SQ4 Gearbox Investigation

Starting the Square Four up mid winter is something I do at least once a week just to make sure it's all working, which it usually is - except when it isn't.

This week I was met with an unexpected noise, like gear-dogs clashing - it sounds like it does when you put it into first gear. Listen to the sound, especially obvious around 28 seconds:

The noise stops as you disengage the clutch (which is dragging badly) as the mainshaft slows to a stop. It's also not present if you run the bike in gear on the rear stand.

Casting to the forum & FB world for advice, I ended up having a chat with AOMCC gearbox guru Geoff - 30 minutes of good advice. To summarise:

• Poor Sleeve gear position leads to dropping out of top gear. It seems I might need an extra spacer to move the sleeve gear towards the mainshaft sliding gear
• Minimise Layshaft end float, centralise in new bushes
• Minimise Camshaft end float
• All the bits are available from AOMCC Gearbox Spares, no bushes need reaming 

Here's the gearbox drawing, for reference:

So on with the work; first job is to get the bike on the lift. This entailed fitting the centre stand springs to the FH, which is a story in itself. Having the half-built FH on the bench is quite a different experience to having the SQ4 up there - you realise how heavy it is, but the lift is quite happy and accommodates the rear stand quite well.

Yet again the bench proves it's worth, supplemented by the new lights overhead. I bought some strip lights to add to the interior light circuit, and when I put them up I moved the old interior lights, which used to face the benches, to face the bike on the lift. Then benches are taken care of by their own circuit, and now I have both sides of the bike illuminated.

Removing the outer cover reveals no issues with the selector or kickstart. I must remember that this box is missing it's kickstart rebound rubber if I recall correctly.

I set up the DTI on the lift deck. This isn't ideal, as it reveals the bike is swaying about a little on it's stand but the arrangement serves to reveal 1 mm end float on the mainshaft, 0.3 mm on the layshaft and 0.3 mm on the selector shaft. The limit specified in Waller for the mainshaft is 1/32", which is about 0.8 mm. There's nothing specified for the other two shafts.

You might recall some issues we were having with the clutch towards the end of last season - I wrote them up here in a post called Clutch Investigation. The end float in the mainshaft possibly contributed towards this problem.

We'll move to the other side now; the first discovery is no surprise - the clutch centre nut has backed off again, though the tab washer prevents it moving too far. The reason for this is the splines, which are very worn and won't take any load without moving a little:

This is a new AOMCC clutch centre. The splines are wider and have square shoulders:

With the clutch off, I was able to get a better quality mainshaft endfloat measurement of 0.88 mm (35 thou) with the DTI magnetic base mounted on the engine/gearbox plate

Moving the DTI to a steel plate clamped to the gearbox produced some better numbers for the layshaft and camshaft:

In summary, I have end floats as follows:

• 35 thou on the mainshaft, a bit over 1/32”. Ariel's stated limits are 1/64" - 1/32"
• 9 thou on the layshaft. Ariel only state a minimum for this end float.
• 9-10 thou on the selector. Ariel only state a minimum for this end float.

So, barring the sleeve gear, I have it all apart and can measure up. The parts washer is such  useful piece of equipment.

In no particular order, here are some pictures. Having stripped the gearbox I have discussed it with various experts on the AOMCC forum and Facebook pages, and twice with Geoff, the AOMCC gearbox spares specialist. These are the two selector pins - the wear is obvious, but not measurable. These are not going to be changed.

 

This is layshaft 2nd; dogs are good, teeth are good.

This is the layshaft sliding 'clutch' - dogs are good, splines are good.

This is mainshaft 3rd; dogs are good, teeth are good.

Selector forks are a little worn but show no sign of bending, burning or major damage. They are a good fit in the grooves on the sliding sleeves.

Two things in this picture - the sleeve gear, which I can't remove yet (big box spanner on the way) and the drive side layshaft bush. The shiny ring shows you where the layshaft output gear has passed it's end load.

The dimensional survey revealed the following: 

• Layshaft 2nd gear bush 1.061” gear 1.065” 4 thou clearance
• Layshaft 1st gear bush 1.062” gear 1.0653” 3.3 thou clearance
• Mainshaft 3rd shaft journal 0.786” gear 0.788” 2 thou clearance
• Sleeve gear bearings: 
• Mainshaft outer journal 0.931” sleeve gear bush 0.940” 9 thou clearance
• Mainshaft inner  journal 0.928” sleeve gear bush 0.941” 13 thou clearance
• Kickstart pawl bush length 0.81”
• Layshaft timing side bush 0.663” shaft journal 0.652”, 11 thou clearance
• For some reason I didn't record the dimensions of the layshaft drive side bush or the camshaft bush, but I guess I had already committed to changing them because of the end float. Well, that's the best excuse I have right now.

And another one from the experts - it looks like the timing side mainshaft nut is wrong, as it doesn't bear on the kickstart pawl at all.

So that's it! New layshaft, camshaft and sleeve gear bearings and a new kickstart rebound rubber.