John Healy
Transmissions of this era have several conditions:

• Shaved and non-shaved gears.
• Stepped and non-stepped shafts.
• Gears in two conditions to fit the stepped and non-stepped shafts.

• The non-stepped lay shaft has a major shaft diameter of 1.123" to 1.1225".
• The stepped lay shaft has a major diameter of 1.123" to 1.1225" and a minor shaft diameter of 1.1190" to 1.1185"

• The non stepped main shaft has a major diameter of 0.9985" to 0.9980".
• The stepped main shaft has a major diameter of 0.9985" to 0.9980" and a minor shaft diameter of 0.9945" to 0.9940"

• Non-splined gears bore diameter are the same for both conditions.
• Splined gear's bores vary to be compatible with their respective stepped or non-stepped shafts.

• The T916 main shaft 2nd small bore (stepped shaft) used up to early 1969.
• The 57-3858 is an alternative for T917 replaced by the 57-3858 up to early 1969. It was used with 57-3860 (22T) lay shaft 3rd for the stepped shaft.
The 57-3858 (24t) can be replaced with 57-3889 (23T) main shaft 3rd + 57-4155 non-stepped lay shaft + 57-3890 lay shaft 3rd (22t). These three must be used as a set when replacing the 57-3858.

It is obvious that you cannot fit a splined gear for a stepped shaft on a non-stepped shaft. Less obvious is fitting a splined gear for a non-stepped shaft on a stepped shaft. This is going to cause all sorts of shifting problems.

The gears you listed are all for a non-stepped condition (up to early 1969) so fitting non-stepped gears could cause you problems.

Originally Posted By John Healy

The 57-3858 is an alternative for T917 replaced by the 57-3858 up to early 1969. It was used with 57-3860 (22T) lay shaft 3rd for the stepped shaft.


The 57-3860 layshaft 3rd is for a non stepped layshaft. There's a chance whoever installed the 57-3858 mainshaft 3rd upgraded to the 57-3860 layshaft 3rd and a non steeped layshaft. If that's the case, the gears you ordered should fit. If you still have the stepped layshaft, I don't see why you couldn't exchange the 57-3890 3rd for the earlier gear with the small bore and the 57-3889/57-3890 ratio (57-2379?). 

"The 57-3860 layshaft 3rd is for a non stepped layshaft. There's a chance whoever installed the 57-3858 mainshaft 3rd upgraded to the 57-3860 layshaft 3rd and a non steeped layshaft. If that's the case, the gears you ordered should fit."