Re: Triumph Herald Overdrive

Actually, no, but I did this on my 13/60, with little trouble. A local place cut down and re-made a spare propshaft to my specifications, and the result was great. I do not know anything about D-Type and J-Type boxes, I just modified whatever I had. Good luck!

A long while ago (somewhere about 1980) I had a Dolomite 1500 with overdrive where the gearbox self-destructed dumping it oil on the clutch and the road. I went hunting round the local breakers and found a replacement overdrive gearbox, but when I got it home it wasn't a straight swap - the dimensions were different (perhaps the difference between the D and J units?) - so I took it back for a refund. There was a non-overdrive gearbox for the Dolomite there, so I took that and the associated propshaft. The splines were different so I had to get a different clutch that would fit the gearbox splines. When fitted, the original overdrive gear lever (with a switch built into the knob) fitted the gearbox but it wasn't perfectly aligned to the hole in the transmission tunnel, so that hole needed to be adjusted with a file.

I don't know if the Herald is simpler, but I do know that the overdrive gearbox is longer so you will need to replace or re-engineer the propshaft. You might have to check that the gearbox mountings are in the same position and adjust if necessary too. I think the difference between the Herald and Spitfire engines are a longer stroke so the engine blocks should be identical.

thank you :)

Er ... I have turned up at this party a bit late, so a bit reluctant to throw a spanner in the works, but .....

Has any thought been given to the rear axle ratio ?

I know that a number of cars which had overdrive units factory fitted had a slightly lower ratio than the "normal" models.

For example, my 3.4 Jaguar Mark 2 has an overdrive and a 3.77 diff whereas the non-overdrive models were fitted with a 3.54 diff

I have no idea what the overdrive gear ratio might be for the Herald, but on the Jag its 0.778:1 - which with the normal diff ratio might make top gear unmanageably tall (obviously I have never tried it)

One other big difference is that overdrive can only be engaged in top gear on the Jag whereas the Herald might allow it in 3rd or even 2nd ? On the Herald it might be used as a ratio splitter, whereas on the Jag it’s just a 5th gear

The gearbox I purchased didn't have a gearlever attached, so I just put a toggle switch on the dash. Worked fine, and it operated on all gears, which could result in an interesting close-ratio box effect.

When I first fitted the box, I didn't know that it had the operating valve stuck in the 'On' position, so when I first drove the modified car, it went on its own from first to first-overdrive, and then second-, third-and fourth-overdrive. The problem was easily fixed.

Note my mention in my other post also about the length pf the propshaft required.

OK

Jag owners are warned against fiddling with the overdrive switch to allow the overdrive to be used in any gear but top.

Theres some sort of torque multiplying effect in the lower gears which can cause overdrive gear teeth to get stripped, which I imagine is insanely expensive.

There used to be a prop shaft balancing specialist on an industrial estate in Wembley, and a good chunk of his work was to do with overdrives - I guess he is long gone by now though :-\

My engine was a 1300cc Spitfire, with one carburettor, so the main difference from the 13/60 engine was the camshaft. Base output: 61 bhp (I think). My car's output was not much more, it just happened at different engine speeds. Ad I usually only used the O/D in the higher gears.

There is a company near me on the Suffolk/Norfolk border which will balance anything you take to them.