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TR4/4A TR4A A-type overdrive and Moss Europe TR2-4A cat.

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HerronScott

HerronScott

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So I didn't realize the TR4A had a different A-type overdrive assuming the following information from the Moss Europe catalog is correct. I think this was a change in the accumulator.

OVERDRIVE UNIT, (series 22/61275) TR2 To TS5979
OVERDRIVE UNIT, (series 22/61374) { TR2 From TS5980, TR3-3A} TR4 (To October 1964)
OVERDRIVE UNIT, (series 22/61712) { TR4 (From October 1964)} through TR4A solid rear axle
OVERDRIVE UNIT, (series 22/61753) TR4A IRS

Note: Overdrives can be interchanged as complete units. It is not recommended that 22/61275 is used on later cars, as it is hydraulically incapable of handling any ‘extra’ torque, especially in 2nd and 3rd gears. It is, however better than nothing, and if used on later cars should be restricted to 4th gear use only. 22/61753 is the only safe unit to use on the TR4A IRS, as none of the earlier units feature the ‘cushioned’ engagement. This modification was introduced in 1965 to smooth out engagement, it also lessened the likelihood of the engagement to rip the differential mountings off the chassis.

I found Moss's Restoration catalog to be pretty interesting and wouldn't mind have a printed copy.

Moss Europe TR2-4A catalogs

cott
 
The Moss catalogs are definitely a useful reference, but I wouldn't regard them as gospel.

For example, it's fairly trivial to convert an earlier OD to use the later accumulator arrangement; and in fact this is sometimes done to save a housing with a damaged accumulator bore. Some folks also like the harsher engagement, so occasionally you'll find one that has been converted back to the large accumulator. (The Healey folk consider this a "performance" mod.) So the number on the plate doesn't necessarily indicate the presence (or absence) of the smaller accumulator.

Besides which, it takes only a modicum of care to avoid shocking the driveline.

Another example, there are many ODs originally fitted to TRs that don't have one of the above numbers. I believe the '6' after the '/' in fact indicates a factory rebuilt unit; and many cars don't have that digit. Some cars have a '2' instead of a '6' (which may indicate a manufacturing location).
 
Randall,

I never had any problem with the TR3 overdrive I installed on my rebuilt TR4A transmission in the course if 75,000 miles or so (with the earlier accumulator).

I certainly wouldn't take the catalog as a definitive source, but it's much more interesting than the Moss US catalog.

Scott
 
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