SRM Primary Drive shock absorber nut query

[shabashow]

Hi guys (and gals)

I thought I'd consult the font of all A7/A10 knowledge before I get my hacksaw out and make a pig's breakfast out of my primary drive.

I had a broken shock absorber spring, and while I had the primary drive apart, I thought I'd use the SRM shock absorber nut as a good replacement to the original cush drive retaining nut, which is tightened using a c-spanner. This has got chewed up, so I forked out and bought the SRM replacement which has a more sensible hex nut arrangement.

Scanning the forum, I see that the nut is recommended to be tightened to 65 ft/lbs, which I did. However, this has caused the nut to abut the sliding sleeve part of the drive, so it doesn't slide anymore. It's pointed out by the arrow in the photo.

The bike is a 1952 A10 plunger. Has anyone else had a similar issue?

My initial solution to the problem would be to machine the offending part down, while still retaining the landing that keeps the spring in place so the nut and the sliding sleeve aren't touching and have some room to slide, allowing the engine shock absorber to absorb the shocks.

What does the hive mind think?

John   

[Greybeard]

To help me understand what's happening will you assemble the shock absorber without the spring and check that the cams can slide against each other;  should not go right over.

[Swarfcut]

 Plunger and S/A nuts differ, as do the drive sleeve lengths. My take is that the SRM nut is to replace S/A versions only. S/A nut is flat faced and uses a tab washer. Plunger nut has an extended nose to allow space for the cush drive sleeve to slide and the spring to compress, this nose bears against the crank drive sleeve and passes inside the sliding cush sleeve The flat faced SA nut on the plunger sleeve simply bottoms against the sliding sleeve locking everything solid. A turned up spacer of suitable dimensions may allow the use of your new nut or turning the new nut  down to replicate the proper part are possible solutions, depending on how much crank thread you have and how much strength can safely be removed from the new nut.

   As always never assume the rest of the cush assembly is correct. For comparison a lift of 13mm on the plunger cush I have here just allows the lobes to pass.
 
 A handy plunger drive sleeve measures 57mm long. This is the plain edged oilseal type. The earlier scroll edged type is slightly shorter at 56mm to account for the metal oil slinger shim. Your bike may have a crank seal or not, depends on original D.O.M.

 Plunger nut here is 19mm long, the nose is 28.5mm dia and 8mm deep. So the max cush lift can only be some 8mm. By contrast a standard S/A Nut (ex 1956 A7) is only 16mm long.

  Part numbers.   Plunger nut.... 67 2054

       S/A Nut, a choice of 67 1098, 42 0071,  67 1137 depending on the model year.  So from SRM, not a simple case of one size fits all. Correct part and a custom peg spanner is certainly an easier fix.

 Swarfy.

 Additional. Later post shows a hex headed plunger type nut, new to me.

 

[shabashow]

I think you're right, Swarfy. And Greybeard, yes, the cams don't slide when the nut is done up.

I've had my mince and tatties, and got enough nourishment now to get the old grey matter working. I've compared the old flat chewed up nut to the new SRM one, as well as the inside diameter of the sliding sleeve and measured the dimensions with a set of calipers and got the outside diameter of the end of the old nut to be 1.120", the new one 1.250, and the inside diameter of the sleeve at 1.165. All measurements are very approximate.

I will get the o.d. of the new nut turned down to match that of the old nut, thus allowing the sliding sleeve to slide as it should.

So near, yet so far, SRM! When I ordered the nut from them, I asked if it was suitable for the plunger A10, and was told by someone in the workshop that it should fit fine. I asked if any customers with plungers had got back to them saying it didn't fit, and he said no, so I assumed all would be fine.

I think an email to SRM is in order to suggest to them to reduce the diameter of the bottom part so it really does fit a plunger model.

Now to persuade my friend with a lathe to get his tool out!

cheers,

John

[Swarfcut]

Whoops. Got that one slightly wrong, that looks like the right shape, but I've only ever seen the flat faced S/A type up to now and can't find a Plunger type of Hex nut listed today by SRM.  But all is revealed in that it is just a case of the non precision fit of (expensive) pattern parts from a trusted supplier. As usual you wonder why they can't manufacture to established published dimensions.

 So, useful to know an improved nut is now available for Plunger bikes....or maybe not!

 Swarfy.

[shabashow]

Swarfy,
it's that raised bit that gives the 8mm travel up to almost the end the ramps. The SRM one will give 7mm travel, I think that should be enough. I could always add in a 1mm thick spacer to get back to what was on the bike before.

John

[Greybeard]

Now to persuade my friend with a lathe to get his tool out!

Filth!