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MG MGB Technical - which overdrive?

Which overdrive was fitting to the 1973/4 chrome bumper MGB?
Nigel

Its the more rugged later one which does not need the vacumn interlock. If I remember right its the O rather than LH
Stan Best

Nigel-
The LH-type Black Label Overdrive unit used from the 1969 through the 1974 model years with an identification number of 22/61972 which had a 21 tooth white speedometer pinion drive gear (BMC Part # 37H 3464) and pinion (BMC Part # 37H 3463) appropriate for the 1,280 tpm (turns per minute) speedometer. The later LH-type Blue Label Overdrive unit used from the 1975 through the 1980 model years with an identification number of 22/62005 with a 20 tooth red speedometer pinion drive gear (BMC Part # 37 H 8844) and pinion (BMC Part # 37 H 8845) appropriate for the 1,000 tpm speedometer. Fortunately for those adapting an Overdrive to a different series transmission, these speedometer pinion drive and pinion gearsets are interchangeable. When engaged, both versions of the LH-type Overdrive unit produce a reduction ratio of 0.82:1, but on 1977 and later models, due to the use of a switch in the shift mechanism inside of the gearlever extension housing, the Overdrive unit can be engaged only with the transmission in fourth gear. Aside from their two different speedometer drive ratios, the LH-type Overdrive units are interchangeable. However, their white and red speedometer pinion drive gears are not easily interchangeable. You have to perform a complete disassembly of the Overdrive unit in order to replace the driving gear on the output shaft, as well as the pinion gear. It should be noted that there should be a copper washer fitted to ensure that the drive shaft of the speedometer angle drive pinion does not bottom out in the speedometer drive pinion, causing premature wear and failure.

Be aware that the later LH-type Blue Label Overdrive unit has a weaker thrust washer for the sun gear. Instead of combining the input shaft (first motion shaft) bushing and the thrust washer into one piece as in the Black Label Overdrive units, these later Overdrive units use a two-piece assembly consisting of a spacer and a thin phosphor-bronze washer with oil grooves in it. These washers tend to fracture along their oil grooves. This thrust washer cannot be replaced. The only method of repair is to have the casing modified in order to accept the earlier and sturdier one-piece thrust bushing of the LH-type Black Label version.
Stephen Strange

Stephen,

My OD unit has the id number 22/61755/000010. What is the proper speedometer for this unit.

Tnx,

Steven.
Steven 67GT

D-type OD was fitted to the 3-synch box and that had the vacuum switch and relay. LH type was fitted to the 4-synch, black Laycock label on chrome bumper cars, blue on rubber. Black label ODs were geared for a 1280tpm speedo, blue label for 1000tpm, these number are printed on the speedo dial. 77 and later cars had the manual switch on the gear lever and slightly rearranged wiring. FWIW UK cars were more straightforward than US keeping OD on 3rd and 4th whereas the US changed to 4th only and different wiring again.
PaulH Solihull

Thanks Paul for the clarification.

I have a blue label LH type connected to a 67' 1020tpm speedo. I can live with a 2% error. One less thing to do to get this GT back on the road.
Steven 67GT

Lucky you have a Mk1 then :o)
PaulH Solihull

This thread was discussed between 04/09/2010 and 10/09/2010

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