Weathersbubble Posted August 14, 2022 Report Share Posted August 14, 2022 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? Link to comment Share on other sites More sharing options...
johny Posted August 15, 2022 Report Share Posted August 15, 2022 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 Link to comment Share on other sites More sharing options...
Josef Posted August 15, 2022 Report Share Posted August 15, 2022 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! Link to comment Share on other sites More sharing options...
johny Posted August 15, 2022 Report Share Posted August 15, 2022 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... Link to comment Share on other sites More sharing options...
Pete Lewis Posted August 15, 2022 Report Share Posted August 15, 2022 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 Link to comment Share on other sites More sharing options...
johny Posted August 15, 2022 Report Share Posted August 15, 2022 Diff change would make first gear useable as well👍 Link to comment Share on other sites More sharing options...
Weathersbubble Posted August 15, 2022 Author Report Share Posted August 15, 2022 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 :) Link to comment Share on other sites More sharing options...
Weathersbubble Posted August 15, 2022 Author Report Share Posted August 15, 2022 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 Link to comment Share on other sites More sharing options...
Weathersbubble Posted August 15, 2022 Author Report Share Posted August 15, 2022 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 Link to comment Share on other sites More sharing options...
Weathersbubble Posted August 15, 2022 Author Report Share Posted August 15, 2022 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 Link to comment Share on other sites More sharing options...
Pete Lewis Posted August 15, 2022 Report Share Posted August 15, 2022 try this www.mintylamb.co.uk/gearspeed/ Link to comment Share on other sites More sharing options...
Josef Posted August 15, 2022 Report Share Posted August 15, 2022 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. Link to comment Share on other sites More sharing options...
johny Posted August 15, 2022 Report Share Posted August 15, 2022 or for same info in a posher format: Gear Ratio Speed Calculator for Standard Triumph Drivelines - RichyRichRacing Link to comment Share on other sites More sharing options...
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