Jumping Out of Gear

A nuisance well known to many owners of 3-synchro gearboxes as used in Magnette Z, MGA and early MGB is jumping out of gear on overrun. What is described here may work for many other boxes as well, though. Workshop manuals indicate a number of items that must be controlled to prevent this from happening such as:
  • locating balls and springs in synchro hubs and shift shafts to be in good condition
  • wear in the locating grooves in those shafts gear change fork wear
  • condition of rubbers in the gearbox mounting that prevent the gear box from moving forwards during breaking (and knocking the gear change lever backwards)

All these and others certainly have an influence but even if perfect will often not remedy the dreaded jumping out of 3rd gear.

Gears1When assembling gearboxes there are a number of tolerances to be respected. It is not always indicated why these tolerances are critical. One such critical dimension is the end float between 3rd and 2nd gear.  These two gears bear on a bronze thrust washer, item 30 in below drawing, part number 1G3331. This washer has slots in it, to locate the 3rd and 2nd speed bushes item 32 , parts 11G3029 and 11G3028 (latter not on drawing). The correct end float must be obtained by selecting one of 3 sizes of front thrust washers item 22, part numbers 11G3127/28/29.

In the past I have invariably found, that even with the thickest washer (2.06 to 2.08 mm) the end float was over the specified limit of .10 to .15 mm. Asking around I was told that this was to be accepted as inevitable.

Gears_2On a recently rebuilt gearbox for a racing MGB that exhibited this problem, I decided to investigate the cause and effect and try to engineer a solution. Looking through the side cover aperture with 3rd gear engaged, I noticed some tilting of the 3rd gear when applying a torque on the 1st motion shaft and clamping the outgoing flange. After dismounting the gearbox internals and stripping the 3rd motion shaft I noticed some wear on the bronze thrust washer (30) and on the 3rdspeed bush (32). My analysis of what happens is the following. The front and rear thrust washers are meant to enclose the 3rd gear tightly and take up any torque caused by the contact of the beveled teeth on the gears. If there is more than the design clearance, two movements are possible: the aforementioned tilting a fore aft movement As the torque transmitted by the gears during the shift down at high speed changes quite substantially in magnitude and direction, any possible movement of the gear results in it being accelerated into a high-speed displacement with high inertia. When the 3rd gear moves it takes the synchro hub with it. If the gear is being stopped dead by hitting the thrust washer, the synchro hub is loaded with kinetic energy and wants to continue moving. The energy seems to be enough to overcome the resistance of the locating balls and springs and the hub disengages itself and pushes the gear change lever into neutral. Wear to the bush and the washer caused by the tilting will aggravate the situation, as will wear to the gear change fork caused by the irritated driver trying to hold the gear lever in the forward position. The correct solution would be to have a front thrust washer (22) with the correct, heavier gauge. That this is necessary at all is  because after years of service there may be some setting in the thrust face of the splines between this washer and the 3rd motion shaft. The washer is of hardened steel and difficult to make for all but the best equipped workshops. Heathrow Sport Cars suggested fitting a shim behind the 2nd gear thrust washer, but this requires matching the profile of the splined shaft. There are better chances to produce a heavier gauge rear thrust washer, in the easier to machine bronze material, as was done in my case. Determine the required gauge to achieve a .10 mm clearance. Make sure the thrust faces are exactly parallel. Carefully file the slots to match the locating clogs on the bushes. I found that a new bush was slightly longer than the gear and had to be ground down to be able to fit the front thrust washer, once the correct clearance was established. Make sure everything turns free when fully assembled. The result is very gratifying. Even drivers who expect  their racing machines to accept cruel and unusual punishment, will be totally satisfied.

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