Cush drive Mk1 how oes it fit

[roger.fellows]

Hi Graeme, yes, I've been doing a bit more thinking, too.

You will see that the sprocket I bought from Bruce is 27 teeth.

The thinking I've been doing is that the backs of both sprockets I have seem to be of similar geometry. I really need to do a dry fit to determine exactly what was wrong. I had not previously worked out what the shock absorber tightened onto. The inference I make at the moment - without the bits in view - is that both sprocket and sleeve must bear against the shoulder on the crank. I'm even wondering whether the shoulder has been machined too far after being built up and then re-splined. I know the coupling gear cover has been affected, the damage is immediately around the boss on the back of the sprocket and sleeve, and I believe a cover was acquired as part of the rebuild. The problem might stem from further back than I'd assumed.

I'll measure the saucer and photograph the cover tomorrow.

[D.Ryan]

Hi all,
Careful with the mk1 sprocket (mine is 1949 build sold in 1950) and the coupling gear cover clearance. Use plasticine to test the fit. I rebanded a front sprocket and thought I had enough clearance after welding. I found that there wasn’t and the sprocket actually swaged the raised portion of the coupling gear case onto the outer bearing ring on the inside of the case closing up all clearance for the bearing itself. It was a devil of a job getting the outer ring out and it had increased the interference fit to .004”. Fortunately I was able to clean the bearing hole up on my mill.
It appears to me that the mating surface of the sprocket and inner ring of the bearing are what controls the sprocket alignment and it makes me wonder what the special washer was for 1950. (.104” thick from memory) I tried a spacer and it put the chain alignment right out. I also believe that if a thinner hardened washer of any sort is to be used then minimum cush drive movement is advisable. I had a weak spring and the movement was evident in the wear to the washer. The raised boss on the inside of the standard sprocket should be enough for clearance.
Regards Dennis

[roger.fellows]

Not really satisfied with my measurements, but the sprocket which was in place has a 5mm dish with a 3mm boss.

The new sprocket has a 5mm dish and 3.5mm boss. Still looking at the rest of it. The bearing in the cover does not turn by hand, the cover has been off for some years. There is no sign of the shaft having been turning in the bearing that I can see.

There is some wear of the cover by the chain, but it looks as though that was prior to last use. Some photos tomorrow.

[Graeme.crawley]

Well this has been interesting. Here is what I think/have found out so far

1. The Sprocket is as noted by Roger, dished such that teeth are canted towards the Engine by about 6mm but the boss of the Sprocket is about 3.5mm leaving a net dish-effect of about 2.5mm. I have looked on the net for a ½ inch pitch 26 tooth Sq sprocket but no luck. I looked to make the sprocket from a blank but the blanks here in NZ have a 70mm boss and you need at least 80mm on the boss to allow the non-boss side to be machined out to leave the raided boss and the offset teeth. I cannot locate a Duplex sprocket due to Covid created shortages but I think one could turn the boss-side teeth off and that would leave enough meat to provide for the dishing on the other side without weakening the sprocket.
2. I am going to use a SQ4 23 tooth sprocket and use a clutch sprocket minus 4 teeth to give me much the same ratio (Thanks to a kind benefactor in UK for the sprocket!)
3. If you do not have a Sq4 gearbox shell you need to remove 9.5mm from the Clutch side of the gearbox lug top and bottom and make a spacer for the other side of the lug to keep the box snug between the engine plates – I will try to attach a picture. I have screwed the spacer to the gearbox case to facilitate installation – I have not welding skills. Imis-measured the first time and removed 7.5mm from the lugs, made all the spaced only to find a needed another 2mm to align the chains. Of course the mainshaft if the 1/2inch longer version as well.
4. The Cush sleeve I have with the raised sprocket hub is Huntmaster. One can tune the raised hub off the end up with a Sq4 Sleeve. The Huntmaster sprocket/sleeve cannot be used as it has not enough dish effect but if you were making the sprocket then with some care it could be made to used the Huntmaster sleeve if a Sq4 sleeve cannot be found.
5. The cush Spring cap seems to be unique to the SQ4 – well it is not the Huntmaster shape or the shape of a Single but fortunately I now have one.
6. The cush spring seems to be shorter than any of the 5 or so that I have. All mine would require the spring to be considerably compressed in order to start the nut which given my experience with the VB and the Huntmaster seems not right.
7. My chains all align now, time to find some bearings and complete the gearbox.
Pics below of spaced our gearbox

geabox spaced over by 9.5mm

Chains now align correctly