The Primary Job
Hutch remembers how the seemingly simple job of replacing a front drive sprocket on a pre-unit bike is not so simple…
Irecently had a BSA A7 in the workshop for a while, having sat dormant for some years (about nine). It had come for recommissioning and a bit of a cosmetic spruce-up.
One carb clean and a magneto recon (may thanks to Alex Barona at Matchless Engineering), plus an oil change, and the bike was running nicely. It was then stripped of all paintwork, and new tyres and tubes fitted, etc. Unfortunately, after ‘resting’ for so long, the clutch was, unsurprisingly, well and truly stuck, so an inspection of the primary drive and clutch strip were required. The owner had also requested that a one tooth bigger gearbox sprocket be fitted to give the bike a slightly less frenetic feel at normal road speed.
Now, let’s be honest, much as many of us love them, the majority of old Brits are a complete pain in the backside when it comes to changing the gearbox sprocket compared to most Jap and European bikes. There’s a lot of striping to be done before you get to the sprocket comparatively.
I actively try to not remember how to do particular jobs on different bikes, preferring to clutter my mind with processes and first principals, so I am a firm believer in RTFM (Read The Flippin’ Manual…) before starting a job, and then constantly referring to said manual while doing the job; so this is where I started.
With the bike stripped of all the ‘big bits’ and up on the bench, the primary oil was drained, which was very clean and boded well. Stripping began, making a few notes of all the parts needing attention before reassembly, such as the bent footpeg stud and the footpeg not coming off until the inner primary was removed. Fortunately, this could be worked around until that point.
All he outer primary cover screws came out easily and, along with the other small parts removed during the strip down, were bagged to make refitting a simpler affair.
With the outer primary cover removed, everything looked very clean and in good order. If it had not been for the requirement of the new gearbox sprocket, I would have gone no further than this and a strip of the six-spring clutch.
Referring to the manual, the split pin in the end of the crank retaining the cush drive nut was removed, and the tab washer inside the spring bent back. The crank end nut was then removed, followed by the engine sprocket and cush drive assy and the primary chain; all parts were inspected, and looked serviceable and bagged.
With further ref to the manual, the clutch was stripped with the aid of a magnet on a stick and a piece of bent welding rod to pluck out the very stuck together clutch plates. The clutch plates were put to one side and were later separated and washed with degreaser.
The few burrs were filed of the clutch plates using an “electric file” or hand-held belt linisher, and the plain plates were de-rusted using a Scotchbrite wheel on a
polishing machine. This gives the added advantage of a slightly unpolished (satin) finish. They were then checked for flatness against each other.
The clutch centre tab washer was bent back and the pushrod withdrawn, leaving the clutch centre nut to be undone. These clutch nuts can be a bit FT (Flippin’ Tight) at times, so with the bike in gear and a glamourous assistant holding the rear wheel I gave it a couple of dug-dugs with the nut runner – which is my preferred method
– and off it came. The centre could then be removed.
A special puller is needed to remove the clutch hub and basket, and although it can hold the job up in getting one it’s not so costly in time, parts and effort as trying to use something unsuitable and causing more grief. Fortunately I found a suitable puller in my collection of ‘useful special tools’ and with a little application, off came the hub and basket.
Now the contents of the primary cases were all removed, there was free access to remove the inner primary cover. The front two inner primary mounting bolts were
removed after the lockwire was cut off, and noting that the rear bolt mounting the cover to the frame was missing. With a wiggle, the inner primary cover was removed, along with the left hand-foot peg, which was put aside for some attention.
Free access to change the gearbox sprocket could now be had, leaving a pile of bits to clean, gasket faces to scrape, plus gaskets and lockwashers to buy/ pull from stock. Assembly, as the best manuals say, is just the reverse process. With cleaned-up threads and new parts, it should be easier, possibly.