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MG MGB Technical - D25 Distributor Re-build question

I recently had my distributor apart - there is a groove near the bottom on the lower bush/shaft. Is there meant to be an 'O' ring in this groove? I can't see any in the manuals. Mine didn't have a 'O' ring when I withdrew the distrutor body from the engine, nor was there any oil leaking from the clamp area, but I would have thought an oil seal would be required. Does anyone know?

Thanks
Elliot Smith

No. There were no O rings used inside the 25D distributor. RAY
rjm RAY

The very early 25D distributors did have an O ring on the lower portion of the distributor body. However, it was found that it was not needed, on MGB engines, due to the design of the distributor drive gear and the distributor housing (the sleeve that fits into the engine block and into which the distributor is fitted). All of the later model distributors were installed without an O ring and none of them tended to leak oil. The distributor drive gear fits into a portion of the engine where there is no oil under pressure and the design of the distributor housing is such that splashed oil cannot easily get up into the inside of the housing.

However, the Lucas distributors were used in a wide variety of engines, not all of them as well designed as the BMC engines. Hence, they retained the option of fitting an O ring when required.

Les
Les Bengtson

I don't think I've ever seen a 25D without an o-ring.
Dave O'Neill2

Looking in the Parts Catalogue although all the drawings show a groove and a O-ring, for both 25D and 45D, when you start reading the detail, distributors for early engines are shown having it, also later ones, but there are a batch in the middle that don't show it as follows:

18G, GA and GB show it, 27H 6547.
18GD to 18GK and 18V 581 to 585 don't show it.
18V 672/673, 797/780, 846/847 and all engines for September 1976 and later all show it, again 27H 6547.
It seems 45D4s always had it.

Note that the groove between the rotating drive dog and the distributor shaft is not for an O-ring, that would get ripped to pieces.
Paul Hunt

Sorry if this is highjacking the thread a litte. One of the distributor clips on my 1979 MGBGT is missing. Does anyone know if these can be bought seperately in the UK please ?
C P Silk

CP,
the Distributor Doctor does distributor cap clips - http://www.distributordoctor.com/
Nigel Atkins

Elliot-
The primary purpose of the O-ring (BMC Part # 12H 6547) is to maintain the partially sealed state of atmospheric conditions inside of the engine so that the fuel induction system can create a partial vacuum inside of the crankcase. Without this O-ring, the purpose of either the vented oil filler cap on earlier engines or the restrictor tube in the rocker arm cover of the later engines would be partially defeated, the -2 PSI vacuum inside of the engine being decreased by leakage through the engine block at the base of the distributor. Its secondary purpose is to assist in establishing the proper alignment of the rotational axis of the distributor action shaft with that of the distributor action shaft prior to tightening it down, thus both prolonging the lifespan of the distributor action shaft support bushing and minimizing “timing wobble”.
Stephen Strange

This thread was discussed between 07/04/2014 and 13/04/2014

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