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.


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.

But back to back with different helix angles they look like this:

I know the difference is subtle but it is pretty obvious when you have a matched or mismatched pair.

BTW  The later 3rd gear (1B3697) is the type where the teeth are very slightly nearer parallel to the shaft.


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:

Whereas a correct input shaft gear match looks like this:


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.


By Michael

Who is this guy? Born in New Zealand some time back. Went to Maori Hill Primary School then Kaikorai Valley High before joining the RNZAF and an Airman Cadet in 1968. Graduated 1972 with an NZCE in Aeronautical Engineering. Then embarked on the typical Kiwis "Big Trip Overseas". Got to see quite a few places, and spent a while in the U.K. "home" as it was refered to by many New Zealanders in those days, before travelling on to New York and then to Canada by bus!! This trip is presently on hold (has been for the last 34 years). Met my dear wife Judy not long after arriving in Ontario and we have been happily married since 1976. After travelling around New Zealand and the pacific in 1979 I started Precision Sportscar andfor the next 23 years grew the business and helped raise 2 boys Drew and Robin.

1 comment

  1. I have a first gear that I believe is for a early A90 gearbox as it has 30 teeth. The old BMC box has no part number and I am trying to find the correct part number for the first gear and slider to make sure what it is. Might you have a A90 parts list that has this info and would you be so kind as to let me know the part numbers? Thanks,


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