Gearbox

Have a '73 850, have a small (at the moment !!!!) problem when shifting from 4th to 3rd , not every time, but annoyingly it doesn't find 3rd needing a couple of tries before engaging the gear, very very occasionally this happens going from 3rd to 2nd, is this a major problem that needs fixing now ?

As it seems to occur when changing gear in one direction (on the down change in this instance) then the ratchet 'hairpin' spring is possibly in need of adjustment.

Hopefully a easy fix with the spring.

In addition, it is worth draining the gearbox and checking the condition of the cam plunger piston and bore it resides in. The bore is in the nut that secures the plunger piston and spring. The cam plunger mechanism sits close to the bottom of the gearbox and can get dirty inside the bore. The grit in the bore slows the piston action down some. The top of the plunger can also get scored or flattened. When the plunger is dirty or the tip is flattened it can cause miss shifts. Once the plunger piston is cleaned up and polished (if it is still in good shape) it can make a world of difference on gear selection. Very simple to clean up the piston and bore. Don't touch the shifter while the plunger is out. Note there is no fiber gasket on the plunger nut, so don't spend any time looking for one that is not missing.

+1 for the hair spring - my dad's 850 was doing the same thing 20 years ago - that was the culprit - it was actually very badly worn from a poor fit, replacement was required

Derek Wilson said:

+1 for the hair spring - my dad's 850 was doing the same thing 20 years ago - that was the culprit - it was actually very badly worn from a poor fit, replacement was required

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Indeed - the spring is quite easy to trap as maintaining it in the correct position on assembly requires some care.

Schwany said:

In addition, it is worth draining the gearbox and checking the condition of the cam plunger piston and bore it resides in. The bore is in the nut that secures the plunger piston and spring. The cam plunger mechanism sits close to the bottom of the gearbox and can get dirty inside the bore. The grit in the bore slows the piston action down some. The top of the plunger can also get scored or flattened. When the plunger is dirty or the tip is flattened it can cause miss shifts. Once the plunger piston is cleaned up and polished (if it is still in good shape) it can make a world of difference on gear selection. Very simple to clean up the piston and bore. Don't touch the shifter while the plunger is out. Note there is no fiber gasket on the plunger nut, so don't spend any time looking for one that is not missing.

Click to expand...

Interesting.
During Mick Hemmings gearbox teardown and renewal DVD, if I recall he doesn't mention checking/cleaning it.

Tornado said:

Interesting.
During Mick Hemmings gearbox teardown and renewal DVD, if I recall he doesn't mention checking/cleaning it.

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I looked at a Hemmings document about gearbox teardown. Ended up not using it and referencing the exploded diagrams of the gearbox in my old Haynes manual when I finally got around to taking the gearbox apart.

I did not have to take the outer cover off and mess with the ratchet spring and pawl to fix an occasional issue upshifting from 2nd to 3rd. I just cleaned up and polished the camplate plunger assembly. It may not help with downshifts. I never had an issue with downshifts.

I would think if someone was going to go to the trouble of taking the outer cover off, they might as well check the condition of the plunger since the pressure of the spring under the plunger against the camplate is what holds the camplate in position. The rachet moves the camplate, but does not hold the camplate in position if the plunger is not working optimally. That's how I see it working anyway.