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Tr3 overdrive transmission

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johnstydo Avatar
johnstydo John Styduhar
Hermitage, PA, USA   USA
I happened to have an older top cover (with the dipstick) and straight shifter for the TR2/TR3 non-synchro gearbox that has two OD switches and was used with an A-type OD. It could be fitted on the subject gearbox and have OD in 2, 3 and top gear. But I would first want to bench test the OD to see if it functions before I would consider a purchase.

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TR3driver Randall Y
Confusion, Los Angeles, USA   USA
The OD was modified internally to develop more force on the clutch; before allowing OD on 2nd gear. So I would definitely proceed carefully when making the change. 2nd gear means roughly twice as much torque applied to the OD, and those clutches can definitely slip if there isn't enough force. There is even a warning in the OD manual.



Randall
56 TR3 TS13571L daily driver
71 Stag LE1473L awaiting engine rebuild
7? Stag awaiting gearbox rebuild

johnstydo Avatar
johnstydo John Styduhar
Hermitage, PA, USA   USA
I'd want to see what pressure the accumulator builds up to on the OD. Does the owner have any information on the bench test?

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TR3driver Randall Y
Confusion, Los Angeles, USA   USA
So, you'd turn the pressure up to make up for the pistons having 50% less area?



Randall
56 TR3 TS13571L daily driver
71 Stag LE1473L awaiting engine rebuild
7? Stag awaiting gearbox rebuild

2long Dan M
Honolulu, Hawaii, USA   USA
The earlier O/D units only worked on top, as they had smaller pistons and steel or cast iron piston rings and smaller or weaker accumultor springs. These units produced less pressure and so were only used initially for top gear. The steel rings were prone to breaking resulting in further loss of oil pressure. Later overdrives have the larger pistons and rubber o-rings.

Dan

TR3driver Randall Y
Confusion, Los Angeles, USA   USA
All true, but the rubber O-ring (on the operating pistons only, not the accumulator piston) didn't come in until during TR4 production. The larger operating pistons and OD on 2nd were introduced during TR2 production (not at the same time).



Randall
56 TR3 TS13571L daily driver
71 Stag LE1473L awaiting engine rebuild
7? Stag awaiting gearbox rebuild

johnstydo Avatar
johnstydo John Styduhar
Hermitage, PA, USA   USA
My early A-type has the large accumulator piston with steel rings, inner and outer accumulator springs, and steel-ringed operating pistons. Puts out loads of pressure and instant engagement. It's now in my TR6.

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TR3driver Randall Y
Confusion, Los Angeles, USA   USA
That's nice, but doesn't seem particularly relevant to this thread unless you're claiming it's one of the first few thousand made for TR2, with the smaller operating pistons.

There are four major change points for the A-type OD as fitted to the Triumph TR series (plus some smaller changes along the way). We're talking about the earliest version, which is quite rare. The slightly later version, with OD on 2nd 3rd & 4th is much more common, having been used on later TR2, all TR3/A/B, and the first 25000 or so TR4 (when OD was fitted, of course).



Randall
56 TR3 TS13571L daily driver
71 Stag LE1473L awaiting engine rebuild
7? Stag awaiting gearbox rebuild

JoeOtero Avatar
JoeOtero Joe Otero
Lexington, Kentucky, USA   USA
1963 Triumph TR3B
1968 Triumph GT6 MkI "Cali"
1972 TVR Vixen
In reply to # 1491843 by 2long If you are looking for it to go to a good home, my Doretti's original early gearbox (TS 1076) is non-overdrive, and I would sure love the parts that make it an overdrive. That would be the overdrive unit, the mains shaft, and there are quite a few other important bits that I would need. Just a thought. And an example of someone who wants an early overdrive that operates on top only. I have that on my TR2 and I just love it as a magic electric top gear when I am on the freeway!

Cheers

Dan

I had never heard of a Doretti.. Had to look it up.. Pretty cool... Wikipedia states an OD unit would get you JUST over 100 MPH.. Looks to be a good fit for your car Dan!

https://en.wikipedia.org/wiki/Swallow_Doretti

"Performance
A car with overdrive tested by the British magazine The Motor in 1954 had a top speed of 100.2 mph"


Interesting..

Cheers
Joe O

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