Three speed gearboxes

[Alan]

IDoes anyone know- are the gear sets and shafts in an early belt drive speedo box the same as in the later version with speedo drive.  I’m guessing the gears are the same but the 3rd motion shaft is different?

[Dave Mann]

Many years ago I was warned not to swap gears in the 3 speed boxes as Austin's changed the gear tooth profile. By whom I can't remember. Andrew Bird would be the gentleman to answer your query.

[Parazine]

Tooth profile was changed in late 1926/early 1927 I believe. It's always best to use gears in matched sets; they've run together for many miles so will have adjusted themselves to each other.

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And, yes, the third motion shaft was lengthened with the gear driven speedo.

[Alan]

Thought as much. But tooth profile apart would the gear sets interchange? Right now I have a good early box with a damaged tooth and a late box with a damaged casing

[Parazine]

I'm pretty sure that, used as a set, the gears will interchange, using the shafts appropriate to the casing. There are some differences to the first motion shaft however (the length was changed at some point, and the length of the splining changed when the clutch plate splined length became longer (1928?)). There were also some changes to the first motion shaft housing, which may cause problems. The very late boxes used an oil scroll at both ends and may be different again.
I guess you'll need to get both gear sets on the bench and carefully compare to see what can be achieved.

[Alan]

Yes that’s what’s needed. It’s not critical I have a spare box of the right sort, just the broken one is the original (I think) and I’d want to repair it.

[Ron Hayhurst]

The question raised about a change in gear profile is answered in Volume 1 No1 of the Austin Service Journal dated April 1927. On page 3 it states:
"In order to ensure longer life and quieter running, the shape of the tooth on the gear wheels was altered on Car Number A3 9000 and all following. As we do not now manufacture gears with the old shape teeth, when fitting or ordering spare gears for any cars previous to this number, it will be necessary to do so in sets. The catalogue numbers have not been altered"
By not changing the part number, a glance through the hand books available in the Archive would lead one to think that there had not been any change! I estimate that the chassis carrying Car number A3 9000 would be about 28500 - say early 1927. This is some time before the speedo arrangements changed.
As Tim and others point out, gears should be kept in their sets - i.e. all gears in a set should have the same inscribed number, which is peculiar to a given box - something we all hope to achieve. However, this raises one or two points that others may care to add to this thread. 1) on stripping a box it may become apparent that the gears are not part of a set! 2) one gear may be badly worn and you have another in your box of bits that looks OK as a replacement; 3) Second gear in a 3 speed box is often noisy these days so just how much noisier will it be with an "old" and "new" gear in mesh? 4) finally does anyone fancy posting a pic of the tooth shadow cast by "old" and "new" gears, and if it is seen to be marginal (or even if it isn't marginal!!!), just how much should we be bothered!
Ron

[Mike Costigan]

Checking the Chassis Register, it looks like A3-9000 could be anywhere between chassis numbers 28000 and 31000! There are two cars, A3-8600 and A3-9536 with chassis numbers 29913 and 31020, which fit nicely and would suggest A3-9000 is around 30500, but another two cars with A3-8253 and A3-8289 have chassis numbers 27160 and 27193 which would put A3-9000 around 28000 - yer takes yer money and takes yer pick!