Herald/Spitfire overdrive gearbox

[Weathersbubble]

Hi all,

Looking for an overdrive gearbox (with overdrive unit) - it will be mated to my supercharged Herald 12/50 engine to try and keep the revs down on a cruise.

I believe most overdrives with small triumph gearboxes were D Type, and I'm keen to stick with a 3 rail gearbox (3 or 4 synchros doesn't matter).

 

Does anyone have/know of one for sale?

[johny]

one on ebay at the moment but price will probably go up some and no guarantee as to condition...

TRIUMPH SPITFIRE D TYPE OVERDRIVE GEARBOX | eBay

[Josef]

Don’t over pay for an overdrive gearbox, the actual gearbox bit. You need a different mainshaft to the non-overdrive but otherwise the actual gearboxes are identical. So converting one you already have may be the way to go. If you do buy a whole unit, make sure all the ancillary bits are present!

[johny]

Also dont forget you have the option of using a different ratio differential to drop engine revs. Probably not a straight swop but should be cheaper...

[Pete Lewis]

if yours is supercharged  a higher ratio diff as Jonny suggests  would be a far better all round idea look out for a spitty  3.63   :1   much cheaper and overall better option with your increased torque

Pete

[johny]

Diff change would make first gear useable as well👍

[Weathersbubble]

3 hours ago, johny said:

one on ebay at the moment but price will probably go up some and no guarantee as to condition...

TRIUMPH SPITFIRE D TYPE OVERDRIVE GEARBOX | eBay

I’m keeping an eye on that listing, just testing the water here too & to get an idea of a reasonable price :)

[Weathersbubble]

3 hours ago, johny said:

Also dont forget you have the option of using a different ratio differential to drop engine revs. Probably not a straight swop but should be cheaper...

I’ll be opening the diff up anyway so this was a consideration

[Weathersbubble]

1 hour ago, Pete Lewis said:

if yours is supercharged  a higher ratio diff as Jonny suggests  would be a far better all round idea look out for a spitty  3.63   :1   much cheaper and overall better option with your increased torque

Pete

I’ll do some maths on the overdrive top ratio and with a 3.63 final drive - doesn’t sound like a bad bet

[Weathersbubble]

3 hours ago, Josef said:

Don’t over pay for an overdrive gearbox, the actual gearbox bit. You need a different mainshaft to the non-overdrive but otherwise the actual gearboxes are identical. So converting one you already have may be the way to go. If you do buy a whole unit, make sure all the ancillary bits are present!

Good point! Just trying to work out the most wallet friendly way of going about this but will rebuild whichever gearbox regardless before it goes in 

[Pete Lewis]

try this www.mintylamb.co.uk/gearspeed/

 

 

[Josef]

Canleys list all the ancillaries (inhibitor switch and cam, adaptor plate, oil pump cam and woodruff key - I think that’s it) so you can get an idea of the cost of new parts. Plus a mainshaft which is by far the most expensive bit, and also not possible to inspect in a used box. 

[johny]

or for same info in a posher format:

Gear Ratio Speed Calculator for Standard Triumph Drivelines - RichyRichRacing