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MG MGB Technical - Fundamental Dizzy question

I have always changed and set points with the dizzy in the car. Recently people tell me they pull the dizzy to do this.

When I asked them why since turning the dizzy sets the timing. A retired MG mechanic claimed as long as you set the rotor in the same position in the keyway it won't change it.

This makes no sense to me. If I want to change the timing I loosen the clamping nut and gently turn the dizzy. So how could you remove the dizzy and replace it without affecting the timing?

Side question - how often does the o-ring need to be replaced and what is its function?
Max71

Max - Do the change which ever way you want or are comfortable with, makes now difference whatsoever. Cheers - Dave
DW DuBois

Hey Dave,

It would be easier to inspect and do the adjustment by pulling the dizzy I'm just suspicious I wouldn't have to re-time it.
Max71

Unless you put the dizzy back in exactly the same position, you will need to check/adjust the timing.

However, having replaced the points, you would probably need to adjust the timing anyway.

Even adjusting the points gap will alter the timing, dizzy in or out.
Dave O'Neill 2

The most accurate way to set the points is by setting the Dwell angle. Setting the points gap is the crude way of setting the dwell. As Dave said setting timing is also necessary to maintain peak efficiency, you just wouldn't do one without the other.
Allan Reeling

When you change the points, or even change the gap of existing points (by using either feeler gauges or dwell), you will need to check the timing anyway which will almost certainly have changed.

RHD cars have the rack shaft very close to the distributor which is a pain, so I always remove the distributor to change the points. I remove it complete with clamp plate, you usually have to slacken at least one of the clamp plate bolts as well as the pinch-bolt to get the distributor out, so I find it easier to undo both. If you do slacken the pinch-bolt don't overtighten, many do, which distorts the distributor body under the clamp and will eventually cause it to jump out when you are running.

I then set the gap/dwell on the bench, using nothing more than an analogue ohmmeter and an Allen key lightly clamped to the distributor shaft, confirming it is correct with a dwell meter when back in the engine. You can mark the block and distributor body or clamp plate before removal so you get back close enough to start the car, then dynamic time it, or static time it first then dynamic.

You can only get the distributor fully seated back into the engine in one position, no matter how you have turned the crankshaft or distributor in the meantime - the offset keyway and drive-dog ensures that. It can seem to engage 180 degrees out, but it won't fully seat. As long as the flange on the distributor body is flush with the clamp plate when the clamp plate is fully bolted to the block, then it is in the right position.
Paul Hunt

Quite interesting description. I'll have to keep that in mind.

I always use a dwell meter after setting the points. Silly question on confirmation. The higher the dwell the tighter to the points and visa vera, yes?
Max71

Affirmative. RAY
rjm RAY

Dwell angle is defined as the rotational angle in degrees for when the points are closed.

So yes, the smaller the point gap the higher the dwell.
Steven Rechter

Thanks both of you! With the Blue Streak I rarely had to check so this time I put a note in where the gun/dwell meter is.
Max71

Max-
The primary purpose of the O-ring (BMC Part # 12H 6547) on the shank of the body of a Lucas distributor is to maintain the partially-sealed state of atmospheric conditions inside of the engine so that the fuel induction system can create a partial vacuum inside of the crankcase. Without this O-ring, the purpose of the vented oil filler cap (BMC Part # 12A 402) of either the 18G, 18GA, 18GB, 18GD, 18GF, 18GG, and 18GH engines or the restrictor tube in the rocker arm cover of the 18GJ, 18GK, and 18V engines would be compromised, the -2 PSI vacuum inside of the engine being decreased by leakage through the engine block at the base of the distributor. Its secondary purpose is to assist in establishing the proper alignment of the rotational axis of the distributor action shaft with that of the distributor action shaft prior to tightening it down, thus both prolonging the lifespan of the distributor action shaft support bushing and minimizing “timing wobble”. Note that the O-ring should never be mistakenly installed onto the driving dog of the distributor. In that position, the rotation of the driving dog would quickly destroy it. The O-ring also has another, although less significant, purpose: to keep oil mist inside of the engine instead of allowing the pulsating atmospheric pressure within the engine to force oil to ooze out around the base of the distributor and drip onto your garage floor. Oddly, most people seem to illogically believe that this is its sole purpose.
Stephen Strange

Not all 25D4 distributors have a groove for an O-ring. The Parts Catalogue shows one for distributors on 18G, GA and GB engines, not for 18GD to 18GK and 18V 581 to 585 i.e. November 67 to August 72, but after that 18V 672/673 engines onwards have it again. All 43/45D4 distributors have it. I have a 25D4 41288 for an 18V 581/2/3 which doesn't have it, which concurs with the Parts Catalogue.

There may be a groove right at the top of the distributor shaft, one between the shaft and the drive dog, and one on the drive dog but none of these are for a O-ring. Where there is a groove it will be about 1/2" from the top of the shaft, as on the attached 45D4.

18G and GA engines had a sealed oil filler cap and no partial vacuum inside the crankcase except minimally from the air filter tube or road draught tube. It wasn't until the PCV system of the 18GB engines that engines had a partial crankcase vacuum and a ventilated cap was fitted - used until the end of production on non-emissions equipped engines, or until 1970 when engines got the vapour recovery system.


Paul Hunt

Fascinating explanations of the o-ring. Thanks Stephen and Paul. Having that purpose what is the maintenance interval where this should be replaced?

I thought I knew most everything about the car after all these years. You both gave me a new lesson.
Max71

Never replaced it :o)

If you find oil leaking from that area perhaps, but if you have a post PCV engine the slight crankcase depression is likely to prevent that anyway. If it's there, complete and rubbery it should be fine.
Paul Hunt

😀 it's a 67 head on a GB block so it has a pcv valve. No matter how hard I tried through rebuilds there's always a film of oil on the engine.
Max71

This thread was discussed between 30/09/2015 and 07/10/2015

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