MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Disributor drive gear clearance

Having rebuilt my distributor, I was frustrated to find that my spark timing still jumped around when I played with the throttle. I pulled the drive gear out and found that there was slop in the gear. I can shim it, but my manual doesn't say how much clearance should there be?
Neal Zick

Neal,
Sorry - can't offer any wisdom on acceptable clearance. Try a call to John Twist (University Motors). You may also want to consider excessive stretch in the timing chain and/or wear to the tensioner pad (slipper head).
Steve Buchina

Steve-
The cam chain and sprokets are new, as is the chain slipper device, so no problems there. The cam clearance is .004", so that can't be it, either. I'm convinced that the problem is the skew gear sliding because it has too much clearance. I found lots of slop. The question is, how much clearance should be left after I shim it?
Neal Zick

Is there any rotational slop in the rotor? If so the springs have probably stretched. But on my V8 what stopped slight timing wander was changing the timing chain and gears.
Paul Hunt 2

Paul-
No the distributor is newly rebuilt, so no slop there. As I said before, the cam chain and sprockets are new, as is the slipper device, and the cam is within spec. All I need to know is the clearance for the skew gear on the distributor drive as there's lots of slop there.
Neal Zick

Nick,

Did you check the distributor gear carefully. I recently had the same problem you are describing, and when I examine the gear carefully the teeth were very sharp and worn in the middle. I substituted a good gear and hoped for best that the cam gear was not too worn. I am switching to EDIS ignition in a month so it will not matter after that.

I did have machine shop make me a thin oil-lite shim between the distributor housing (the steel sleeve that the distributor body goes into) and the distributor gear.

It is holding up nicely. My concern is still how much wear the cam skew actually has.

Thanks,
Shareef
Shareef Hassan

Shareef-
How much clearance did you use use between the shim and the skew gear?
Neal Zick

I set the dist. drive gear clearance at .015 on my '68 Camaro. I would think that .015-.020 would be fine on a B.
Carl Floyd

Carl-
Thanks. I'm sure that that clearance would work on your Chevy motor, but I need to know what the factory spec is for my MGB engine.
Neal Zick

Neal. Welcome to the wonderful world of out of production cars. There is no "factory spec" for the amount of end-shake-the up and down movement of the part being discussed. If a mechanic felt that, based on his experience, there was excessive movement, he would replace the parts, as necessary, and see what happened. If he had questions, he would check with head mechanic at the dealership. If the head mechanic did not know the answer, a telephone call to the factory rep normally answered the question. Watched this process happen with a distributor problem on my 79 MGB back in early 1980.

The purpose of limiting the movement of the distributor drive is two fold. First, as the cam starts spinning, it forces the distributor drive upwards and against the distributor housing. Think about this for a minute, it is a fairly dramatic thing as the drive dog is moved from zero rpm to about 500 rpm in the space of a second. The second reason for limiting the travel is that the end of the distributor drive has a teat which fits into the engine block. This teat and the upper area, where the dizzy dog engages, for the bearings for the system. Thus, if the distributor dog is allowed to move too much it tends to batter the distributor housing (causing even more end-shake) and, in extreme circumstances, could allow the teat to disengage from the block and quit rotating the distributor. Damage to the cam and drive gear would be probable in such an instance.

Zero clearance, achieved by extending the distributor housing by shimming or welding, would not be desirable. As the parts heat up, they expand and what was a zero tolerance fit becomes an interference fit, causing damage to the parts and, perhaps, the block.

Since there are no factory specifications available, we have to search for similar applications and see what other factories used for their engines--ones where they did publish specifications.

Carl provides a starting point which I would feel comfortable using. My range, base on other experience, not testing of these parts, is that the desired range of movement would be about .010" to .025". But, this is just a guess based on similar experiences and could well be incorrect.

Have you used a dial indicator to measure the exact amount of movement that is present? If not, it might be worth doing so. It would tell you, exactly, how much movement you have. I have rebuilt five engines of the last few years and all have had between .010" and .015" of movement on the distributor drive. This has never bothered me because, when the engine is turning, the distributor drive is pushed upwards, firmly, and it cannot move downwards as long as the cam is turning it.

Most cars will, at certain rpms, show some form of timing variation. About plus or minus one degree, with lower rpms tending to show this more often due to the characteristics of the distributor springs commonly used. (One light and one heavy in many cases.) At certain engine rpms, when the springs are transitioning from the light spring only to the light and heavy springs in unison, there will be a slight "timing wander" evident when using a timing light.

My experience is that such minor variations are well within the range of accepted tolerances as defined by the factory in their later model manuals.

Les
Les Bengtson

FWIW, I have spare a block and all the associated parts. I installed a used cam, distributor drive spindle and distributor housing in the block. The end play of the spindle measured .021" using a dial indicator and a magnetic base. I also measured the clearance between the slot in the spindle and lugs on the distributor drive dog, it was .013", measured with a feeler gauge. The parts I used a used parts, there is no wear ridge in the top of the spindle where it contacts the housing. The housing is smooth on the contact area but I have no way to determine if there is wear. I realize this information doesn't answer your question, I'm passing it on from measurements on used parts that I have, I wouldn't use the cam but I would use the other parts if needed.

Clifton
Clifton Gordon

The critical measurement has to be how much rotational slop is there in the *bottom* half of the distributor drive shaft (slop in the top half can be due to streched springs) with the distributor installed. Ideally there should be none. Slop will definitely come from wear in the gears on both drive gear and cam, no matter how little up and down play there is in the gear with everything assembled, but could also come from up and down play even when fully down there is no rotational play. I suppose all you can do is shim out any up and down play with the gear fully installed (no shims are indicated in the Workshop Manual or Parts calatogue - that I can see), then if you still get rotational play it must be in the gear teeth themselves or the drive dog.
Paul Hunt 2

Neal, I must ask who rebuilt your distributor? If it were truly rebuilt, the timing issue would have been dealt with and you would not have any issues! As you play with your throttle, the timing is SUPPOSED to jump around. That's the mechanical advance funtioning! If it jumps around at idle, there's a 90% chance that the problem is with the springs. Are they new? They fatigue over time and lose strength. In some cases the primary spring is extremely light and can't hold the weights in place if the rpm is too high.

Also, were the weights tested to see if they both seat solidly under the pressure of the advance springs? If not, and its typical that they don't, you may always have this problem.

Other considerations are not only the end play in the main shaft, but in the cam assembly. Lateral wear in the cam assembly is common as well and that can also cause timing fluctuations. Wear in the points plate center pivot can aslo cause dwell variations which can be seen as timing variation. It sounds like you may actually need a distributor rebuild!

By the way, I set the end play of the main shaft from .002-.005" upon rebuilds. If the distributor does overheat (which I've never seen upon extensive testing) the nylon washer under the advance assembly will flex and allow for extra clearance, at the expense of slight wear. The 45D distributors came with a .016" shim under the nylon washer, and all MGB dizzys came with a .036" brass shim washer between the gear and housing. As the shim washer and shaft bushing wear, the need to reshim becomes greater. If you have a replacement shaft bushign from Moss, the length is shorter than stock and it DOES need to be shimmed under the advance assembly to compensate!
Jeff


Jeff Schlemmer

"As you play with your throttle, the timing is SUPPOSED to jump around"

Well I wouldn't put it quite like that. The timing *will* change as you alter the throttle and revs, and I suppose it will move pretty smartly if you pull and release the throttle smartly. But it should move gradually and smoothly according to pre-determined curves. If it jumps around at *any* steady throttle opening and engine speed, not just at idle, there is something wrong somewhere, like springs (easily checked by gently twisting the rotor back and fore and checking for slop)cam and drive gears, timing chain and gears, etc.
Paul Hunt 2

Now I remember why it is that I don't visit this board often. Semantics are argued way too much. I'm just here to offer help to Neal, so please play nice Paul!

Twisting the rotor back and forth will not tell you much, since the advance weights need cetrifugal force to push them out. Without that force present, the weights will hit the outer spring posts before you travel the full working distance of the spring (in most cases). Unless you work on these distributors freqently, rotating the rotor will tell you little but the amount of spring tension. With nothing to compare it to or any feasable way of measuring it, its useless information.
Jeff Schlemmer

Trying to be nice, if there is slop in the rotor such that it doesn't return all the way under spring tension is a simple check that *does* tell you that the primary spring is either missing or stretched. What you then do about it is another issue!
Paul Hunt 2

Jeff-
You said "all MGB dizzys came with a .036" brass shim washer between the gear and housing." The distributor itself doesn't have a gear as a part of it. Do you mean the brass shim washer between the drive dog of the distributor and the dog? There was no brass shaim washer between the housing and the gear inside the engine.
Neal Zick

Jeff-
Are you still out there?
Neal Zick

Sorry Paul! I let my frustrations get the better of me! Lets all try to focus on putting forward good information rather than critiquing each other!

I sometimes use universal terminology since I work on a lot more than British distributors! I meant to say that the brass washer fits between the drive "dog" (aka gear) and the aluminum distributor casting on the distributor shaft. In the case of older distributors, the shim is a phenolic resin, and many rebuild companies use steel or occasionally NO replacement shim. The most important info is that the drive dog not have more than a few thousandths of an inch end float, and the same goes for the advance assembly.
There are no shims on the steel distributor housing or the drive spindle.
Jeff
Jeff Schlemmer

By 'eck, been away a week and this one is still going :o)
Paul Hunt 2

Thanks, Jeff. Thought that's what you meant, but I needed to be sure. My distributor is missing that shim washer, so that might be the source of my problem. You may have just saved me the cost of a distributor.
Neal Zick

This thread was discussed between 24/01/2007 and 12/02/2007

MG MGB Technical index

This thread is from the archives. Join the live MG MGB Technical BBS now