Race Engine Forum UK

Can anyone tell me please why 5th gear on a 131 5spd dog-clutch gearbox would slip out of mesh during racing?

Sorry to be clueless but I'm not really into gearboxes..

GC

Am I right in thinking that this is a 'bow wow ' box?

no syncro

in your title? A filter has redered the usual phrase dog!

If so, suspect that either some part of the selector mechanism is damaged or bent (this could even include part of the remote) or that the dogs themselves are worn or damaged. Good on line description here -- look for the subheading Crash gearboxes or dog boxes. If the dogs are just worn or chipped (they should have fairly sharp and well defined edges) it may be possible, depending on the quality and treatment of the material, to have them re-ground. You tend to get rougher engagement, as there's more slop, but it could usefully extend its life.

Hope that helps -- generally I know nothing about syncro boxes!

Yes, baulked by my own word censor!

Thank you very much indeed for your very helpful reply and that link to that marvellous website!

G

Hi Guy,

Some experience based potential problem points. Very similar to motorcycle boxes with the exception of the cam driven shift forks in a motorcycle to the linear sliding shaft in a typical auto box. Usually second gear was the issue there but any selection can suffer. I am not familiar with your particular box but I think I can help.

Only 2 things I can think of that prevent the gear dogs from backing out of mesh.

I) the incident /thrust angle created by the mating faces on the dogs. Effected by the dog face that can be beaten round or worn to a slight bucking angle that will push them out of mesh. Main shaft bearing play can in some instances allow relative movement across the mated dogs that can cause an unmate force.

2) The shift shaft detent and shift fork. Detent force is important and the shift fork needs to fully mate the dogs while the detent maintains the engagement. Bent forks are common in lower gears. Make sure the gearchange is free and not loaded by a foreign object like a boot ETC. Even a ruptured engine or transmission mounting can be the culprit.


Hope I helped.
Robert Kenney

Interesting how "shift-er" is posted as "gearchange"

Thanks Rob

another prime suspect (I have heard from the previous owner of the gearbox) is a loose ball and socket joint in the remote (see photo), I understand that the amount of movement of the gearchange is critical. That assembly has to be inspected and tightened up periodically.

G