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MG MGA - distributor drive spindle end float

The notes from John Twist's MGA engine rebuild class say the distributor drive spindle end float to the spindle drive housing should be .002-.003". It recommends reducing the housing flange thickness to get the proper end float. That's not easy to do. I can't find anything in the workshop manual about this, what do you guys think?

Thanks, George

G Goeppner

A related topic - is there supposed to be an O ring between the dizzy and the block? Some diagrams show it, others not.
Art Pearse

I've not heard of that one George but I can see how doing it would remove a small amount of backlash from the drive to the distributor. As you say, doing it would not be easy and would require something like a lathe. You would have to take tiny amounts off at a time and keep checking it, if you went too far you would end up having to shim the housing away from the block. Is it worth it? I'd say, make sure there's not too much wear on your spindle thrust face at the top and on the bottom edge of the housing and leave it at that.
Lindsay Sampford

Art, per Barney's web site, an O-ring is not required.
G Goeppner

With due respect to John Twist, I can't believe that end float is very critical. I'm all in favor of good running tolerences but I can't see where there is any axial load on a distributor shaft. Did he indicate what the concern is?
G T Foster

George. The first thing is what method are you going to use to measure the end float on the distributor drive gear? A dial indicator, placed into the hole at exactly right angle (90 degrees) to the upper surface, zeroed out, then a bolt inserted into the dizzy drive gear and the drive gear lifted would be the most accurate method of measuring. Minor problem though. Most dial indicators may not have a shaft of sufficient length to allow them to be inserted into the bottom of the opening and zeroed out.

As to why it might be of interest, the distributor drive has a skew gear which meshes with a skew gear on the camshaft. Thus, as it begins to rotate, it is forced upwards against the bottom of the distributor housing (the piece which fits into the block and limits upwards movement of the drive gear). Over some period of time, end float (also called end shake) will develop with the float developing faster when there is greater end shake and developing slower when there is minimal end shake.

As we are all aware, the distributor is inserted into the engine when the distributor drive is at the bottom limit of its ability to travel (i.e. zero end shake) and the distributor is inserted until it will not longer move downwards, then clamped in place with the distributor clamp. Thus, when the engine is started, the skew gears cause the distributor drive to move upwards, pressing against the distributor drive dog, which is attached by a cross pin to the distributor shaft, and forcing the drive dog and shaft upwards against the bronze washer between the upper side of the drive dog and the bottom of the distributor body. This will cause the washer to wear at a faster rate the more end float there is and will, in turn, lead to end float of the distributor shaft which is undesirable because it will affect the position of the points cam in relationship to the points and the rotor in relationship to the terminals of the distributor cap. This could be, under some rare circumstances, a problem.

Under the majority of circumstances, there will be no problem associated with slightly larger distributor drive gear end float than John notes. But, John has an international reputation for the quality of work that he does and, when an engine came out of his shop, it needed to be the very best possible rebuild. Thus, he worked/works to a higher standard than most hobbyists do. Having trained as a gunsmith/machinist, and holding both a degree and an instructor certificate in the area, I do not worry about this on my own rebuilds unless I can feel a significant amount of slop when assembling the pieces together. I have never had to machine the distributor housing to reduce end shake. I have adjusted the end shake on the distributor shafts, which I do feel is important.

As to the rubber O ring associated with some versions of the Lucas distributors, they are somewhat useful on some applications, like the Triumph, which uses a distributor drive shaft which is pinned to the bottom of the distributor. The BMC engines do not have an oil feed to the distributor drive gear and lubrication is supplied by oil splash/slinging off of the camshaft/drive gear skew gears. Very little oil is thrown upwards and what oil is hits the rotating drive gear and is thrown outwards towards the side of the hole. Nothing to force the oil to move upwards, along the upper side of the drive gear, then take a 90 degree turn to slip between the tight fit of the upper surface of the drive gear and the lower surface of the distributor housing, then make a 90 degree turn to move upwards inside the distributor housing and along side the distributor, etc. etc. Not a real or potential problem. Anytime one sees oil "coming from the distributor", the culprit is the oil galley plug just below and behind the distributor--the galley plug you cannot see until you remove the distributor and clean the area carefully.

Les Bengtson

Thanks both for the O ring comments.
Les- is the distributor drive gear not retained by the plate that bolts to the block? And it end float is fixed there. Then it cannot rise and force itself on the distributor, surely.
Art Pearse

"Les- is the distributor drive gear not retained by the plate that bolts to the block?"

Art. That "plate that bolts to the block" is called the "distributor housing" (as opposed to the distributor body which is the outer, aluminum part of the dizzy that holds all of the other parts inside it). Yes, the distributor housing controls how high the distributor drive gear can move upwards in its hole in the block. In other words, the depth to which the distributor housing depends into the hole in which the distributor drive is located is the controlling factor in the end float of the distributor drive gear.

The distributor, when installed, is pushed down as far into the hole as it can be. It will be stopped either by the lip of the enlarged section (just below where it flares outward) being stopped by the distributor hold down clamp, or it will be stopped by the lower end of the distributor dog pressing against the upper end of the distributor drive gear. Which will happen is dependent on the length of the lower portion of the distributor body in relationship to the depth of the distributor housing.

If the lower portion of the distributor body is longer than the internal length of the distributor housing, the distributor can be forced upwards by the distributor drive as it attempts to move upwards until it hits the lower portion of the distributor housing. This is undesirable in that it can cause increased end float in the distributor shaft and the change in relationships I mentioned in my original post.

If the distributor body bottoms out on the distributor hold down clamp and the distance between the bottom of the distributor drive dog and the top of the distributor drive gear is greater than the distance between the top of the distributor drive gear and the bottom of the distributor housing, there can be no effect on the end float of the distributor shaft. The top of the drive gear and the bottom of the drive dog will not be able to make physical contact, thus no upwards pressure from the meeting of the two.

Again, this is the theory behind why the end float of the distributor drive gear might be important in certain, high end, applications. I have never found sufficient end float between the distributor drive gear and the distributor housing that it caused a problem on an engine designed for street use.

But, John has built more engines than I have and has a much more valued reputation to defend when he does so.

Les Bengtson

All Les said is true. However, the really relevant point is that as the shaft moves up and down, the timing changes. Sloppy can lead to a significant difference between static settings and dynamic ones, one reason specs now usually call for dynamic timing. One might suppose that once the engine is running, the shaft would stay in whichever direction the skew gear pushes it, but extensive investigation by serious race guys shows that the cam and everything driven by it jump all over the place when running. This gives timing scatter and beats parts to death. Worn parts and excessive clearances anywhere in the timing drive/cam/distributor/oil pump system just aggravate this. So the circle increases.

FR Millmore

Dear Fletcher:

"All Les said is true."

I am a friend of John Twist, Bob Muenchausen, Paul Hunt, Roger Parker, Chris Betson, Peter Burgess, Daniel Stapleton, Barny Gaylord, and a host of others, including yourself.

Watch "Second Hand Lions". I, too, have commanded men and had the love of only one good woman in my life.

I am glad that you will take the time to verify what I know and can teach to others, and will take the time to teach all of us more than we/I know.

Thank you, my friend.

Les Bengtson

Great discussion, this is what I was looking for (always something to learn here). Les, how does one "adjust the end shake on the distributor shaft"?

Thanks guys...
G Goeppner

This thread was discussed between 22/06/2009 and 23/06/2009

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