Lucas 25D4 Distributor and Spring Specifications

by Les Bengtson

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INTRODUCTION:

The Lucas 25D4 distributor was used in the MGB, and other, automobiles from 1962 until late 1974 . Each distributor, while retaining the basic model number, will also have a "specification number" stamped into the side to identify the exact application it was intended for. There is also a date code stamped under the specification number which gives the week and year of production (e.g. 5 68 or 49-72). The internals of all of this model distributor are interchangable. Hence, it is possible to have an early specification distributor with later specification internals. Thus, each distributor must be individually examined to determine whether it is suitable for the intended application.

    A second problem area is the centrifugal advance mechanism. This is composed of two weights which swing outward as the speed of the distributor shaft increases and moves the ignition timing forward, thus advancing it. The weights are returned to a stationary position and their rate of advance controlled by two springs. These springs may be either identical or of different specification depending on the particular advance curve desired. Exact replacements for the original springs do not appear to be available to the general public from any known source. They may be made available to certain rebuilders, but this has not been verified. We do know, however, that rebuilt distributors are available, but have not been able to test them on a distributor test machine to see how well they perform against the original specifications.

    Several distributors, believed to be fully original, have been purchased and/or examined and the markings on the centrifugal advance system noted. Further, the springs have been examined in an attempt to begin to locate suitable replacements. Not all of the distributors listed as being used in the MGB have been obtained. The ones found were taken from several running vehicles and found in an MG wrecking yard. They appear to be the ones most commonly encountered in the US. The following information has been obtained:


Distributor Specification Number 40897 (11-64)

Weights: Not marked
Cam Arm: 10 degrees

Springs:   Number 1   Number 2
Diameter of wire:   .020   .030"
Number of turns:   12   9
Spring diameter:   .185"   .211"
Coiled length:   .260"   .250"
Overall length:   .632"   .661"

Distributor Specification Number 40897E (10-66)

Weights: not marked
Cam Arm: 10 degrees

Springs:   Number 1   Number 2
Diameter of wire:   .021"   .039"
Number of turns:   12   6
Spring diameter   .186"   .203"
Coiled length   .256"   .255"
Overall length   .625"   .764"

Distributor Specification Number 41400 (49-72)

Weights: not marked
Cam Arm: 18 degrees

Springs: Number   1 Number   2
Diameter of wire:   .031"   .033"
Number of turns   5   5
Spring diameter   .244"   .243"
Coiled length   .278"   .285"
Overall length   .695"   .696"

Distributor Specification Number 41491 (not marked)

Weights: 19 degrees
Cam Arm: not marked

Springs:   Number 1   Number 2
Diameter of wire:   .030"   .030"
Number of turns:   7   7
Spring diameter   .204"   .204"
Coiled length:   .320"   .323"
Overall length:   .735"   .734"

CONCLUSIONS:

    It may be possible to have these spring produced on a custom basis to be sold as sets to be used in the rebuilding of the Lucas 25D4 distributor. Whether such custom made springs will work properly will only be determined by having them produced, then installing them in several test distributors. The test distributors will then have to be tested on a distributor test machine to see how closely the new springs mimic the factory supplied springs. Timing tolerances are available for the later production distributors, but not the early production versions. Hence, some form of tolerance must be developed to determine whether the newly produced springs can be marketed. These tolerances should be developed along with an article covering the rebuild of the distributor and such article included with the distributor rebuild packages.


ADDENDUM:

Anglo Parts Spring Kit for the Lucas 25D4 Distributor

Dia. Of Wire   .020"   .027"   .030"   .0305"   .031"
No. Of Turns   6   6   7   6   6
Spring Dia   .190"   .198"   .195"   .200"   .196"
Coiled Length   .281"   .286"   .301"   .305"   .289"
Overall Length   .658"   .682"   .710"   .711"   .689"

 

ADDENDUM II

Auto Zone rebuilt Lucas 25D4 distributor for 1968 MGB part number 31-402

Specification number 41234 H (15-73)

Weights: Not Marked
Cam Arm: 19 deg

Springs:   Number 1   Number 2
Diameter of wire   .026"   .027"
Number of turns   7   9
Spring diameter   .226"   .196"
Coiled length   .185"   .296"
Overall length   .630"   .695"

This monograph may be reproduced only for non-commercial use without other permission of the author. Reproduction for commercial use only by written permission.

Copyright 2002 by Les Bengtson


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