1955 xk140 overdrive question?

Stupid qustuion here…how is it possible to have overdrive with a manual transmission?

All intelligent questions are welcomed here.
The overdrive is a unit attached to the rear of the trans.
It’s the round thing to the rear of the red case, which is the main gearbox.

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‘Overdrive’ is a two-speed gearbox on the back of your four-speed gearbox.
So that’s an eight-speed gearbox, if you discount the external control switches.
Four speeds with the overdrive switched off, and four higher speeds with the overdrive switched in.

Others are more technically knowledgeable than me, but on an XK 140 the overdrive unit is a hydraulic system that when actuated by a solenoid reduces the final gearing by about 28 percent reducing the engine speed. On an XK 140, overdrive can only be engaged in 4th gear. A similar system on Austin-Healey 3000s works in 3rdand 4th gears.

Bob, you are correct…
The solenoid is electrically switched by a dashboard mounted switch, but the circuit goes via an isolation switch that is mechanically enabled by the gear selection rods, thus enabling the solenoid circuit in only TOP GEAR. When in the intermediate gears the isolation switch ‘protects’ the overdrive from engaging.
The reason being of course that the torque capability of the cone clutch in the overdrive is limited, thus Jaguar and Laycock deemed the torque of an XK motor was such that if the overdrive could be engaged in the intermediate gears, the net torque multiplication was beyond the capacity of the cone clutch, thus not allowed… Also the reason why correct operation of an Overdrive in an XK is only to be engaged at a steady state cruising speed, and not whilst accelerating as this prematurely wears and can eventually damage the cone clutch. So an Laycock overdrive is not really like a five speed gearbox, where you can accelerate though all five gears where the torque is always mechanically accommodated.

As you say, in Austin Healeys they do not isolate either 3rd or 4th gears as the Healey engine torque even when multiplied through 3rd gear was deemed within the capacity of the overdrives cone-clutch, and indeed sharing a house yeras ago with a friend who had six ‘big Healeys’ the party trick was to fully enable all gears by bypassing the isolation switch, and claim an eight-speed gearbox, but actually engage overdrive in first or second at your peril…
A Laycock overdrive is intended for more relaxed/fuel efficient high speed cruising, by dropping your rpm at your desired cruising speed, and is not meant to be used for acceleration as per a mechanical five-speed gearbox. Thus never fitted to racing XKs, extra weight for no performance benefit…

And this is what happens to them if you violate those rules.


I think the original poster is asking “how you can have an overdrive on a manual gearbox”…you cant just bolt an overdrive to the rear of a standard gearbox…you need a longer mainshaft out the rear of the gearbox…easier to get a complete unit as photo above…Steve

And a 1 inch alloy riser spacer between the red box and the lid so it can clear the extra height of the O/D. Longer selector forks will also be required.

Hi Roger, maybe splitting hairs here, but my experience has been that I get the smoothest engagement and disengagement if I have slight pressure on the accelerator, if I toggle the OD on or off while cruising with no pressure on the gas pedal there is a noticible lurch, especially evident when slowing for a highway exit and realizing when about to shift to 3rd that the OD is still engaged, followed by a quick unloaded flick of the switch… is this normal?

Hi Dave…from the Jag operating service handbook

With a nice typo. :slight_smile:

50mph is not 800kph (km/h), more like 80.5.

100mph (which you can not do that often nowadays) is correct about 160kph (161km/h to be more exact.)


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The later versions of the overdrive on the 140 include a throttle switch and relay which will not allow you to drop out of overdrive without depressing the throttle and applying some power.

Almost agree, except when ENGAGING overdrive, you need to lift off on the accelerator…, but yes when DISENGAGING overdrive you do need to squeeze down on accelerator…
Its something you can always do totally smoothly with skill and practice and have no discernable jolt/speed change, just watch the revs change on the tacho. But there are those who don’t seem to be able to master it fully, so if too jerky, I suggest they use clutch to help …, but then there are those who cant properly drive a manual gearbox car either…