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Technical Curiosities:
The Laycock de Normanville Overdrive

October 29, 2011 by Matt

Laycock de Normanville Overdrive OD Gearbox Transmission

This one was used on a pretty wide variety of cars, but I had to include it because of the name. I mean, c’mon. How I could I not write a post about an automotive component with a name like that? It’s awesome.

Invented by a Briton, one Edgar de Normanville, and manufactured by automotive supplier Laycock Products, the unit basically consisted of a solenoid-activated planetary gearset residing between the standard manual transmission and driveshaft, offering a reduction in gear ratio at the driver’s command. At the press of a button or flip of a switch (depending on the car) in the cockpit, the overdrive would engage and lower the engine speed relative to the driveshaft, markedly improving fuel economy.

Advantages? Transmission development has always been pricey, and as fuel economy began to become a priority for automakers, the idea of a fuel-saving external add-on to an existing 3- or 4-speed manual transmission was an appealing one. Not only that, but the nature of the Laycock Overdrive meant that it could be engaged at any time, even in the lower gears, effectively doubling the number of ratios at a driver’s disposal. A 4-speed tranny became an 8-speed, for instance.

Downsides? Added complexity, mainly. The solenoid in particular could be finicky on higher mileage cars, and frequently the overdrive unit had its own fill and drain plug separate from the “main” transmission, requiring a unique maintenance interval. As all-in-one 5-speeds with integrated overdrive became de rigeur, the Laycock Overdrive faded from the scene.

Laycock de Normanville Overdrive OD Gearbox Transmission Diagram Schematic

I first learned about it from its presence on the Volvo M46 4-speed + overdrive transmission. Installed from the factory on Volvo 240s and later 740s, it’s one of only two manual transmissions (the proper 5-speed M47 being the other) available stateside for those of us interested in a bolt-in manual swap for a Volvo 200-, 700- or 900-series. Remarkably, up until the late ’80s, the Laycock Overdrive was fitted to a wide variety of cars, not just Volvos: Jaguars, MGs, Austin Healeys, Alpines and Triumphs, among others, were available with the unit. Not too shabby for an obscure bit of engineering with a funny name.

Editor’s note: This post is part of an ongoing series spotlighting obscure automotive engineering solutions. Read the other installments here:

Filed under: Technical, Technical Curiosities


  1. Joel S. says:

    First car was an ’84 Volvo 240 Diesel wagon w/ this transmission. It was one of those with the square push-button right on top of the shifter. Never knew how it worked, and didn’t know I could use it below 4th gear while cruising long city streets. Didn’t like the car at the time because it was slow, but would love it’s practicality now. Diesel-stick-overdrive = great fuel mileage.

  2. Chris says:

    For the record, I have read that de Normanville was an American who gave the idea for a planetary OD to Laycock, a British engineer, during a conversation in a pub after WWII. The model shown in the photograph looks like a J type which superseded the D type fitted to Volvo 1800s during the ‘sixties. It was matched with the M41 gearbox in the early ‘seventies. Over a million Volvo’s are said to have been fitted with these ODs. Other European car makers that used them include Ferrari and Bentley.

  3. Ken says:

    He was a Brit, he was born in Coventry !

  4. Jack DeShazer says:

    The borg warner t85n is a laycock design and was used throughout the 50’s and 60’s by Ford, GM, and Dodge maybe more. Gearvendors an aftermarket overdrive unit is also a laycock. I had the borg warner t85n in a 69 Ford F100 truck my grandfather left me and it was by far the most fun to drive because of the overdrive unit. The t85 is just a 3 speed standard transmission but a t85 “n” is a standard 3 speed but with an overdrive attached to the rear of the transmission.

    • F. R. Eggers says:

      The Borg Warner OD was completely different from the Laycock deNormanville OD.

      The LdN OD would shift under power and never had freewheeling.

      With the BW OD, it was necessary to close the throttle and wait for the engine to slow down before the BW OD shifted into overdrive. Moreover, when the under-dashboard lever was in the “in” position, the BW OD had freewheeling until it shifted into the OD ratio.

    • tim t says:

      whilst the BW iron case overdrive is similar in principal to the laycock its a very different unit..( BMC sedans such as the ‘austin westminster’ used the BW unit, whilst their sportier cars such as the healey 3000 used the laycock) the BW is a very heavy iron cased unit.. the laycock much lighter with an all alloy case.. the laycock actualities electrically via a solenoid.. bw is mixture of electical/mechanical with 2 control systems.. an electric ‘governor’ and an cable operated pawl..the laycock engages almost instantly..the BW is ‘variable’.. with the cable control set to overdrive engage position, the overdrive will engage above 38mph on average and cuts out , going into ‘freewheel’ mode below 35mph.. the laycock is generally installed with an inhibitor switch to prevent to operating in all gears .. commonly they are set to work on 3rd and 4th only..on cars such as jaguar it works on 4th only..there are warnings that the layckock must NOT be engaged in reverse.(hence the lock out switch) as it will severely damage the unit.. the BW will engage in any gear, but it also has a lock out so it wont work in reverse gear ( in this case the ‘governer’ determines when it will engage)
      the laycock engages instantly..the BW slowly with a ‘clunk’ after lifting often gas..or declutcing.. the BW is a great unit to add to a non synchro trans as one can downshift to any gear when in freewheel mode..

  5. John says:

    Curious to know when deNormanville “invented” this OD. There were American ODs starting in 1934, Chrysler products, I believe. They were mechanical. Electrics came later. Laycocks were standard on the BN-1 Austin Healeys I had back in the 50’s. And,as I recall, there was one on the tailend of a Dodge Dakota P/U I bought for one of my jobsite managers back in ’92, or so (it was the first year of the V8 Dakota). It was behind the 3 speed auto, making it Chrysler’s automatic O/D. I don’t recall any trouble with any of them. My current MG-B has one. Works well. Complex, yes, compared with just adding an O/D gear or two to an existing gearbox. But, they were all reliable. And none of them were babied. Probably more like they were abused.

    • Donald H Dool says:

      I had a 1941 Packard 160 convertible with an electric Laycock overdrive. To engage (as I remember 67 years later) you let up on the gas and then punched it.

  6. David says:

    My 1959 Austin-Healy model 100-6 had this (owned in 1966). Worked in all gears as already pointed out. Loved that car. My recollection however is that it was labeled “Havilin de Laycock”. Probably a false memory.

  7. I note all the comments about Laycock de Normanville overdrives, but do not forget that most Jaguars with manual gearboxes of 1960’s also used the overdrive. This also applied to Rover, where the overdrive was available on the Rover 90 from 1955 onwards, and was fitted to the 105s, 100, 80 and 110 models and then the 3 litre from 1958 to 1967.

  8. I had an original AMC Pacer with three on the tree and a Laycock de Normanville overdrive. It greatly improved the mileage on road trips, was very smooth, and trouble-free.

  9. Stuart Fisher says:

    I’ve been driving a 1972 Triumph TR6 with a factory installed A-type overdrive for over 41 years. It’s very rugged and reliable. Oddly, it seems that one of the inhibit switches has failed and the OD can be engaged in reverse. Having owned several TR6s with OD over the years, I learned to check that the steering column switch is “off” before selecting reverse as a habit even though OD was inhibited automatically by a switch. This automatic inhibit feature worked for 39 years

  10. Stuart Fisher says:

    (Continued from previous) Who can tell me which of the inhibit switches has Failed? I understand that there is a 2nd gear inhibit switch and a 3rd and 4th gear inhibit switch, but which switch inhibits Reverse? I also see a reverse lamp switch and a seat belt switch but neither of these is wired into the OD circuit. Since the OD otherwise operates perfectly, why is the unit engaging in reverse after all these Years? Fortunately I learned from owning another
    TR6 how to release a “locked” A-type OD…because I forget…

    • Stuart – Hi there! Glad to hear your overdrive has worked well for so long as this is unusual. the 3rd/4th and 2nd gear overdrive enabling switches are hooked in parallel, so if either switch fails in the “on” position it can cause the overdrive to engage in any gear – including reverse – which, as you mention, is usually fatal to the overdrive. There is no inhibitor switch for reverse, only the enabling switches for 2nd and 3rd/4th overdrive. You are also correct in that the reverse and seat belt switches are independent of the overdrive switches.

    • Jeff Law says:

      I inadvertently left the OD engaged when engaging reverse in dad’s Standard Vanguard. The ‘box locked up solid and the gearbox needed to be removed to fix it!

  11. Aw, this was a really nice post. Finding the time and actual effort to
    make a really good article… but what can I say… I procrastinate a whole lot and
    never seem to get nearly anything done. Billiga Fotbollströjor

  12. David Landers says:

    Interesting article. Laycock de Normanville overdrive(s)(various models) actually operated hydraulically. A little pump pressurises gearbox oil to about 400psi and stores it in an accumulator. The electrical solenoid simply opens a valve to release the oil, which then forces out pistons (rather like brake wheel cylinders) to move the cone clutch and operate the planetary gear change. BW overdrives, by contrast, use the solenoid itself to make the gear change. Other overdrive units (add-on Land Rover units for example) are basic two-speed boxes with a separate gear lever. Incidentally, Laycock overdrive was available on Ferraris (250 series), and Volvo was the manufacturer who used most of them, in total. I think Volvo was also the last manufacturer to specify them. I know that, towards the end, Laycocks tried a sales push to get their overdrives fitted on the back of auto transmissions in big American RVs (for fuel saving) but it wasn’t successful, unfortunately.

  13. Ronnie says:

    Hello people i have one question, is it possible to replace an A whit an D cant see anny differens on them. Ore typ J for exampel

  14. John Adams says:

    I owned a Swallow Doretti sports car fitted with a Laycock de Normanville overdrive which could be engaged in 2nd, 3rd and Top gears. The car was built from a so-called Monkspath set of components and was completed and registered for the road in 1959. It is reputedly the last Doretti to be registered. It had a 1955 Salisbury axle which when combined with a 1959 TR3A engine resulted in it being over-geared. The best speed achieved on a level road was 114 mph, at 5000 R.P.M. As others have reported, the solenoid was not very reliable until I cured the problem by incorporating an intermediate relay

  15. Doug Klein says:

    My friend and I rallied a 140 back in the 80’s. We found a Laycock equipped wagon sitting in a yard and purchased the trans, complete. After removing the ‘detent’ switch on top of the trans, we fitted a microswitch on the rear to detect reverse and lock it out. A ‘splitter button’ fitted to the side of the shifter provided power as needed in any gear, effectively creating an 8 speed. Takes a bit of practice, but. boy, was it fun to drive.

  16. Nigel Clarke says:

    My grandfather had a Humber Hawk MkVI, a Hawk Series II, and a Humber Sceptre MkI. Later I had a Sceptre Mk I, and a Super Snipe Series Va. All of them were fitted with Laycock de Normanville overdrives. I love them

  17. Geoff Sprigings says:

    My 1954 Standard Vanguard Phase 2 was fitted with the Laycock de Normanville overdrive.

  18. David Leslie Lucas says:

    Regarding the reply by John Esposito concerning the two ‘enabling’ switches for 2nd and 3rd/4th overdrive operation; if either of these fails in the “on” position, the O/D will engage in ANY gear – including reverse.
    This happened to a 1970’s Triumph 2.5PI rally car just before an event, and not having a replacement switch to hand, I had to figure out a solution quickly.
    I wired a Normally Closed relay into the 12v feed to the ‘enabling’ switches, and the relay coil was energised from the switched side of the reversing light switch on the gearbox. The relay thus cut the 12v supply to the ‘enabling’ switches whenever reverse gear was selected.
    This “temporary” fix lasted for many months, until the gearbox was removed for clutch maintenance.
    So if anyone has a problem obtaining new ‘enabling’ switched for their O/D, you could always fit a ‘disabling’ relay as outlined above.

  19. Peter Jestin says:

    My Austin Healey when switching to engage o/d usually takes about 10 secs to engage. If someone could advise me how to rectify this I would be most grateful. It is very dangerous as it will freewheel on a moderate downhill.

  20. Reno says:

    I had been told that Gear Venders purchased the Laycock and improved it. Perhaps that was apocryphal.

    In the Volvo Turbo fanatic world, it’s known that a either the M46 or M47 will suffer under the torque that an overboosted, overfueled B2nnFT can deliver. Stock, it’s fine.


  21. Berlina says:

    In the 70s, when I was a teen learning how to fix cars and sweep floors, we had a 122 wagon with this OD, and, once working on Volvos, remember them 142s as well.

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