Dommy distributor housing fixings

Hi Robert, 

From your description, it sounds like a Torx bolt, but to my knowledge these are only made in metric sizes. Also you say 'nut', then describe as like an allen 'bolt'..

If it is indeed a nut, is the shape like in the pictures I have attached? These are of 'E' sockets and are in imperial sizes, but I feel these are more modern than would have been used on a 99. Anything like what you have?

Regards,

George

The Maintenance Manual describes them as Allen nuts.

The studs used for holding a Magneto / Distributor in position are supposed to be 5/16" x 18 BSW. The opposite studs ends having 5/16" x 26 CEI threads.

However, my equivalent thread tables show that 5/16" x 18 UNC has a close enough thread pitch to fit in the cases.

In which case a full-length 5/16" x 18tpi stud might fit. Robert mentions nuts that look like sockets with multi-point socket heads. Supposing these are actually  5/16" x 18 UNC Torx bolts???

A similar fitting change was made on the Commando cylinder head when the rocker spindle end-plate bolts changed from 1/4" x 20 BSW to 1/4" x 20 UNC.  Try playing spot the difference with Atlas head rocker  fittings?

Wonder what caused Norton to use these nuts for some years, until replacing them with ordinary hex nuts. Anyhow, as far as I know there is two different types of 12 point screws/nuts. The triple square ones (XZN) which I've only seen in bigger dimensions. And the double hex ones. Of course point angle is different. Don't know if any of these where made in imperial sizes.

Hi Robert,

See: https://www.fastenerdata.co.uk/fasteners/nuts/more-nuts/nuts-a-to-c/12-point/unc-12-point-allen-nuts-alloy-steel.html

Is your nut like shown on this link? If so, then a splined socket bit like as seen towards the top of my first picture I posted earlier might be what you need? The ones I have are metric, but I now wonder if imperial ones are available too?

George.

Hi Robert,

Is it worth sacrificing a flat blade screwdriver by grinding the end to fit? A gentle 'persuasion' into the nut and a strong twist might get it moving? Alternatively,  sacrificing the next size up Torx bit? Chamfer the end and same gentle 'persuasion' as before? Both have worked for me with rounded off allen bolts.

As to replacement, if 'correct' nuts aren't available,  would something like clutch spring screws/nuts work?

Good luck,

George.  

Hello again Robert - Those internal double hex nuts are !/4 inch BSCycle (26 tpi).  Two are used at the top in the recesses in the mount,  for the 18D2 distributor,  each with a small 1/4 inch thick  washer. (The washers have a small o/d - maybe 3/16 inch washer but with a 1/4 inch hole?  I personally use 1/4 inch spring washes adjusted with two pairs of pliers to stop the springing.)  The bottom nut is usually a standard 1/4 inch BS Cycle nut again with a thick washer.  If you're not careful undoing the nut (if you can get at yours) you can undo the stud from the case instead and then you're in trouble.  I have a specially altered spanner that allows me to undo it - most people don't. Unlike the suds for fitting the magneto the studs for the distributer are parallel (1/4 inch) all the way to the collared end that screw into the cases.  Good luck, Howard

Both 1/4" BSF & BSC threads are 26tpi. The slight difference in thread angle is next to impossible to make out.

The 2 studs that have the allen nuts are waisted which allows the housing to be adjusted by rotating slightly around the bottom stud. Enough to adjust the drive chain.

Hello again Robert -  I don't believe the studs are waisted as Paul suggests.  The nuts are 1/4 inch and the holes in the dynamo housing mount are 5/16 inch.  So there is already plenty of movement to adjust the drive chain.  On a magneto the nuts are 5/16 inch so the studs are waisted down to 1/4 inch to permit the movement required for adjustment.  If they are indeed waisted what diameter remains of the stud - 3/16 inch ?  I suspect a previous owner must have done it rather than buy a new chain.  Cheers, Howard