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MG MGB Technical - when setting point gap

I am curious as to the different methods everyone uses to bring the distributor cam lobes into position to set the point gap. My usual method: put the car in gear and rock it. I put the car in 3rd or 4th gear. What other methods are used.

If you are setting the gap with feelers, take the distributor off the car.

If you are setting the gap with a dwell meter then I put the car in neutral and rock the engine. Easy if the plugs are out, which they may well be if you are working on the ignition.


Interesting. All my dwell meters have expected the engine to be running at the time.
David "not dwelling on this point" Lieb
David Lieb

"Interesting. All my dwell meters have expected the engine to be running at the time.
David "not dwelling on this point" Lieb"

Very dwell put David Lobe!

I use feeler gauges, put it in 4th & rock it while eyeballing to make sure it gets right up on top the Lobe.
Rick Bastedo

As the others said rock the car in fourth though sometimes turning the fan by hand is OK. If setting with feeler gauge when installing new points and you have a tolerance 14-16 set as close to 16 as you can, as the points will close up 2thou as they bed in very quickly.
With a dwell meter do the same. Set for minimum dwell and by the time they bed in the dwell will still be in spec. Old bedded in points can be set and forget.
If your doing a full tune you will usually have the spark plugs out so do the distributor then.


I adjust points approximatively to 0.015" and fine tune with a dwell meter. But as points is a medieval invention, it is a task that should be done frequently. A good substitute is pertronix, mainly when distributor bushing is a bit old. And it takes 10 min. to come back to original if electronic fail.


Jean Guy Catford

Yeah, but you can set the dwell with a dwell meter whilst cranking the engine on the starter motor with the plugs out. This is very much quicker than faffing around the hit and miss method of start stopping the engine.
Iain MacKintosh

If you are going to set the points by gap it is easier to do it out of the car especially on RHD.

However *checking* of the gap using dwell is definitely the way to go as it can be done without doing anything to the engine except clipping on the meter, and ignores any spiking and pitting that might have occured.

I've just changed a set of points in the 45D4 on my roadster that had done 18k miles over 5 or 6 years and were still well in spec for dwell from first fitting. Another set on the V8 had done similar mileage, I did have to tweak those back in a year or so ago as they have a much smaller tolerance than the 45D4 points, as do 25D4 points, but that is a doddle as there is an external adjustment for dwell with the V8 distributor. If you are having to adjust them more frequently than that there is something wrong. The old points were Lucas on both cars, the replacement on the V8 were NOS Unipart, whereas the replacement for the roadster was new Unipart with 'Turkey' printed on the box. I did hope this was the country of manufacture and not a comment on their quality, and after a summer of use they are still in spec.
Paul Hunt

My original was made in the 70’s and I have not seen them since. However anyone with access to metal working facilities could make one, or if you are lucky, 19mm i/d tube might work!

It is metal cylinder about 32mm high, with an internal diameter that allows it to fit snugly over the distributor shaft (when the rotor arm has been removed!), and a “wall” thickness of 10 “thou” (though this is not critical –see below)

The device is slipped over the shaft by holding the heel of the points off the shaft with a finger /thumb nail. When the heel is released, the effect is to increase the points gap by the thickness of the wall of the device. Because the device has created a uniform cylinder around the shaft, the cam lobes no longer affect the setting.

The gap is set using a 25 thou feeler gauge (or one that equals the gap required + the thickness wall of the cylinder)

When the tube is removed the gap is 15 thou…..You can always rock the car to check…..!
Michael Beswick

Does anyone else miss the Delco distributors like I had on my 1972 Chevelle with the small-block Chevy V8? It had a sliding window on the side of the distributor cap through which you could insert an allen wrench that adjusted the points gap. Hook up your dwell meter, start the car, turn the allen wrench as required until the meter said it was happy and you were done. Then they came out with the HEI electronic distributor and dwell adjustment was a thing of the past ;-(
David "I don't miss the rest of the car..." Lieb
David Lieb


As far as the later Delco points distributors are concerned, this was a clear case where the design engineer/s were forced to try to adjust the points on a customers car at a dealership with the engine at operating temperature!

I have also switched to 'Petronix' on both M.G.'s


Larry C. '74 B/GT & '69 midget
Larry C '69 Midget


How do you keep the engine running with the distributor cap off?


That was the beauty of it, you didn't need to remove the cap. Simply slide up the little metal cover on the side of the cap, insert the allen wrench and adjust it while the car was happily purring away.
David "I use a 123/Mini on my Midget and points on my Bugeye" Lieb
David Lieb

I have attached a picture of the points plate where you can see the spring-loaded screw that adjusts the gap. This lined up with a 1" window in the side of the distributor cap that had a little metal flap that slid up to grant access. You were nowhere near the high tension part of things, so I never heard of anyone getting zapped. Note that the mechanical advance was above the points so you could readily tell when it was frozen. I also used to have an old distributor body with the plates and cams stripped out of it and all the teeth ground off of the gear at the bottom of the shaft. Since this gear was turned by the camshaft and the slot in the bottom of the distributor shaft turned the oil pump, you could chuck the top of the shaft into a 1/2" drill, slip it into the block, and crank the drill over until you had oil pressure in your newly-rebuilt engine.
David "I have learned to upload the picture befor etyping the text" Lieb

David Lieb

I, always, remove the distributor from the engine when installing a new points set. This allows me to mount it in a padded vise and not worry about dropping the small screws holding the points and condenser in place while removing those items. I have that happen to others. It also allows me to throughly clean the points plate, the cam lobes and to measure the side clearance that has developed between the distributor shaft and the bushing. It also makes it easier to inspect and lubricate the distributor, something which is supposed to be done on an annual basis.

When installing the points, secure them down to the points plate and set the gap at .016" on one of the lobes. Then, rotate to each of the other three lobes and measure the points gap. I had one distributor in the shop which had a difference (bent shaft and bad points cam) from one lobe to the opposite lobe with the remaining two lobes showing about .017" variance from the low lobe. This problem would not have been caught if I had not had the friend pull his distributor and bring it in. If the points had been set on the highest lobe, they would not have opened on the other three lobes. If set on the lowest lobe, they would have opened too soon on the other three lobes. We replaced both the points cam and the shaft to get it back into working condition. This explained why he was having "missing issues". It also explained the minor pinging/pinking issue he had--when the points open too soon, the timing is advanced depending on how much too soon they open.

It is easy to remove the distributor (at least for those of us having left hand drive cars) and the benefits outway any small increase in time that it takes to remove and replace the distributor.

Always check the ignition timing, preferably with a strobe light, after installing a new points set.

As Paul notes, periodic quick checks, using the dwell meter, will tell you if your points are functioning correctly.

Back in the old days, the Lucas points sets often had the contacts misaligned and all of the professional mechanics used to remove the distributor so that they could "adjust" the contacts, by bending the arms, until they were making proper contact. Less common today, but I have still seen some sets that needed adjustment. Another good reason to pull the distributor, clean and lubricate it, and inspect it as part of the points replacement/setting process.

Les Bengtson


This is a picture of the metal cylinder and feeler guages from your article (at least I think you wrote it) which you quote above.

DK McNeill


Thats the one! I tried to upload it but it didn't work-probably should try to upload pic before writing text

many thanks


ps I suppose I should admit to having changed to pertronix, so don't now use the device......
Michael Beswick

The V8 distributor doesn't even need a window, it has a hex shaft sticking out. The distributor on the V8 also drives the oil pump, so it is simplicity itself to remove that and insert a suitable bar in a drill to prime the system after any work. The only drawback is that one can insert the distributor in as many different positions as there are teeth on the gears, so a bit of fiddling or calculation is required to reinsert the distributor correctly, unlikde for the 4-cylinder where it can only go back properly on one position. Swings and roundabouts.
Paul Hunt

This thread was discussed between 25/11/2008 and 28/11/2008

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