BN1 GEARBOX GEAR IDENTIFICATION
There are essentially 2 types of gear sets for the BN1 gearbox because, in an attempt to improve durability of the gearbox, BMC changed the pressure and helix angles of both the input shaft gear and the 3rd gear (and of course their mating gears on the laygear) in later gearboxes.
When rebuilding these gearboxes, it is critical that matching gear pairs are used as mismatched gears will fail very quickly.
This is the method I use to check that the correct gears are being used.
3rd GEAR IDENTIFICATION
When you present the 3rd gear up to its matching gear on a laygear, if it is the correct gear, it will nest parallel when the gears are meshed… like this: If it is the wrong gear it will sit at an angle when the 3rd gears are meshed like this:
As a further check, if you have two 3rd gears, you can check that they are the same helix angle by putting them back to back and looking along one tooth. If the gears have the same helix angle they look like this: note the straight line.
BTW The later 3rd gear (1B3697) is the type where the teeth are very slightly nearer parallel to the shaft.
INPUT SHAFT IDENTIFICATION
The same test can be used to ensure that the input shaft is the correct type for the laygear. An incorrect input gear match looks like this when the gears are meshed:
If you happen to be using gearbox parts from an Austin A70 or early A90 a further complication can arise with respect to reverse gear.
Gears from these early boxes, although not originally used in a BN1, can be used in the Austin Healey gearbox however particular care must be exercised with the reverse and 1st gears.
The reverse idler used in the A70/90 box has 14 & 18 teeth and it must be used only with a matching 1st gear which has 30 teeth.
The later A90 and BN1 reverse idler has 13 & 18 teeth gear and must be used with a 1st gear having 29 teeth.