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Trailing arm stud jig?


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I've decided to helicoil my trailing arm studs and have ordered a 5/16 UNF helicoil kit. I'm a bit concerned about drilling the holes straight and true. Has anyone done this and made/used a jig to drill out the arms ready for the tap?

Thanks

Simon

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Hi Simon,

I think you need a 5/16" 'UNC' helicoil kit not UNF.

 

It may be possible to use the hub as a guide.

The 5/16 helicoil taping size drill is very close to 5/16.

 

Get the kit, see if it works, if not then thinking caps on - as you observe it needs to be square.

 

Roger

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Simon,

 

I guess you are probably aware of this available in the States:

 

http://www.pattonmachine.com/TAK.htm

 

Do you have an immediate issue to resolve, or is this part of the plan to replace your driveshafts? I was thinking that we could do with something like this jig as a local group resource. Maybe we could sort something out at the next meet?

 

John

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Nice looking Jig and with all the options covered in it by various interchangeable inserts ie it will allow redrilling for 3/8th UNC tapping or with helicoil , 5/16 th UNC tapping or helicoil which just about covers the variations required.

This is a good example of an almost essential tool or jig which after you use it once, because it does the job correctly is not needed again and so lies rusting in the garage.

 

Whats that...a galloping noise in the distance ...whoa boy - steady ...snort..snort Hi AlanT unofficial Armourer for all things TR I thought you would be along, my cheque is in the post if we can cover whatever you decide is the cost between us for you to make such a jig and if you think a good idea open it out to other forum members on a loan basis also subject to a suitable contribution from themselves.

 

Mick Richards

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Hi Stan,

 

Yes made in Alloy but it will still have a surface reaction to the atmosphere, I might suggest to Alan we anodise the jig if we do it...I like Blue.

 

Mick Richards

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Hi Simon,

I think you need a 5/16" 'UNC' helicoil kit not UNF.

 

It may be possible to use the hub as a guide.

The 5/16 helicoil taping size drill is very close to 5/16.

 

Get the kit, see if it works, if not then thinking caps on - as you observe it needs to be square.

 

Roger

 

Thanks Roger, I intend to use standard studs so they would be 5/16UNF wouldn't they?

 

Simon

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As they've lasted 43 years and are only in need of replacement due to my ham fisted efforts to "tighten" them, I think they'll probably see me out!

I've been told that a helicoiled stud (if not over tightened) should outperform the standard item. I think Stuart said somewhere he torqued one to over 60 ft/lbs without it giving up.

Simon

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Hi Simon,

I believe Moss do UNC/UNF studs. Although the original is UNF, UNC would be a better bet.

 

 

Have a look in the Moss Cat - back pages under hardware. There is a PDF file in the on-line Cat.

 

TE505161 is a 2" long unc/unf stud - there are other lengths.

 

 

The helicoil insert will improve the strength of the thread compared to standard.

 

Roger

Edited by RogerH
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Simon,

 

I'm with Tony on this one.

 

UNF because of it's fine thread profile is not a suitable thread for soft materials such as Aluminium or even an alloy, because Triumph got it wrong it's not a reason to perpetuate it and try and "do the wrong thing righter". As you pointed out the Helicoil will provide a superior torque application, but that is BECAUSE IT'S A COARSER THREAD INTO THE ALLOY, and the circumferential increase (it's a bigger dia to allow another interior thread to be used inside it) will increase the torque loading.

 

However there are other better ways to sort it. As Tony suggests fitting a 5/16th Helicoil or Timsert insert with a UNC profile will give much better grip in the softer material, or if you want to retain as much material in the trailing arm as possible counterintuitively increase the hole dia to a 3/8th UNC tapped hole which is smaller than a 5/16th UNC (or UNF) Helicoil dia required for a 5/16th stud, (by 16thou).

Then use a stepped 3/8th UNC stud into the alloy with a 5/16th UNF thread upon the Hub end to retain originality and fitting, this is where the importance of a jig becomes apparent, I've seen enough trailing arms with bodged helicoils fitted "on the cock" which then requires the hub to be drilled clearance on the misaligned studs.

 

Mick Richards

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There ya go, how many times do you want tellin?? ... :DUNC it!.... :lol: :lol: :lol:

 

Tony

 

PS. Thanks Mick, you said what I was trying to say (my wife does that too just like Rogers)... :D

 

Tony

Edited by Tony Millward
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Simon,

 

Nonsense, all you have done is garnered different opinions, you own the car and you decide which method you want to use. it's just that sometimes you get this nagging little feeling that...AAwwwwwww.

 

Mick Richards

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I thought that I read somewhere on here that to fit UNC studs you just need to tap the thread with a 5/16 unc tap.

 

Is this correct ?

 

A couple of my studs look a bit dodgy so I was contemplating changing them to Unc/unf.

 

Steve

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Oops, duplicate!

 

Steve

Edited by SDerbyshire
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Think you are confusing fitting a 5/16th UNC helicoil with tapping a 5/16th hole in the parent metal.

 

There's only 15 thou difference in material for parent metal hole size between UNF and UNC at 5/16th size, so not enough to hold a thread if you are fitting it to a damaged hole. Refer back to Rogers post if you wish to fit a UNC Helicoil but beware some of the trailing arms are a bit problematical on material around the stud holes, the casting blocks that carry the studs are not all centrally positioned accurately and the holes are closeish to the edge on some castings. You may well find that the holes break through on the inner edge, the Helicoil tapping size is .328 for the hole (carrying a 5/16th UNC inner tapped hole) that's why I prefer a 3/8th UNC stud.

 

The drilling size for 3/8th UNC is only .312 and that 16thou less on dia may be all that you have before breaking through, so a 3/8th UNC stud with a longer length than standard (about 8mm) into the casting with a 5/16th UNF standard length stud on the other end is my choice when I can get round to fitting them into my Stag (same rear trailing arm casting) and after making/begging a jig to drill it. You'll only get one chance to achieve a workmanlike improvement on this and I don't want to risk hand drilling and keeping it bang on 90 deg in two planes at once, not into alloy where the drill will get snatched or want to wander if the original hole is damaged.

 

Mick Richards

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