SQ4 gearbox rebuild

 Now we know what we have to do, we can get started with the rebuild helped by these lovely new bushes from Geoff at AOMCC Gearbox Spares

First job is to remove the two Welch plugs over the layshaft and camshaft; I drill a hole in these and lever them out:

With some heat in the casting, you can drift the layshaft and camshaft bushes out from the drive side:

The sleeve gear bushes come out with a bit of tube; the new ones go in with a large flat drift. They go in quite easily with a hammer (you don't hit the bush directly), but it's times like this that I wish I had a press.

The new drive side bushes can be pulled in with suitable studs and washers:

You need to be careful with the layshaft bush. It's easy to get it misaligned, and there is an anti rotation pin designed to keep the grease groove at the top. At the factory this was drilled and fitted from the drive side, but with a replacement bush you must align the slot in the bush when you put it in.

All this work culminated in 0.13 mm end float (5 thou) on the layshaft. That's better than the 9 thou I had earlier, and I have the proper gasket fitted now.

What's not so good is the 0.37 mm I have on the camshaft. That's about 15 thou and will have to be sorted with shims.

I've also decided to discard these two nuts and make some new ones. The threads are poor and the nuts would be better twice that thickness.

We'll fix the camshaft end float with this 0.3 mm shim. It's a commercially available 26 mm ID shim that I've opened out to fit:

This shim fits inside the inner cover. Unfortunately, I found out later that this is a 0.5 mm shim in a wrongly-labelled bag - testing revealed a selector shaft that was firmly clamped in place.

Here's the selector shaft assembled and greased ready to go in.

A trial fit, with shakeproof washers under the BSW nuts (to prevent them coming loose) revealed the shim problem. I went through two bags of shims with a micrometer and found that all the shims in the 0.5 mm bag were 0.3 mm and all the ones in the 0.3 mm bag were 0.5 mm...

Easily found and fixed, fortunately. It pays to measure and test at every step. 

With that fixed and the selector shaft showing about 0.05 - 0.07 mm (2 thou or so) end float we can set up to measure the mainshaft and sleeve gear end floats.

The mainshaft endfloat still measures 0.88 mm or 35 thou: not surprising, since we haven't changed anything.

The sleeve gear shows 1.11 mm when in top gear:

A call to AOMCC gearbox guru Geoff suggested I make a spacer to reduce that to 0.11 mm, 4 thou, to maximise spline engagement. Geoff provided a useful spacer to make that from:

This is the spacer that sits under the gearbox sprocket, inside the oil seal. It parts off quite easily on the mini-lathe, once you remember that you don't want the lathe in high gear!

The spacer fits neatly on the inside of the main sleeve gear bearing, pushing the sleeve gear splines into further engagement with the mainshaft sliding gear splines.

To deal with the excess mainshaft end float, I have machined a 0.5 mm recess into the mating side of the nut. This will restrict mainshaft travel a little more and should bring the end float down to somewhere near 0.38 mm, or about 1/64".

Reassembling the gearbox reveals sleeve gear movement at 0.8 mm; I must have measured that wrongly as I thought I had 1.11 mm and added a 0.75 mm spacer. Mainshaft end float is now 0.6 mm, down from 0.88 mm and around 25 thou. This is 1/40", within Ariel's stipulated range of 1/64" - 1/32".

All good news; however, since tightening the inner case nuts for what I thought was the last time, I've realised that the camshaft is too tight. It turns, but that 2 thou end float has clearly been taken up by gasket compression - I may relieve the bush a little or I may fit shims to 0.2 mm (8 thou) rather than the 0.3 mm currently fitted (12 thou) which should give me what I need.

As it turned out, smoothing the surface of the camshaft bush was all that was required to return to normal operation, so I tightened up the end cover. The last step was to fit the two Welch plugs, which have to be done before the sprocket goes on for the last time. It would have paid to ensure the bores for these were clean before the bushes went in, as enthusiastic staking at the factory had left a lot of material in the layshaft bush bore which had to be scraped out with a twist drill ground flat, like an end mill.

Next job is to close up the other end of the gearbox, but I can't do that without the clutch in place as I can't hold the mainshaft to do the nut up on my own.

More next time.