Overdrive options for an MGB fall into 3 types. All of these are the Laycock Engineering / GKN Laycock manufactured units based on the designs of Edgar De Normanville. Also known a Laycock de Normanville overdrives.
- 1962-1967 MGBs with a 3 main synchro gearbox used a “D-Type” Laycock overdrive
- 1968-1974 (including some late 1967) MGBs with the 4 synchro gearbox used a “LH-Type” Laycock overdrive also known as a “Black Label” overdrive because the plate under the solenoid was black
- 1975-1980 MGBs, also with the 4 synchro gearbox also use a “LH-Type” Laycock overdrive, but there are some internal differences and these are also known as a “Blue Label” overdrive because the plate under the solenoid was blue
Let’s review theses overdrive options in a bit more detail.
D-Type Overdrive #
I don’t have much experience with the D-Type overdrives. While they are functionally the same they are an earlier design. Visually the shape is slightly different and the solenoid is external to the main bulk of the unit.
LH Black Label Overdrive #
Used on the 4-synchro (all sychromesh) gearboxes for which the speedometer uses 1280 Turns Per Mile (TPM). These are very robust and the overdrive adapter section has a stronger and better design of thrust washer, based on my experience. The black label units are linked with the 1280 TPM speedometer and is controlled by the gearing in the overdrive which drives the speedometer cable. Typically these would be “top fill” gearboxes. The filler has a dipstick and is on the front top of the gearbox
LH Blue Label Overdrive #
These are on 1975 and later cars. These cars have speedometers that are using 1000 Turns Per Mile (TPM). Also mated to the 4-synchro gearboxes but had a different design in the overdrive adapter related to the thrust washer. This thrust washer design wasn’t as robust as the earlier black label design and could break. A later change to the gearbox would limit the use of the overdrive to just 4th gear rather than the previous option of both 3rd and 4th gears. Typically these would be “side fill” gearboxes. The filler hole is on the side of the gearbox directly above the drain plug.
Mixing and Matching #
You should not assume that based on the year of the car will determine what gearbox and/overdrive is in use. It is possible to switch the bell housing in a rebuild. Equally a blue label overdrive fits into an earlier car and vice versa with the black label. If this is done it will affect the speedometer. But this is mitigated by changing the speedometer to support the appropriate turns per mile. The only real speedometer challenges are the 1977-1980 MGBs. The different size speedometer fitted in 1977-1980 was not available as a 1280 TPM unit. A re-calibration may be possible or some European specification options may also be available.
LH Thrust Washer Differences #
This is documented up in a article. Follow this link to understand more about the thrust washer differences.
Other Materials #
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