Final Drives, are they interchangeable?

[turbo stephan]

Wonder if there's enough material to machine a 5 speed pinion shaft down to suit a 4 speed box...

I understand early boxes were 3.89 too.

I've heard people putting 5 speeds into the chill-cast boxes. It must be possible then.

[KoogiA17]

Wonder if there's enough material to machine a 5 speed pinion shaft down to suit a 4 speed box...

I understand early boxes were 3.89 too.

There maybe, but I reckon the shaft material is possibly a bit of a pain to machine. Would have to cut a thread into a bearing surface on the 5sp shaft. Also have to ensure that the lengths of the splined interfaces were the same

HA, that's a good point. I recently created an Excel sheet which allows to select gearbox, gear # , RPM (in 250 steps) and tire size, and then give you the resulting speed in kph and mph.

I've got one of those. Also used it to calculate accelerations and terminal velocity.

However, as Luke mentioned, this would only make sense if you have quite a bit of power and would be especially bad if you're in city traffic.
On another note, IIRC, the very first 900 gearboxes up to 1980 came at least in Europe with the 3.89 pinion and gear set.

Well, the plan is to have quite a bit of power and thankfully I don't live/drive in a city, only use public transportation in the city.


Yeah, the early 900 boxes did come with 3.89 pinion

[nutcase]

Oh, you can change the primary housing and put 5 speed innards into a 4 speed box. Think there's some minor machinging to do as well. But to keep as a 4 speed (with the presumably slightly stronger primary housing with a 5 speed pinion set would take some machining - if the material is there to do it.

[KoogiA17]

I'm sure the bearing surfaces on the pinion shaft will be hardened so machining through those might be fun, although once through it might not be so bad. Can work out if you can machine a 4spd pinion from a 5spd if have access to measurements from both.

Nutcase, you have a passion for collecting gearboxes, got any 900 4speeds?