Gearbox selector forks

[Peter Naulls]

Last winter I rebuilt a 4 speed "3 Syn" gearbox for the Ruby - new bearings, intershaft bush, balls and springs etc. and refitted it expecting all the nasty noises to have gone. All quite successful apart from a loud persistent rattle /hammering sound on the over-run in top, especially above 45 miles an hour.

I replaced it with another rough old gearbox as a test but this makes similar rattles.

The engine and gearbox are out for other reasons so I've taken the top off the rebuilt box and had a poke around. All I can find is a fair bit of play sideways in the selector forks (in other words they swing on the selector rod a little more then I would like). They make the right kind of noise so I suspect these are the culprits.

Anyone had similar problems? Anyone know what the clearance between fork and gear should be? Should I change them?

Incidentally I'm about to change the propshaft as this has more slop in the splines than I would like but I now think this is unrelated.

Peter.

[Chris KC]

Hi Peter,

Always difficult diagnosing noises by e-mail; but I'm struggling to think this could be the selector forks. They don't do much once you're in gear but sit there; so if they're rattling something else is shaking them about. Then if you've changed the box and it still makes the same noise, it means you have the same 'fault' in two different boxes.

If the propshaft looks sloppy I'd sort that out and try again. Check the splines, also that the CV joints are good and aligned correctly with each other. I had a propshaft joint fail so long ago that I don't remember much about it but it certainly rattled quite a bit. Are you sure the propshaft isn't touching the handbrake?

[Ruairidh Dunford]

I am not suggesting this is your issue Peter but it may amuse.

A car came in - having been transported a huge distance at great expense - with a reported rattle above 40mph, that "nobody" was able to identify or cure.

I started the car, which was very quiet and smooth, and proceeded carefully to come up to the reported problem speed of 40mph. Sure enough a rattle appeared and got worse on overrun.

I leant down pulled the oil gauge pipe away from the firewall and the problem was cured F.O.C!

[Peter Naulls]

Thank you Chris. 

Certainly I want to eliminate the possibility that the propshaft is causing it, though I'm concerned how any external problem can cause lateral movement in the gears or forks when the bearings are new. The propshaft clears the hand brake ok and there's no vibration associated with it, just noise.

The rattling can be lessened slightly by holding the gear lever tight. My first though was a worn lever but I've minimised the wear in the ball at the end of the lever by building up with weld and grinding back, also tried a different lever, spring, top cover plate etc.

I know what you mean Ruairidh, sometimes we can be too clever and miss the obvious. Odd acoustics can make you search in the wrong place for a long time!

Peter.