooth, drill new hole for M6 tap.
Over size the hole, plug and weld then redrill.
If access allows, fit longer bolt, M6 penny washer with M6 nut at the back

An M6 helicoil is too small for an existing M8 hole

 

[PCUK]

Drill the hole oversize and then fit a 6mm rivnut?

 

[DinghyMan]

it depends... If its 8mm fine, 8mm x 1mm pitch, you could as the 6mm inserts would be 6mm x 1mm pitch, and would fit - going to double check when I go down to the workshop later today as I think I have both sizes of helicoil - but more likely it will be 8mm x 1.5mm pitch so wouldn't work as you would be trying to put a 1mm pitch inserts into an 1.5mm threaded hole

I'm guessing you can't do it the other way around and use a 8mm bolt?

I checked a 8mm x 1mm tap against the 6mm helicoil tap and the helicoil tap is about 0.3mm smaller so reckon 6mm x 1mm helicoil into 8mm x 1mm hole would work, but the idea of drill it out and pop in a rivnut is probably the easiest way to go, and no special tools needed either