Technical Information for Gunsmiths and for Classic Car Enthusiasts

MG Restoration

Lucas 25D Distributor Shaft

Bushing Replacement

Les Bengtson

The Lucas 25D distributor was used in both the MGB and Midget/Sprite. In 1975, it was replaced by the newer Lucas 45D4 distributor, another points type distributor. Thus, the 25D was fitted to several hundred thousands of MGs over a 13 year period. In addition, the 25D was fitted to several million other British cars while it was in production. Most of these distributors have seen tens of years and hundreds of thousands of miles of use and would benefit from a cleaning, inspection and, in most cases, a through rebuild. Certain portions of a rebuild, installing new centrifugal advance system (mechanical advance) spring, followed by testing and tweaking on a distributor test machine, are beyond the capability of most hobbyists. Other portions of a rebuild, such as cleaning and lubricating the distributor can be done by anyone. Until recently, another portion of the rebuild procedure, replacing the old, worn distributor shaft bushing, has been another of those “best left to the professional” procedures. Now, it no longer is.

In this article, I will discuss why bushing fit is important, why there have been problems with replacing them in the past, how those problems have been “solved” and cover the tools and procedures needed to replace the distributor shaft bushing.

For many years, the only replacement bushings available have been those made by Lucas and sold by some of the major parts suppliers. There are some problems with this bushing. The outside diameter of the Lucas supplied bushing is .750” (3/4”) along its entire length. The bushings I have removed from a number of distributors are of a different external shape. These bushings have been turned down to .742” for the upper two-thirds of their length. This allows the bushing to be dropped into the distributor body, aligning itself with the hole it fits in, then pressed firmly in place with the lower one-third of the bushing being a press fit into the hole in the body. With the Lucas supplied bushing, the outside had to be turned down to the proper shape before it was fitted. After the bushing was press fitted to the distributor body, the hole through the center of it had to be precision reamed to just over the size of the distributor shaft to ensure a perfect fit. The tooling to remove and replace the distributor shaft bushing and the tooling necessary to allow the new bushing to be precision align bored to the proper size is not available and has to be custom made. The tooling, or jig, to hold the distributor body for align reaming of the bushing is time consuming to make. It must be a precision jig of very close tolerances if the bushing is to be reamed correctly and on the exact centerline of the distributor body. After it is made, it should be used in a lathe having a four jaw chuck. (With a four jaw chuck, each jaw is adjusted independently of the other three, which allows the piece being chucked to be precisely centered along the center line of the lathe. When reaming the bushing, the “run out”, or side to side movement of the jig when making each complete revolution, is held to less than .0005”. The run out of the more common three jaw chuck, in which all of the jaws move at the same time, can average .003” on a good quality chuck. Not a problem for most work, but not what we are looking for when precision reaming.) The problems of having to make the tooling to ream the bushing, the incorrect external shape of the bushing and the need to have a good quality lathe and four jaw chuck made the job of replacing the distributor shaft bushing beyond the capability of most hobbyists. The tooling to simply remove and replace the bushing, however, is relatively simple to make, does not require the precision that the align reaming jig does and, for a single application, might be replaced with some “field expedient” tools. The only problem with this is that the basic Lucas bushing was the wrong shape. If a properly shaped bushing which did not require reaming could be produced, it would make the bushing replacement something the average hobbyist could do without needing a machine shop. Peter Caldwell, of World Wide Auto Parts in Wisconsin, decided to do just that.

About a year ago, Peter contacted me to inform me that he was having some Lucas 25D distributor shaft bushings made up. He offered to send me two of them for testing and evaluation, which I gladly accepted. One of the bushings, stock number 419430 is the straight .750” outside diameter of the currently available Lucas produced bushings. This bushing would still require the outside to be turned down to the shape of the original bushings used in the distributors. The second bushing, however, duplicates the external shape of all of the bushings I have removed from old distributors. It has a major diameter of .750”, but is turned down to about .742” over the upper two thirds of its length. Here, at last, we have a true “drop in” bushing available to the hobbyist. There is only one problem with the bushing, a relatively minor one—the internal diameter of the bushing is about .002” larger than the external diameter of the distributor shaft. This means it will not last quite as long as a bushing reamed to just .0005” over the distributor shaft size.

When I precision align ream the inside of the bushing, after it has been installed in the distributor body, I ream it to the basic diameter of the distributor shaft plus .0005”. My testing indicates that, set up this way, the bushing will give good service for about four years use in a daily driver being drive about 10,000 miles per year. After that, a new bushing should be installed and properly reamed. Based on my experiments, I would expect the World Wide Auto Parts bushings to require replacement about every three years or 30,000 miles. To those of us who use our MGs as daily drivers, this is not all that significant. To the person who uses an MG only as for pleasure driving, covering only a few hundred or thousand miles per year, this is not even a factor worth considering. Thus, since the introduction of the bushing from World Wide Auto Parts, the hobbyist can now replace the distributor shaft bushing at home and enjoy many years of improved motoring. The fact that the WWAP bushings currently cost less than one half of what the Lucas bushings cost makes the experience even better.

The basic source of information on replacing the distributor shaft bushing is the factory workshop manual. Section B of the MGB factory workshop manual and chapter 4 of the Haynes MGB manual cover this procedure. Since these basic reference documents should be available to all owners, I will limit myself to the specific procedures of replacing the bushing rather than cover the entire distributor tear down and inspection procedure.

The first step is to prepare the new bushing. It should be soaked in engine oil for 24 hours prior to being installed. An old pill bottle or a small glass jar can be used for this purpose. Soaking for longer than 24 hours will do no damage. After soaking, remove the bushing from the oil, wipe off the outside, and then, let it drain on some paper towels or old newspaper. While it is draining, disassemble the distributor as per the instructions in the workshop manual. Set the parts out in the order they were removed to make re-assembly easier. Clean each of the parts, one at a time, then, put them back in the correct order. I use either an old tooth brush and soapy water or a good grease solvent depending on how dirty the parts are. If one of the grease solvents, such as Castrol Super Clean or Gunk is used, I wash the parts in hot, soapy water, and then blow them dry with compressed air. After all the parts are clean and dried, it is time to remove the old bushing and press in the new one.

The bushing is best removed using a piloted bushing driver as noted in the workshop manual. A piloted driver is a piece of steel (or aluminum for occasional use) which has a diameter of somewhere between 9/16” (.5625”) and .725”. A four inch long piece of 5/8” (.625”) cold rolled steel is ideal for a driver. One end has to be turned down to form the pilot—this is the piece that fits into the center hole of the bushing and aligns the driver so that it will push the bushing in a straight line. The pilot should be about one half inch long and have a diameter of .480” to .484”. The bushing driver is used to drive the old bushing out and insert the new bushing into the distributor body. It may be used with a hydraulic press, a large bench vise or, carefully, with a hammer. The body of the distributor must be supported when removing the old bushing and inserting the new bushing.

The best support for the distributor body is a piece of aluminum round stock of about two inches diameter and about two and one half inches in length. Through the center of this piece, you need to drill and/or bore a hole larger than three quarters of an inch. I made mine .800”. This allows the old bushing to drop through the holder when being driven out. The top of the holder is counter bored, one quarter inch in depth, to a diameter of 1.075”. This forms a pocket to hold the bottom of the distributor, when it is in its upright position, and aligns the body of the distributor with the center hole the bushing drops through. When the new bushing is inserted, the body of the distributor is inverted and placed on the support. Then, the counter bored area allows the bushing to be pressed fully home, at which point, the bushing will be protruding slightly above the inside of the distributor body.

Thus, the stripped distributor body is placed, upright, in the support/holder and the piloted bushing driver is used to remove the old bushing. The body of the distributor is then inverted on the support and the new bushing slipped into the hole for about two thirds of its length. The bushing driver is then inserted and the bushing is driven home until it is just flush with the bottom level of the distributor body. At that point, the distributor shaft is slipped into the bushing to check for fit. It should turn freely in the new bushing. Remove the distributor shaft from the distributor body.

At this point, one more thing may need to be done. Some of the earlier model distributor bodies had a hole drilled in the side of the round section which houses the distributor shaft bushing. Later models did not. The workshop manual states that this hole should be used as a guide to drill through the bushing using the appropriate sized drill bit. The purpose of the hole is somewhat obscure, especially since it was deleted in later versions of the distributor. It may have been used to allow oil from the engine to lubricate the distributor shaft. However, with the BMC B series engine little, if any, oil is splashed into the area of the block housing the distributor. There is no oil passage or galley going to the distributor and, in my opinion, drilling the hole is not necessary. If you decide to drill the hole, secure the body of the distributor in a vise, or have someone hold it securely against a piece of wood, and drill the hole. Then, clean out the interior of the bushing and test fit the distributor shaft again. There will be a raised edge on the inside of the bushing where the drill bit came through. This protrusion, if it interferes with proper fit, will need to be relieved using a flap sander or fine needle file, then the inside hole in the bushing thoroughly cleaned. With the later distributor bodies, this step need not be considered.

After the bushing is installed and, if you feel it is required, drilled, it is time to re-assemble the distributor. This is done according to the procedure listed in the workshop manual. The only thing I do differently is to lubricate the lower end of the distributor shaft, the part fitting through the bushing, with a good quality Teflon grease. I also use some Teflon grease on the distributor base plate where it moves against the spring on one side and the brass stud on the other side. This will allow the vacuum advance feature to work more smoothly.

Field expedients and possible problems.

It is possible to use a non-piloted drift to remove the old bushing and push in the new bushing. If this is done, the field expedient drift should be less than three quarters of an inch in diameter and at least one half inch in diameter. Care should be used to make sure you keep it straight and do not damage the distributor body when removing or replacing a bushing.

It is possible to use a block of wood, with a hole, larger than three quarters of an inch, but less than one inch, drilled through it to support the bottom of the distributor body when removing the old bushing. It is also possible to use a deep socket of at least three quarter of an inch internal diameter, to support the body when installing the new bushing. The distributor body needs to be supported from the inside when installing a bushing because it has several small, somewhat delicate, projections above the rim level which may be damaged if they are used as support. While these field expedient measures will allow one to do the job, if used carefully, I much prefer to use the proper tooling.

There are two possible problem areas not mentioned in the distributor re-assembly instructions. The first is that the hole for the cross pin in the distributor drive dog and in the distributor shaft is normally drilled slightly off center. This means that they are in proper alignment in one position, but slightly off when assembled 180 out. I use a scribe to make a reference mark, or witness mark, on the drive dog and a Sharpie marker to make one on the distributor shaft. The shaft is hardened steel and often does not take a scribe mark.

The second possible problem is when the points cam is installed on the distributor shaft and engages the centrifugal advance weights. The points cam may be installed one of two ways—correctly and one hundred eighty degrees out. To install it correctly, remember that the distributor drive dog “ears” are offset to one side. If the ears are offset towards you, the notch in the points cam should be to your left to allow the rotor, which engages the notch, to point to the number one distributor cap terminal. Other than these two items, I have found no problem areas associated with replacing the distributor shaft bushing.

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Copyright © 2005 by Les Bengtson