[Healeys] Gearbox jumping out of 4th gear coasting downhill or decelerating

A H List austinhealeyslist at gmail.com
Tue Nov 7 18:20:51 MST 2017

Hi Bob,

Yes the Healey synchro design is slightly different to conventional
but the principle of operation once in gear is the same. The
engagement teeth rely on the internal hub tooth faces being parallel
to prevent them from trying to push back out of gear and when worn
they tend to be slightly tapered in the wrong direction. The later
(probably Japanese) dovetail design fixes that by making them tapered
in the direction that pulls them into gear.

Those sloped ramps you refer to are a half-assed attempt by BMC to
stop the central hub part moving towards the opposite gear (eg 3rd
when driving in 4th) and wearing out the synchro cone. In the early
boxes there is nothing to stop that apart from the three ball bearings
and springs holding it in place against friction. The idea is that
those lozenge shaped radial pins in the synchroniser assembly ride
down the ramp and lock the central hub onto the mating grooves on the
mainshaft which prevents it from being able to slide backwards. It
doesn't work as you'll find you can engage 4th gear and with a
screwdriver, push the central hub backwards until the 3rd gear synchro
rubs on 3rd gear cone face anyway...

I used a BN7 mainshaft and ground the grooves in the shaft shallower
so the system can work as intended- for better or worse. I'm sure you
could just leave out those lozenge shaped pins and nobody would be any
the wiser. I bet though if you got the synchro hub on the main shaft
in the wrong orientation so the notches and pins weren't aligned right
it would work fine until one day when you were in the middle of
nowhere, jam permanently in 3rd gear.


On 11/8/17, Bob Spidell <bspidell at comcast.net> wrote:
> The Healey hub looks a little different, with 3 'notches' where the
> synchro ring 'ears' fit, and there is an inner groove cut in all the
> teeth, presumably where the detent balls reside. Curiously, two of the
> teeth are 'sloped'--for lack of a better word--one on each side, and we
> can't figure out why.  Our best guess is that the slope relieves some of
> the pressure from the detent balls, making it a bit easier to move the
> slider in and out of gear (which, of course, would contribute to my
> problem).  I'd post a photo, but the car is at my folks' house and I
> won't get back down there until next week.
> Thanks again for the help.
> Bob

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