TB/TC Gearbox Synchromesh

It’s a universal truth that anyone on possession of a TB/TC gearbox must be in need of good synchromesh. Although worn synchromesh can be overcome by “double declutching”, this isn’t always possible to carry out effectively and usually benefits from any residual synchromesh.

Knocking shards of “dog teeth” into the oil to form a distributed “grinding medium” is not something one wants to worry about every time the “gears” are graunched. Sticking a magnet in the sump plug should help preserve the gearbox as well as regular oil changes.

Synchronising the speeds of the “gears” before they are “meshed” depends on the friction between two interlocking cones forced together. It’s not really a “meshing of gears” as second and third gears on the laygear cluster are permanently meshed with the mainshaft/output shaft gears (Fig.1). These are free to rotate independently, but can be locked to the mainshaft by engagement of “dog teeth” attached to the actual gear. Fourth gear is a direct drive through the engagement of “dog teeth” and first is a “crash” meshing of spur gears.

Fig. 1: the TB/TC gearbox internals.

The use of “dog teeth” to engage the various gear ratios offers a more robust way than trying to mesh actual gears, as all the “dog teeth” are engaged simultaneously. Any direct meshing of gears would have to be achieved one tooth at a time, would be difficult to synchronise and would also need noisy straight cut gears as opposed to the quieter helically cut gears.

Photo 1 shows the sliding hub mechanism; this consists of an inner hub free to slide along the mainshaft splines, and an outer sliding hub keyed to the inner hub also by a series of splines. The inner and sliding hubs are semi-locked together by six spring loaded balls. These are buried in the inner hub with just the balls slightly protruding into a recess machined in the sliding hub’s inner bore.

Photo 1 showing the sliding hub mechanism.

The whole hub assembly can be moved backwards and forwards by the selector fork which fits a groove in the outer sliding hub. (Photo 2).

Photo 2 (see above text)

When a gear is selected, the whole hub advances towards the male synchro cone which engages the female synchro cone ring secured in the inner hub. These two cones lock together but the outer sliding hub can continue sliding over the inner hub after forcing the spring loaded balls, previously held in a locking groove, to be displaced. Any further inner hub movement would have been arrested by the engagement of the cones. The spring loaded balls are used to determine the force applied to the cones to achieve synchronisation between the main shaft (continuously driven by the rear wheels) and the selected gear driven by the laygear cluster (in the case of 2nd and 3rd gear) which in turn is driven by the first motion shaft and clutch.

The synchromesh has to adjust the speed of not just the selected gear and the whole laygear cluster but also overcome the inertia of the input shaft and clutch. No wonder anyone sensitive to mechanics continues to “double declutch” when changing down.

As mentioned, the sliding hub continues to slide over the inner hub’s splines until its own internal splines engage the “dog teeth” just beyond the male synchro cone. At this point the clutch can be released to allow transmission of power through the selected gear with its attached “dog teeth” coupling the drive to the main shaft via the splined hub assembly.

The design of the synchro cones needs to be a compromise between a taper angle that’s too shallow, which might “lock up” the cones, preventing them from releasing when the selected gear is disengaged, and too obtuse an angle which would not benefit from the tendency to “lock up”.

A further complication arises due to the synchro cones running in oil, this is overcome by a series of grooves cut in the female cone ring which displaces the oil in much the same way that a tyre’s tread prevents aquaplaning. The female cone ring is made from a copper-based alloy and consequently wears down in time, reducing the groove depth and effectiveness of the synchromesh. The male synchro cone is machined and hardened from steel and then ground to give a polished finish.

Modern cars have floating synchro cone rings which can be easily replaced, unlike ours which are fixed in the inner hub. However, it seems that MGA synchro cone rings can be machined to fit our hubs. On the strength of this advice, a new MGA cone ring was tried but found to be too big. Surprisingly, an old MGA cone ring was found to fit but with a slight “wobble”. After measuring the taper angles of the cones (photo 3) we discovered that whilst the TB/TC taper is 7 degrees, the MGA taper is 10 degrees. Old MGA synchro cone rings are therefore likely to work to a limited extent but with a reduced life.

Photo 3: measuring the taper angle of one of the cones.

We decided to machine our own cone ring, but what material to use? MGA rings seem to be made from a hard brass whilst the TB/TC rings seem to be bronze. We chose a leaded bronze as opposed to the harder wearing phosphor bronze because of machining considerations. The original grooves have a form similar to that of an Acme thread and their pitch was about 30 thou. needing a cutting tip some 15 thou. wide. We were tempted to increase the pitch to 40 thou. as modern rings have a coarser pitch making the thread form a bit more robust.

After setting the top slide of the lathe to match the taper of the male cone on the input shaft at 7 degrees, the internal taper of the bronze synchro ring was turned and then checked with the cone on the input shaft (Photo 4) using “Engineer’s Blue”. A special cutter tool was ground to produce an Acme profile with a side angle of 7 degrees, the same as the taper angle to reduce loading on the cutter (Photo 5) and a series of grooves cut about 35 thou. deep.

Photos 04 and 05 supporting the above text.

The outside diameter of the synchro ring was then machined to give a 2 thou. interference fit in the hub once the old synchro ring had been removed, curiously this was found to be split.

Any oil channelled into the ring’s grooves needs to be expelled by a series of axial furrows which were cut using a 3mm end mill in a milling machine set at an incline of 7 degrees (Photo 6) to match the taper angle.

In conclusion, we were anticipating that the synchro cone might take a while to bed in, but found the new cone to be quite effective, which has allayed some concerns that I was dabbling in yet another “black art”. It’s possible that anyone involved in model engineering stands a good chance of being able to machine a replacement cone.

Photo 6 – machining furrows in cone.

Photo 7 – close up of the finished synchro.

Ed’s note:      Another superb technical article from Eric Worpe. Thank you, Eric.

When was synchromesh invented and when was it first used in a car?

Henry Thompson’s 1918 invention of a synchromesh manual transmission using a tapered cone synchronizer to prevent gear clash (US Patent US1435430 in March 1922) would make shifting a manual transmission faster, easier and safer.

The first usage of synchromesh was by Cadillac in 1928.

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