List of articles on issues with the overdrive on an MGBGTV8
List of the articles and notes we have online to the overdrive topic with rollover links to each item. It's been drafted in the format we have used for what we call an "Information Gateway" on technical topics - essentially it provides a convenient way readers visiting the V8 Website can see all that information easily without having to do detailed searches via the search box at the head of the website or by using the detailed Index to the V8NOTES series.
Search box & Detailed Index to the V8NOTES series
Solving a problem with an LH overdrive cycling in and out
A fault experienced by some MGB and V8 enthusiasts with a Laycock LH overdrive unit is that when the multi-function stalk switch has been operated to engage overdrive, the overdrive then begins to cycle in and out - engaged and disengaged and engaged again - and continues to do so as the car is driven at a steady speed. The essential question is "what can cause an overdrive to malfunction and how do you cure it?" The likely causes for an overdrive cycling in and out are a:
> Faulty overdrive inhibitor switch
> Low oil level in the gearbox
> Ageing seal causing an oil pressure leak
> Faulty seating for the valve ball bearing
> Faulty multi-function stalk switch operating the overdrive
> Elelectrical fault related to the solenoid.
Generally it is more likely the fault will be electrical, but two of the early steps in the diagnosis are to check the inhibitor switch and check the oil level in the gearbox and overdrive.
See pages 1 to 3 in V8NOTE457 via the link alongside. V8NOTE457 (Nov 2012)
How does a Laycock Type LH overdrive work?
The Laycock Type LH overdrive fitted between the gearbox and propeller shaft is a self-contained gear unit which provides a higher overall gear ratio than that with the final drive. The overdrive gears consist of a central sun wheel and three planet gears which mesh with an internally toothed annulus. Fitted inside the annulus is a uni-directional clutch. A sliding clutch member is secured to the sun wheel and is free to move forward and backward on the sun wheel splines. Attached to a ball bearing, secured to the sliding clutch by a circlip, is a static thrust ring. The thrust ring is actuated by two hydraulic pistons and returned by primary and secondary return springs. An electronically operated solenoid valve, mechanical pump, relief valve and low pressure valve comprise the main components of the hydraulic system.
The article looks at the overdrive disengaged, so direct drive applies, and then with overdrive engaged with the diagrams alongside.
See pages 3 to 5 in V8NOTE457 via the link alongside. V8NOTE457 (Nov 2012)
Overdrive on third gear on the MGBGTV8 is not so rare
A writer compiling a new buying guide to British sportscars included a draft section on the MGBGTV8 model with the comment - "overdrive only on top". Well many V8 enthusiasts with a chrome bumper V8 have overdrive on third gear as well as top so know this is a much quoted but mistaken claim. With the help of Peter Beadle and Ron Gammons, Victor Smith clarifies some of the myths with overdrive on third gear with the MGBGTV8. Link (Jan 2008)
What do you do to restore overdrive on third gear?
On an "overdrive on 3rd gear" survey form received from Kent Brown from Australia he indicated his Factory MGBGTV8 did not have overdrive on 3rd gear but added that as he was used to his MGB Mk1 with OD on both third and fourth gears, it would be great on the V8 too. He asked "what needs to be done to make OD available on 3rd gear please?" The answer is an "engine out" so you can open up the gearbox and change the gearbox selector plate. This article is V8NOTE417. (Aug 2010)
Link to V8NOTE417
Solving malfunctions with the V8 overdrive
This note brings together information from several postings on the V8 Bulletin Board of members' experiences with overdrive malfunctions. The most recent posting was from Marcus Schopen from Germany in August 2011. This article is V8NOTE433. (Aug 2011)
Link to V8NOTE433
Overdrive problems query
A query about overdrive problems from a member with an MGBGT was included in the "Problem Solved" section in Safety Fast!. A reply was provided by technical experts at Brown & Gammons, the MG specialist service and replacement parts provider at Baldock in Hertfordshire. See a copy of the Problem Solved item in Safety Fast!. Link (Aug 2011)
MGBGTV8 overdrive failure
Andy Goves describes the process he adopted to diagnose and successfully deal with a complete overdrive failure on his 1973 Factory launch MGBGTV8. He says "following the annual health check on my MGBGTV8, with a full service and MOT in July 2017, I looked forward to the hour long drive home across the Somerset Mendips - perfect MG road conditions! Unfortunately, due to road closures and traffic, it was impossible to use the overdrive (which is operational on 3rd and 4th gears on my Factory 1973 car) until I was quite close to home. That is when I found that no overdrive could be selected! I chose to complete the journey, return home and then he sought help from Chris Smith, owner of CCS Restorations at Chilcompton in Somerset, as he has worked on my car since 2010, restoring it to an original Factory specification to my instructions." This article is V8NOTE551. (Nov 2018)
Link to V8NOTE551
Laycock Type LH overdrive components diagram
Useful detailed diagram of the Laycock overdrive from the MGBGB Workshop Manual. (Mar 2011 & Aug 2011)
Links to: Laycock LH Type overdrive unit note & Overdrive components diagram & parts list
Inline fuse protection for the V8 overdrive
I have heard that the lack of an inline fuse on the ignition power feed to the overdrive switch on an MGB and its derivatives is something that has always been a significant oversight in terms of protecting the wiring from a burn out should a short develop. How do you do this modification? This article is V8NOTE460. (Dec 2012)
Link to V8NOTE460
Head up display with overdrive on at night
With many chrome bumper V8s with overdrive on third gear, it is useful to have a reminder as to whether overdrive is on or not - particularly when driving at night. In a thread on the V8BB Victor Smith noted a useful modification on his V8 that was made many years ago by the late Geoff Allen. This note is V8NOTE474. (Mar 2008)
Link to V8NOTE474
What oil for the MGBGTV8 gearbox and overdrive?
Note on the classic gear extreme pressure oil for an MGBGTV8 gearbox and overdrive. Recommended oils to EP90 specification. (Aug 2018) Link
MGBGTV8 - renewing the oil seal in the overdrive output shaft
Jim Livingstone had some overdrive issues recently and traced them to a low oil level in the gearbox caused by a leakage. The procedure to change the various seals is relatively straightforward but he found the information available in the official Manuals is sparse to non-existent. He thought fellow V8 owners might appreciate the information he gathered from his research into the subject. This article is V8NOTE681. (May 2025)
Link to V8NOTE681
MGBGTV8 - readjusting the overdrive inhibitor switch (and a surprising discovery)
In his previous V8NOTE681 Jim Livingstone suggested that the cause of his V8's overdrive cycling in and out of engagement was low oil level in the gearbox caused by a leaking output shaft seal. This conclusion appeared to be confirmed by subsequent testing on local roads. However, the cycling returned on the first long run with extended use of overdrive. On revisiting the problem last month he discovered that adjustment of the inhibitor switch has proved more long lasting. He has recorded the procedure he adopted in this note. This article in V8NOTENNN.
Link to V8NOTENNN
Laycock Type LH overdrive specialist
The specialist Overdrive Repair Services usually has a stand at the MG Spares Show at Stoneleigh on the Sunday with replacement solenoids on display. They are remanufacturers of all the Laycock overdrive units and their spare parts so have an extensive range of spare parts. They are staffed by ex-Laycock employees and offer a professional service to all classic car owners. Link
Updated: 250714  
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