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Fork slider query

Submitted by peter_bowman1 on Tue, 29/10/2019
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Heavy Twins

The worlds slowest Triton build limps on!

The current query is about a 7&3/8’’ lhs fork slider leg I bought some time ago. I imagine this item may be either a featherbed or commando item but can anyone advise what sizes of pinch bolt were used on these wider forks. 

I notice that RGM sell a 5/16” unf bolt for this application but I have tried a bolt of this size without success. I have also noticed a reference to a 5/16” whit bolt.

I originally bought a the RGM item for the older featherbed forks and this doesn’t wish to fit either.

The threads may of course be damaged but it’s not too easy to see any signs of damage peering down a 5/16” hole.

Any suggestions would be gratefully received before I buy a tap to run through the threads.

many thanks

Peter Bowman

Submitted by philip_hannam on Tue, 29/10/2019

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The pinch bolt slider should have a a double thread stud of 5/16" x 26tpi. 1 & 5/8" long. The original part number was 16664. Later changed to 067671. This being another rare example of a Cycle thread cut into alloy on a Norton. Another is the oil pump studs. 

The corresponding nut is the same as that on the Dominator carb 2 into 1 manifold. Again 5/16" x 26tpi.  Part number E3231.

I may be wrong but I believe that Commando sliders had an oval profile whereas pre-Commando sliders were all round in section.

Submitted by peter_bowman1 on Tue, 29/10/2019

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Hello Philip

Thanks very much for your speedy post in reply. The 16664 stud is the one I refer to in my post above which is BSC and this doesn’t want to fit either.

My slider doesn’t appear to be oval so is presumably late featherbed. Perhaps I just put a drill through and use a nut and bolt as I believe the later Commandos do.

Many thanks

Peter

Submitted by peter_bowman1 on Wed, 30/10/2019

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Yes I apologise, I should have mentioned that it was drum brake.

Thank you for explaining the oval/round situation.

To summarise then it must be either 5/16” bsc or unf. I have tried both of these in the thread. They both engage for half a turn or so then tighten up in that way that usually tells you that you have the wrong thread.

Guess I’ll buy two taps and see which one feels right but I thought I’d ask the forum first.

Anyway thank you all very much for responding

Peter

Submitted by david_comeau on Wed, 30/10/2019

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Took 5 minutes to look at each specimen...results are

1 Commando (disc) clamp side oval legs are clear through hole for bolts,  fender and stay mounts are UNC

2 Commando drum/round legs are 5/16 UNC stud at axle clamp with unf nut, fender mount 1/4 UNC thread holes and 5/16 UNC stay mount.

3 Featherbed clamp side legs are... axle clamp stud and nut, fender mount and stay mount all 26tpi.

7 or 7-3/8 C-C difference is on the drum brake side.

 

Rather than CUT aluminum which is a loss of metal, I would first  try an antiseize compound and cold form the metal back by screwing in the correct threaded bolt you will usually save more metal that way. If it fails, then clear hole bolt or helicoil.

Submitted by peter_bowman1 on Sat, 02/11/2019

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Well I decided to bite the bullet on Wednesday and bought a 5/16” bsc taper tap on eBay. It arrived this morning and I applied it as gently as I could to the fork leg.

it went through with very little resistance and just a few tiny scraps of aluminium swarf. All is now well and the 16664 stud now goes fully home.

Thank you everybody for your help

Peter

 


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