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MG TD TF 1500 - Helicoil or Retap?

I drilled out a broken locator bolt for the layshaft in a TF gearbox. In doing so, I shaved the threads enough to not grip the locator bolt.

>> Should I have a helicoil installed in the hole to accept the original M6 fine thread locator bolt? Is fine thread helicoil available? Is there enough cast iron to allow this? Any suggestions on having one installed?

OR

>> Should I drill and tap for an M8 fine thread bolt? I would need to machine the tip to match the original locator bolt? This will drive the next owner crazy if he tries to replace the locator bolt.

Any other suggestions?

Thanks,

Lonnie
TF7211
LM Cook

Lonnie - I would opt for a Helicoil for the original thread, but then I am anal in that respect. Tapping it our to the next size would probably work just fine and you could also document all of the repairs like that (or when the time comes, just sell it as is and chuckle at the surprises that the new owner is in for). Cheers - Dave
DW DuBois

Hi Lonnie

I opted to install helicoils on a couple of occasions and quite happy with it.

I did try a large helicoil to accept the bumper bolt to the chassis and I didn't feel it worked so well.

In my very limited experience larger helicoils work very well on smaller bolt sizes.

They are quite easy to do also. The kits I bought had the drill bit a dozen helicoils an inserter and a tool for breaking off the part used to install them.

Regards Darryl
D Lamb

Lonnie, like Dave, I would Helicoil it. You can get an M-8 kit, if that's the thread you need. You probably already know this, but using a drill guide to keep it straight is recommended. When I install one, I make a drill guide using a 1 inch square block of metal, usually aluminum, easier to drill. PJ
Paul Jennings

Sorry that was ment to read larger helicoils don't work so well.
D Lamb


Lonnie;
I looked up the thread pitch on both the 6 mm and the 8mm screw. If they are french metric, I would guess they are, then they are both 1 mm pitch.
A healicoil for the 6 mm bolt would then also be 1 mm pitch. Thus the tap for the helicoil to fit the 6 X 1 bolt would be almost identical to a tap for an 8x1 bolt.
So the work involved in fitting a helicoil vs an 8 mm bolt would be almost identical

It would cost you a bit more for the helicoil kit but a future owner would thank you.


Jim B.
JA Benjamin

Thanks guys -

You confirmed my gut-feeling ... Helicoil with original size locator bolt. I'll have a machine shop install it. I've never worked with Helicoils so I don't want to learn on this gearbox case.

I need to confirm that the locator bolt is M6X1, which is Metric course thread. Helicoils are not available in Metric fine (M6X.75)

Lonnie
TF7211
LM Cook

Lonnie, I'm not sure which thread it has, but here's a image of a new layshaft bolt vs. a worn one. One could probably determine the thread by scaling the image if one knew the bolt diameter. Bud

Bud Krueger

Thanks Bud -

I have a couple of them. I'll try to find them today and determine the pitch.

Lonnie
TF7211
LM Cook

Lonnie - Both the 6mm and 8mm bolts have a 1mm pitch. Cheers - Dave
DW DuBois

This thread was discussed between 10/12/2016 and 11/12/2016

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