Second Test Drive

Around the block this time, about a half mile.

Brakes are good.
Generator is good.
Oil pressure is good.
Water temperature is good.
Transmission is noisy in 1st, 2nd and 3rd, ok in 4th.
Trans oil level is ok.
I understand about 1st, straight cut gears.
This is my first experience with double helical gears.
Is that noise in 2nd and 3rd characteristic of double helical gears?
I would have thought not, as quietness was supposed to be a feature of helical gears over straight cut.
Or is it characteristic of 96,000 miles?
Thoughts from the more experienced?

Hi Rob,

You can compare it with mine here:


2nd/3rd should be quiet: sounds like a worn gearbox.

Congratulations on the drive!

Thanks for sharing the double helical experience, I can’t recall hearing of them in car transmissions. Not being experienced, I can’t say what they should sound like in good or bad fettle.

Closest experience for me is straight-cut second gears in cars from the '20s and '30s can sound like a kitchen disposer. Straight-cut has zero axial thrust but is noisy. Maybe the double helical was a step towards quiet and also keeping zero, or small, axial thrust compared to single helical gears. And maybe manufacturing costs, lifetime, and thrust washer costs, for car applications caused the single helical to win out?

Anyway, your drive is fun to learn about. Cheers.

Thanks for the drive, Peter.
I should have mentioned that in my Mark V and XK120, I have maybe 20,000 to 30,000 miles experience with JH single helical gearboxes.
I went around the block again to compare with Peter’s, this time after dark so with the P100s on. Hmm, not very well aimed but there was no other traffic.
Peter’s is a bit louder than my XK120 box in 2nd and 3rd, but my SS double helical is louder than that, and only when driving; it is quieter when shifting or rolling with the clutch pedal depressed.
So my first thought is needle bearings in the counter shaft.
Looking in the '46-48 Service Manual, the double helical seems to be nearly identical in construction to my JH boxes, so the C.918 needle bearings in the counter shaft should be identical.

Rob I thought the double helical boxes had bronze bushes on the lay shaft. I know the clearance is critical.
Also the SS had a rubber gearbox cover, a bit like a jaguar version of those in T Type MGs. These should do a bit to insulate noise and heat.
They stopped making repros recently but you might be lucky enough to find one. They are quite neat and excellent mouldings.

Hi Rob,

Ed is right, the layshaft has bushes rather than needles. I don’t think there was ever a repro of the rubber cover for our gearboxes but Derek Sharpe made repros for the coachbuilt cars and these can be modified to fit reasonably in the ours.

When I took the video of my car it was just after I reconditioned my gearbox.

Apart from the synchromesh work I also replaced the ball and roller bearings. If memory serves me right don’t think I did anything with the layshaft bushes or thrust washers. I guess I don’t need to tell you that my car was carpeted in the video. That said, I have been in a 1937 coachbuilt car whose gearbox was noticeably quieter than mine.


Yes from a practical point of view, it wouldn’t be economically feasible to make the various versions. but they can be cut and shut. WE have fiddled them to fit in 100 models and steel bodied saloons ans well as the coachbuilt models

And here’s how we mounted them , Cut off the horizontal flange. Made 2 L shaped sheet metal pieces that were mounted about 1/4"apart and pushed thevertical edge of the rubber in between
A useful thing we found on the 1940 model was a semi-circular sheet of rubberon the tow board whic pressed against the bell housing and deflected hot air from coming along the space above the gearbox , and into the cabin.

Ok I see the bushes called out.
Then the next candidate is the two main roller bearings and more likely the ball bearing in the tail case, as the noise seems to be coming from the rear.
Hoffman MS12, also used in JH boxes.
My synchros seem to be good.

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I’m curious here. Is top gear a straight through lock up?

Yes. That is generally the case in vehicles that are not front wheel drive.

I hadn’t heard the term lock up, but yes, the 4th synchro engages the input shaft directly, so the ratio is 1:1.

Most 4-speeds do that: VW (Bugs) were a notable exception, because you were driving through gears in all forward speeds.

Early Datsun 5-speeds, also used 5th as a 1:1.