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Stripped threads

Submitted by andrew_fox on Tue, 04/07/2017
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Commando

On my mk3 I have got stripped threads on the throttle body, where the master cylinder locates and on the ally seat retaining knurled knob, I would like to identify the thread type of these items and purchase thread repair kits soI would be grateful for anyinformation.

Regards Andy

Submitted by martin_freeman on Tue, 04/07/2017

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hi Andy,

Seat Knobs are 3/8 UNF same as Morris 1000 head nuts - I've turned a few down over the years and pressed into work seat knobs.

As for Master cylinder to twistgrip don't know

regards Martin

A second hand knob may well be cheaper than Helicoil kit if you only want to do one.

Submitted by neil_wyatt on Wed, 05/07/2017

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Regards the seat knob, these are alloy and the threads wear very quickly. Years ago, an old engineer, now sadly in that workshop in the sky, made brass inserts for them and no more wear, the knob (knobs on pre Mk3's as far as unscrewing in concerned) have remained a tight fit for more than ten years.

Submitted by christopher_winsby on Wed, 05/07/2017

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You can buy a new one at Andover Norton Cheaper than the cost of a Helicoil kit. But if it was me I would drill it out and fit a larger screw with Loctight . Cut off excess and face then drill and tap 3/8th UNF and re-drill roll pin hole if needed.

New Knob £11.70 plus Vat+P&P.

Fitting steel inserts is a common engineering practice or it use to be.

Helicoil's were only used when there was not enough material for a made insert. You use to be able to buy steel inserts not just Helicoils.

Submitted by andrew_fox on Wed, 05/07/2017

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Thanks for the info. everyone.

I have got a lathe (myford 300)& so far my only experienceis turning some ally wheel spacers, so an insert for the seat knob would be a goodexercise to improve my turning skills. I am sure I have the 3/8 UNF nuts mentionedas I have owned a few old Brit cars in my time.

Are the bolts that attach the master cylinder to the throttle body 3/16 UNC (also known as 10-24)? I have seen recoil kits for this size but do not want to purchase until I know this is the correct size! If I can't findget adefinitive answerI will be forced to commit sacrilege & use a metric helicoil of 5mm (I've got loads of these)!!

P.S. isn't that roll pin in the seat knob an awkward little so & so!!!

Thanks once again Andy

Submitted by christopher_winsby on Thu, 06/07/2017

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You need a good and right sized punch to remove the roll pins without damage. replaced mine with new as the bike had suffered from a hard and amateur mechanics. having done a complete strip and rebuild I have found it would have been cheaper to replace rather than refurbish some parts.

Is your bike a MK 3 as in Electric start or Mk 111 as in a 750?

Submitted by andrew_fox on Thu, 06/07/2017

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Hey there...Chris I have an electric start, it's an interstate in silver. I have tried to attach a piccy of the wooden jig I made to hold the seat knob, it should show everything clamped up in a vice with my 1/8 pin punch (now ground down to 3/32!) in position. I found the job awkward because the roll pin is offset on the axis causing the seat knob to rotate and also because of the small size of the pin, in fact I had to use the eye piece of a microscope (that I use for reading jet sizes) just to see what was in there! I agree it's cheaper to buy new but I do enjoy repairing things & I think that a helicoil (or insert) would make a stronger thread.

John that fastners list is excellent, I did what you suggested & was able to solve the conundrum, I need a 10-32 UNF, so I will now have a browse around the web & see what I can find.

Thanks again everyone

regards Andy

Attachments seat-knob-png

 


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