Overdrive question..?

[Ian R]

Are all A Type Laycock overdrives the same and fit a 100/6.
The BN4 I'm restoring has had its overdrive removed for some reason and a
non overdrive box put in. I would like to change it back.

 

[Healey 100]

You will have to change the complete transmission with another one that includes the OD. I believe the main shaft on the non-OD tranny is quite different from the OD version, so it is not a matter of just adding an OD to back end of your non-OD transmission.

Years ago I did change another 100/6 with a non-OD transmission. It was not difficult and it worked OK. But it is harder today to find a rebuildable 100-6 transmission with an OD.

PS you probably have one of the non-OD Healeys that were built that way from the factory, there were some. I understand these came with a lower numerical rear axle which is a plus whether you make the conversion or not.

 

[British_Recovery]

Gearboxes can be swapped, but adding an overdrive to your existing 'box would require additional modifications. The main shaft, for example.
Bob

 

[Bob_Spidell]

If you can scare up the gearbox/mods Denis Welsh offers rebuilt ODs for sale outright:

https://www.bighealey.co.uk/healey-overdrive

If you can wait until the effects of Brexit fully register, the exchange rate might be more favorable.

 

[John Turney]

...

PS you probably have one of the non-OD Healeys that were built that way from the factory, there were some. I understand these came with a lower numerical rear axle which is a plus whether you make the conversion or not.

A BN4 that came with overdrive will have an "O" in the serial number. No "O", no overdrive.

 

[Ian R]

It was originally built with an overdrive it has the "O" in the chassis number, is recorded on the heritage certificate and still has the wiring & switch gear.
I've been offered an unused "A" Type overdrive at a fraction of the listed price but he is unsure what vehicle it was actually intended for. I realise the main
shaft has to be replaced but as part of my restoration I will have the gearbox overhauled I thought this would be the time to do the change.
So, back to the question "are they all the same"...?

 

[Bob_Spidell]

I'm getting ready to tear into my BJ8's OD, so I've been studying the parts catalog, and I've stayed at a Holiday Inn before--bad US TV commercial reference--so I'll go out on a limb and say, with a couple exceptions, they should be interchangeable. In particular, the gearbox->OD adapter plates vary (DWR has them). Earlier Healey ODs have a different brass drain plug with a (very handy) square stud for wrenches, whereas later one have a notched perimeter that requires a special wrench (or hammer and drift). The actuating piston sealing mechanism--rings vs. O-rings--varies between models, but they appear to be interchangeable. Healeys used a smaller, weaker accumulator piston so that OD engagement wasn't as traumatic, but some substitute the larger accumulator and spring--found, for example, on TR4s--for quicker OD engagement. I just had a conversation about gearboxes and ODs with Tom Monaco at Tom's Import Toys (https://tomsimport.com/), and he said the TR ODs they overhaul usually have more wear/damage to parts like thrust washers due to the more dramatic engagement. Driver technique can determine wear as well:

https://www.quantumechanics.com/

I would try to contact these people (above), as maybe with a photo or two they could identify the OD you are considering buying.

 

[Randy Forbes]

To the best of my recollection, any Healey overdrive unit will fit on any__side or center-shift__Healey XMSN that has the ovd-specific mainshaft and adapter plate.

It has been my most recent experience that the 100/6, specifically mine (5/14-15/1958 build date) requires an overdrive with exposed solenoid linkage. When I attempted to fit an overdrive with an enclosed solenoid linkage, it did not clear the chassis.

I'm not sure when the change-over date was, but it would stand to reason that cars before that date *might* run into interference there.

This is an early style with exposed linkage (solenoid not yet mounted).

Another 100/6 unit.

I'm at a momentary loss for a good picture of the later type overdrive (somewhere, I have a picture of the fresh Quantum Mechanics ovd NOT fitting into my car...) but here's a bonus TR3 trans with a very similar overdrive; primary difference is the tail housing/mount, but many of the internals interchange, though they may be of different ratios, so some components would have to be swapped in sets.

 

[Ian R]

Thanks for all the info and pictures...(very helpful)..
I think I'll take a chance and buy the overdrive I've been offered, if it's wrong I can
always move it on.
I will get a specialist to do the overhaul / conversion for me.
Thanks again to all......