Early seventies gearboxes use a leaf spring to index the selector cam plate, presumably cheaper to manufacture. It didn’t work very well and they went back to the pre 1970 sprung plunger later on. If the bottom of your gearbox has the long nut underneath the gearbox as in the photo on the left you’re okay, a blank as in the photo on the right you have the leaf spring.
A worn leaf spring may well be the cause of your selection problems. To replace the gearbox and clutch don’t need to be dismantled completely. Remove gearbox outer cover, undo mainshaft nut and inner cover bolts noting that one of the inner cover bolts is an Allen head and the other a conventional head – stupid. Ever wondered why?
I’ll let Nigel Sprocket explain – Trev Bolt and I were both working in the Meriden draughting office at the time and were discussing outer cover fixings – Nigel: “Ah lads we’ll be ‘aving an Allen key bolt ‘ed ‘ere”. Trev; “Nah mate an or’nary ‘ed were good enough for me granddad it’ll do for a Trumpy”. After an hour or two of this good natured banter things began getting heated and before they came to blows Edwin Turner stepped in with his unparalleled people management skills “Righto chaps we need this cover designed by tea time so we’ll have one of each”.
Replacing the spring is easy part no……. but assembly does require timing the gearbox, see further Tech Note if you have trouble with this