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Helicoils.... !


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And other thing...! 

Now fitting rear hubs and drive shafts and I have two stripped threads in the swing arms...I know I can remedy this by drilling out and inserting Helicoils... but I don’t know what size kit I need..? Still can’t get my head round imperial sizing and any help would be greatfull..

fixings are 1/2 inch 

thanks 

Nick 

 

 

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HI Nic,

If we are talking about the 6 x studs which hold the hub onto the trailing arm ? They are 5/16th UNF studs both ends, that's why they've stripped. The UNF fine thread doesn't have a lot of material in the "throat" of the thread, more threads = more material removed between them to form the thread = less cross sectional thickness.

RogerH has a theory that the UNF is more resistant to vibration loosening and that's why the Triumph engineers picked that thread form, personally I think they were on the funny baccy ! 

You can get 5/16th Helicoils in UNF (Fine) or UNC (coarse) form, however there is not much wall thickness around the threadform in the arm (often breaking through). The helicoil is self evidently an inner thread inside an outer thread, which means it needs a bigger tapping size drill than the standard hole size...which removes more of the precious trailing arm material. There are 3/8th UNC studs which can be fitted into the parent metal with 5/16 UNF threads on the other end available, the 3/8th UNC has a tapping size of 5/16th (.312 thou) which conveniently is the size of your stripped hole, no drilling necessary, just tap it 3/8 UNC and screw in the 3/8-5/16 studs and you are good to go.

The 3/8 UNC - 5/16 UNF studs are available from CDD and he will loan you a drilling and tapping jig to ensure the studs go in square in two planes, they are very easy to lean over, the holes are only 5 thou clearance on the studs and if a stud leans you can't get the hub over it.

Counter intuitively the 5/16 Helicoils need a .328 hole drilling which removes more material for it's tapping size hole. 

Mick Richards

Edited by Motorsport Mickey
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Hi Mick,

really greatfull for the info..  I kinda understand and made it clearer.

i am  contemplating having the holes welded up and tapping new holes and possibly now from you post, using a stud with the corsa thread, makes sense...

or.......

Daivd,

How much are you asking for swing arms?

cheers

Nick 

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Mick,

who is CDD?

Nikc 

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Nic,

No, I wouldn't bother welding the trailing arms. You have no control over what metallurgical changes will happen with the welding process, I regard it risky in case "hard spots" are induced and a possible fracture starting point when exposed to constant vibration.

Whether you go for a helicoil 5/16th or a larger 3/8th UNC into parent metal which actually needs a smaller hole,  both are preferable in my judgement to welding. 

Mick Richards

PS: Thank you Waldi for the comment.

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Keenserts are also an option. These are steel cylindrical inserts, 5/16” UNF on the inside, 7/16” UNC on the outside, so a good coarse thread to bite the Aluminium. They also contain built-in pins to lock the insert in place, so it won’t back out. I did helicoils years ago and still managed to strip one when reinstalling a hub, so I retapped for keenserts. They seem like a much more robust solution. Patton Machine sells a jig for installing them

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HI Nick, I agree with the comments to go down the CDD route, I have just completed providing new 3/8ths studs. Alistair at CDD provides a complete jig set , taps, drill and studs. The first jig is used to drill the holes and a second jig to tap the holes,  two taps are used a taper and a plug, the set comes with comprehensive instructions. I believe it would be  impossible to get 6 studs drilled and tapped perpendicular to take  the hub.

Regards Phil

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I too fitted the cdd studs, its very easy even with the trailing arms on the car.

steve

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