install overdrive switch

[Dkalel]

I am adding an overdrive to my 77 MGB and I was wondering about the gear shifter and how to put the overdrive switch onto the old gear knob. I saw online that they have a cap with the switch you can put on top of the shift knob. Can you switch the old top of the knob with the one with the overdrive switch? I don't have my car at home as it in the shop being worked on or I would be able to figure this out on my own or take a picture to show what kind of knob I have. Has anybody else done this? Any advice will help if you can considering I don't have the knob or pictures. The knob I have has MG on the top and the knob is leather or vinyl and I don't see any way to replace the top from what I can remember what it looks like.  I believe I will have to get a whole new knob for my car or buy an used gear lever with the overdrive switch. Any ideas?

[june2020]

My old B had a toggle switch on the steering column to control the OD
I kinda liked it that way
Tom raddatz

[june2020]

Doesn’t the windscreen wiper lever also have a OD switch?
John Hennessy

[june2020]

In order to put the overdrive switch on the shifter, you would need the original shift knob, with the top for the switch. They are getting hard to find. If you wanted to put the overdrive on the wiper stalk as was done prior to '77, it will require some rewiring and modifications of the steering column.

George Van Horn

[june2020]

If you go with the OD shift knob you also need the OD shift lever. It has a groove for the wiring to the switch.
Steve Phillips

[june2020]

The switch already a part of the wiper control is the simplest way to go. Can even get a factory type wiring harness for it.
Robert-Amy Gehweiler

[Art]

I'd say to go with the gearshift switch.  Besides being original (and looking like the ejection seat switch in Bond's Aston-Martin), the 77-80 MGB were only wired for the shift-top OD switch. 

If your new trans is either missing the shift lever or you're using an older transmission, you'll need to find the complete gearshift assembly from any of those years.
 
The major catalogs do sell the switch (which is shared with the Triumph Spitfire), gearshift top cover, shift lever wiring harness and other small parts.

I didn't see the OD gearshift knob or the grooved gearshift lever itself offered new, but those can be found used, if with some effort.  There's a grooved shift lever on EBay for $60 OBO right now.

Hope that helps.  Good luck with the conversion.