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Anti-backfire device on commando 850Mk3

Submitted by alexander_caldwell on Mon, 10/08/2020
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Commando

I recently discovered the shouldered nut on the anti-backfire device has broken in half inside my primary.  I'm in the process of accessing the device and when released and inspected I'll order a new nut plus any other parts showing excess wear.  I have read the piece in 'Old Brits' describing the setting up of the device to allow it to slip when 50ft-lbs of torque is applied.  

In the article the gentleman describes making a tool by welding a thrust washer to an old socket.  I don't have welding kit so am looking for advice on any/all other options to accurately set up the required tension on the device.  Does anyone produce a suitable tool I could buy?  

One option I'm considering is buying a thrust washer and cutting it on the outside edges to fit a suitable size socket, then simply using the socket and thrust washer 'nut' to apply the necessary torque force, any views on why this wouldn't work?

Any and all help appreciated. I'm located in Norfolk, England.

Regards,

Alex Caldwell  

Submitted by peter_shand on Tue, 11/08/2020

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I made mine doing what you said, welding an old socket to a thrust washer. I don't see any reason why making the thrushwasher hexaganol won't work either. 

I also altered the way of locking the shouldered nut by facing off the 'peening edge' and making a thin locking nut to take its place.  Then by using a thin jawed open ended spanner on the shouldered nut and another spanner on the thin lock nut, makes it a doddle to set the correct release torque.

I got fed up trying to set the correct torque with the original set up. Using the welded tool and attempting to see what torque the unit has been set to actually altered the adjustment. The action of the overload operating (lifting and turning the mechanism) undid the adjusting nut!   

Regards

Peter

Hi I am trying to get information on the backfire set up on my commando. I am building this bike from a box of bits all the primary Chain side parts were missing but have managed to get all the parts over time from Ebay and Andover norton. The backfire parts form Ebay looked ok but dismantled it to clean and check but I can not find any information on reasembl and how to reset torque settings. Any info or pictures would help please

Am I missing something here I have found a photo of how to reset the torque to 50lbft but this doesn’t seem right as the gear is held in a vice and a torque wrench on the end of the shaft will never move unless the inner part of gear can rotate which it doesn’t seem to. I have taken some photos but I am not very good with computers and don’t seem to be able to download them.  My guess is the inner spline bit is seized or is it a pressed fit. Any suggestions appreciated 

 

Submitted by alexander_caldwell on Tue, 11/08/2020

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Thanks Peter, you're clearly more ably skilled or equipped than i am, I could face off the peening edge, but don't have the kit to make a thin lock nut to suit.  Thanks for the vote of confidence on the Hex thrust washer, that's probably my favoured route at the moment.  It may change as i progress!

Regards,

Alex.  

Submitted by peter_shand on Wed, 12/08/2020

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While you have the stuff in bits also modifiy one of the thrust washers (the one that is nearest the engine) by turning a bigger chamfer or a concave radius to provide more clearance from the primary chain. I always see evidence of the chain touching this washer on The MK3s i have worked on.

Regards

Peter    

Submitted by alexander_caldwell on Fri, 14/08/2020

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So having taken the device out, I noticed the ball bearings had dislodged from their seat and the whole thing had jammed, no doubt forcing the collared nut to shatter.  This made releasing the remaining half-collared nut difficult as the spring washers were under high tension, so i needed to turn the device as if it were in use to re-seat the ball bearings, then releasing the tension and freeing the broken nut.  So i made the old thrust washer into a 7/8 W nut by grinding the shape.  That did the trick, now i've also got it to use to set the torque up.  Result.  Now to order the new parts! 

 

 

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Submitted by peter_shand on Thu, 14/03/2024

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Look at the drawing on the AN Mk3 pages. It has an exploded drawing showing the assembly order. To set the overload torque you will need the tool mentioned earlier in this post.   

Submitted by peter_shand on Fri, 15/03/2024

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Have you read the old brits article in Alans post? Do not tighten the adjusting nut to 50ftlbs. If you have done that it will be solid and won't operate as it should. The 50 ftlbs is the breakaway torque that the unit should operate at.

 



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