Overload backfire device

Hi Tony. I've had this device off a few times and reset the torque, but as it was a few years ago I don't have a step by step guide to give you. Removal is all nicely described in the Clymer Publications Norton Service Repair Handbook, 2nd Edition 1975 onwards which details a Mk3 Commando stripdown. This is a slightly better manual than the Haynes Book that shows a Mk3 Commando on the cover but describes the Mk3 changes as a last chapter, and incorrectly states the rear hub lock ring as left-hand thread in chapter 9/22! But, to be fair, the Haynes manual has a nice diagram of the starter drive mechanism and states that the device should give a slip torque of 50 ft/lbs.

If the anti-backfire device (ABD) is slipping then you will see its outer teeth not rotating while the starter and large drive gear behind are (primary outer cover off and press the starter). If however this gear is turning and the crankshaft is not, then the fault lies in the sprag that is on the crankshaft.

The ABD has a thin locknut to hold its torque setting that can easily slip. Drive is obtained by two plates being forced together by hefty dish washers with ball bearings between the plates and small ball bearing holes (6?) in the drive faces. The torque nut is rotated down to compress the dish (spring) washers until that 50 ft/lbs is achieved.

You will need to remove the alternator and its alloy support frame to take off the ABD. Put it in a vice and take it apart. The balls will have rounded off the holes in the drive faces but it might well still be useable. You will need a new torque locknut. By gripping the gear in the vice you can fit a suitable socket on the input splines and use a torque wrench to measure the slip torque. I tightened mine to a bit more than 50 ft/lbs. Be careful - go too far and a backfire will transmit through it and break the sprag. Since many people experience broken sprags it is questionable whether the ABD ever does its job anyway. The sprags fail because the clearances between its drive faces are too great. These can be reduced (see other threads here on SPRAG).

Best wishes!

Previously norman_lorton wrote:

Hi Tony. I've had this device off a few times and reset the torque, but as it was a few years ago I don't have a step by step guide to give you. Removal is all nicely described in the Clymer Publications Norton Service Repair Handbook, 2nd Edition 1975 onwards which details a Mk3 Commando stripdown. This is a slightly better manual than the Haynes Book that shows a Mk3 Commando on the cover but describes the Mk3 changes as a last chapter, and incorrectly states the rear hub lock ring as left-hand thread in chapter 9/22! But, to be fair, the Haynes manual has a nice diagram of the starter drive mechanism and states that the ABF should give a slip torque of 50 lbs/ft.

If the anti-backfire device (ABD) is slipping then you will see its outer teeth not rotating while the starter and large drive gear behind are (primary outer cover off and press the starter). If however this gear is turning and the crankshaft is not, then the fault lies in the sprag that is on the crankshaft.

The ABD has a thin locknut to hold its torque setting that can easily slip. Drive is obtained by two plate being forced together by hefty dish washers with ball bearings between the plates and small ball bearing holes (6?) in the drive faces. The torque nut is rotated down to compress the dish (spring) washers until that 50 ft/lbs is achieved.

You will need to remove the alternator and its alloy support frame to take off the ABD. Put it in a vice and take it apart. The balls will have rounded off the holes in the drive faces but it might well still be useable. You will need a new torque locknut. By gripping the gear in the vice you can fit a suitable socket on the input splines and use a torque wrench to measure the slip torque. I tightened mine to a bit more than 50 fl/lbs. Be careful - go too far and a backfire will transmit through it and break the sprag. Since many people experience broken sprags it is questionable whether the ABD ever does its job anyway. The sprags fail because the clearances between its drive faces are too great. These can be reduced (see other threads here on SPRAG).

Best wishes!

Hi Norman,

I have been reassembling an electric start mechanism etc etc with new parts and tried to make a suitable tool to lock the assembly using a large socket and a spring disc as suggested by another member. Unfortunately, this was not a success (poor welding skills!) and am now still at the point where I need to set this assembly up. You describe using a socket on the splines. Where would I get one of these? I wouldn't have thought an ordinary socket would do. Also, the parts list shows a circlip retaining the torque nut (which doesn't look right if the torque nut is supposed to be peened over into a recess that is on the shaft. Would I do better asking a local engineering firm to help me out?

Any thoughts/guidance would be most welcome.

Andy

Hi Andy

I'm relying on memory, it's the suff that leaks out of holes as you get older. I know that the parts diagram shows a circlip out side of the tensioning locknut, but I don't remember it, and it's not going to stop the nut rotating is it? That nut relies on being peened over to lock it and I used to see the nut material cracking, which is not what you want.

Re. the torque applied to the splines. Maybe I found a (multi-point mm/AF/Whit?) socket that just happened to fit, or maybe to get the length a tube spanner squeezed over the splines with a socket on the end of that? I have lots of AF and Whit tube spanners that keep coming up with the answer to 'odd jobs', they are useful!

Once the holes get 'peened over' by the balls jumping then the torque drops because they now have a easier ramp to climb; but easliy overcome by tightening a bit more.

Look at oldbrits site herehttp://www.oldbritts.com/e_start_backfire.html which has a full photo set - it's a cracking site for their other tech articles - Alan

Previously antony_ryan wrote:

Hi, I have had trouble for a long time with starting, when I push the button it makes a horrid grinding noise and I have to repeatedly push the button till it catches, I have just taken off the outer case to see what's going on, the noise is the backfire device churning round. How do I check it and adjust it? Help again please, Tony.

Previously norman_lorton wrote:

Hi Andy

I'm relying on memory, it's the suff that leaks out of holes as you get older. I know that the parts diagram shows a circlip out side of the tensioning locknut, but I don't remember it, and it's not going to stop the nut rotating is it? That nut relies on being peened over to lock it and I used to see the nut material cracking, which is not what you want.

Re. the torque applied to the splines. Maybe I found a (multi-point mm/AF/Whit?) socket that just happened to fit, or maybe to get the length a tube spanner squeezed over the splines with a socket on the end of that? I have lots of AF and Whit tube spanners that keep coming up with the answer to 'odd jobs', they are useful!

Once the holes get 'peened over' by the balls jumping then the torque drops because they now have a easier ramp to climb; but easliy overcome by tightening a bit more.

Hi Norman,

Thank you for this.

I didn't think the circlip was real!

The starter bits are all new so hopefully will perform to best of their intended ability. I'll have a look and see what I have that would fit the bill wrt the splines. Failing which it'll be getting someone to weld up the tool with a thrust washer and socket as per the hyperlink in the next post from Alan Sessions. Lifetime of learning as they say!

Andy

Previously alan_sessions wrote:

Look at oldbrits site herehttp://www.oldbritts.com/e_start_backfire.html which has a full photo set - it's a cracking site for their other tech articles - Alan

Previously antony_ryan wrote:

Hi, I have had trouble for a long time with starting, when I push the button it makes a horrid grinding noise and I have to repeatedly push the button till it catches, I have just taken off the outer case to see what's going on, the noise is the backfire device churning round. How do I check it and adjust it? Help again please, Tony.

Hi Alan and apologies to Antony if this is deemed butting in. Just to say thank you for the hyperlink. My own voyage of discovery continues.....

Andy