IDENTIFYING BN1 GEARBOX GEARS

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.

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.

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:

Whereas a correct input shaft gear match looks like this:

REVERSE GEAR

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.

 

About 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.
This entry was posted in Healey Stuff, Restoration Techniques, The Restoration of Healey #174. Bookmark the permalink.

3 Responses to IDENTIFYING BN1 GEARBOX GEARS

  1. Unquestionably believe that which you said. Your favorite justification appeared to be at the net the easiest factor to remember of.
    I say to you, I definitely get annoyed whilst folks consider worries that they just do not
    know about. You managed to hit the nail upon the top and
    also outlined out the entire thing without having side effect , people can take a signal.
    Will likely be again to get more. Thank you. Also visit my invention best cord electric mowers #agreenhand

  2. I’ll іmmediately grab youyr rss as Ӏ can’t tο find yoսr email subscription link ᧐r e-newsletter service.

    Dօ you have any? Pleaѕe let mе realize so that I mmay ϳust subscribe.
    Thɑnks.

  3. Aԁmiring the time and energy you put іnto your site andd detaileed information you present.

    It’s great to come across a blog every once іn a while that isn’t tthe same oⅼd rehashеd material.

    Wondеrful read! I’ve saved your ѕite and I’m incⅼսding yⲟur RSS feеds
    to my Google account.

Leave a Reply

Your email address will not be published. Required fields are marked *