XPAG engine, use of a Lucas 25D4 distributor with vacuum advance

1 Nov


The Lucas 25D4 distributor is freely available and economical, both new and used and is easily repairable with a good spares supply.

The centrifugal advance/retard characteristic is adjustable with the necessary bob weight control springs, which are available with a wide variety of spring strengths (Hooke’s Law constants, i.e. extension per unit load in lb/inch, gm/mm, or N/mm.)

A range of vacuum capsules is available to provide more economical part-throttle running (higher miles per gallon).

The resultant increased ignition advance at mid-throttle power settings (1500 to 3500 rpm) also gives improved motor flexibility and higher torque.

The octane demand of the modified engine is at least no worse than a standard XPAG motor and no additional stress loads are imposed (see Fig.1 for a general arrangement of the 25D4 distributor).


Small modifications must be made to the 25D4 distributor to make it fit the XPAG motor dimensions and configuration. These include:

*penetration of the body into the crankcase

*conflict with the vacuum capsule position and the dynamo rear face and a tappet chest cover bolt

*conflict between the distributor low tension terminal and the engine breather pipe on the tappet chest cover

*modifications required to the advance/retard adjustment screw on the capsule

*provision of a sensing point on the engine inlet manifold ……. and

*a small modification to the distributor drive shaft pinion fixing method.

Care is needed to determine and select the appropriate centrifugal advance control springs and the operating ranges of the centrifugal advance control springs and the operating ranges of the centrifugal advance mechanism and the vacuum capsule.


In order to make the distributor fit, the following modifications are necessary. Note that there may be small dimensional differences between my TC and TD and TF variants, but the installation is, I think, practicable for all types.

  1. The penetration length of the 25D4 distributor into the XPAG block is too short. About 3/16 inch must be cut from the base of the distributor body flange, (see Fig. 2). Ideally, this should be done in a lathe, but I achieved an acceptable result with a fine hack-saw and careful use of files. I have used a standard TC pattern clamp to fix the distributor body into the block. The TD and TF may dictate a different arrangement.
  1. The vacuum capsule positioning on the distributor allows very little margin in its circumferential orientation to fit between the tappet chest cover centre bolt on one side and the dynamo rear face on the other. The arrangement shown in Fig.3 should work, but the clearances are very tight. I do not have a tachometer drive gearbox on my TC, but it should be possible to arrange clearance (just) between it and the vacuum capsule.

The spring behind the vacuum capsule head is designed to place the contact breaker moving plate in such a position that the points can be made to open at or just before top dead centre. Without this spring, the distributor cannot be turned enough clockwise to achieve this condition, owing to interference between the capsule rim and the tappet chest cover centre bolt.

With the capsule arranged thus, the existing advance/retard adjustment screw is unable to engage with the threaded rod on the capsule body. A screwed sleeve is therefore required with a longer threaded portion to fit the rod, (see Fig. 5).

  1. With the orientation dictated by the vacuum capsule, the distributor low tension terminal is very close to the tappet chest cover vent pipe, so there is a risk of an electrical short circuit. I have modified the terminal nylon carrier by removing the terminal completely and drilling the carrier to accept a longer cable between the 12 volt supply point and the contact breaker, (see Fig. 4 inset to Fig 3).

  1. Following experiments on the road, I have arranged the distributor mechanical advance to be a maximum of 28 crankshaft degrees. With a static advance of 4 degrees, this gives a total of 32 degrees, which is the standard TC maximum setting. This has been achieved by placing a brass sleeve (6.2mm outside diameter) over the advance stop pin and using a 16 cam degrees advance cam rotor from an MGB 40897 distributor, (see Fig. 6).

(Figs.1 – 8 follow with text in between).

(5) The centrifugal advance curve I have aimed for is shown in Fig. 7. This characteristic required two springs of tension 617g/mm (34.5 lb/inch) taken from a MGB distributor number 41491, used on engine nos. 18V672Z and 673Z (1972 -1974 EEC specs). Note that the standard maximum centrifugal advance on the 41491 distributor is 19 degrees camshaft. Cam rotors with a maximum of 14 degrees or below are also available.

The spring dimensions are:

– Wire diameter 30 thou inch
– No load length (inside coil ends) 18mm
– Coil OD 5mm
– Number of coils 6
– Both coils ends are circular loops, not oval

(6) The engine intake vacuum is transmitted to the vacuum capsule by a copper capillary running from a fitting on the centre of the intake manifold. Conventionally, on most vehicles with vacuum advance, the pick-up point is on a carburettor body and is masked by the throttle plate when closed. This is designed to avoid the full engine vacuum being applied on starting to give full vacuum advance. The motoring technical press asserts that this is done to avoid engine back-fires on start-up, but in practice I have found no difficulty with starting. By contrast, the engine always seems eager to go when I pull the starter and will settle down to a smooth tick-over when warm of about 900 rpm.

The vacuum capsule I use is that for the standard chrome bumper MGB (distributor 40897) stamped “5-13-10”

5-13-10 translates as:
5 ins mercury vacuum, ignition advance is at a minimum (zero), i.e. large throttle opening.
13 ins mercury vacuum, ignition advance is at a maximum, i.e. small throttle opening.
10 degrees camshaft (20 degrees crankshaft), maximum advance.
With these values the car is flexible and economical, particularly at mid-range throttle settings 1500 to 3500 rpm, but will still rev well when above these figures.

(7) A distributor helical gear was purchased to replace the standard drive dog of the 25D4 distributor. The drive shaft is the same diameter as the XPAG standard device. The fixing pin holes are smaller (1/8 in) than the pin port in the distributor shaft (3/16 in). It would be best to drill out the gear holes to match, but I used a roll pin and a sleeve to take up the clearance (see the inset in Fig 2.) Note that the standard 25D4 drive dog pin is parallel sided, not tapered.

(8) With the 5-13-10 capsule used there is substantial ignition total advance at low and mid-range engine speeds, 1200 to 2500 rpm, of 45 to 50 degrees crankshaft with a 20 degree maximum vacuum advance, (see Fig.7). In spite of this, with a compression ratio of 8.8 to 1, the engine cannot be made to pink on 94 octane petrol. In a car with a lower ratio a higher static advance than the 4 degrees I use could be applied with confidence if desired.

(9) Fig.8 illustrates a comparison of my advance characteristic with that for an MGB (high compression 8.8 to 1, engine type 18G or 18GA, distributor 40897) and a Triumph Spitfire 1500 of 1979.

My car runs very well with a static of 6 degrees crank advance up to about 4000 rpm but then becomes harsher running above that. I suspect I could be better off with a couple of degrees less on the centrifugal advance, say 26 degrees, but with a static of 6 degrees to retain the standard TC maximum of 32 degrees total. I have yet to try this. On a car with a lower compression ratio the harshness would probably not occur, even with a 6 degree static and 28 degree maximum centrifugal advance.


The figures given above are based on my experience over the past few years. The Lucas distributor is not a precision instrument, so a different operator’s findings could differ. In particular, the advance curve shown in Fig.7 is from calculations of the mechanical dynamics of the distributor springs, bob weight masses and moments of inertia, and rotation centres, not from a commercial tester device for example.

To establish the success or otherwise of changes, I use the usual subjective impression, backed up by observation of the tachometer (electronic in my case) and the speedometer (ditto) and confirmed by a careful check of the fuel consumption over runs in excess of 70 miles. I have a tank calibrated in litres and I buy fuel in multiples of 5 or 10 litres so I can derive a reliable mpg figure after each run over my standard test course.

In addition to the distributor modifications, I have made alterations to the carburettors; weighted pistons of 175g (6.2oz) total, no piston springs or dampers, and FU needles. Together with these settings I have a high ratio axle (18.3 mph/1000 rpm) and can obtain 45 to 50 mpg over a run, using cruising speeds of 45 to 55 mph and bursts up to 60+ mph.

Without these carburettor and gear ratio changes I would expect a T-Type still to benefit from the distributor modifications alone; 35 to 40mpg at least should still be attainable.


The following references could be useful for more information:

Marcel Chichak, Tuning the Lucas Distributor. https://www.scribd.com/doc/137215689/Lucas-Tuning
Hammill Des, How to build and power tune distributor type ignition systems, Veloce Publishing (1997).
Vizard David, Tuning the A-series engine, Haynes Publ. (1999)
Southern Springs and Pressings Ltd, https://www.southernsprings.co.uk Stern Lane, NEW MILTON, Hants, BH25 5NE (01425 611517).

John Saunders

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