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MG MGB Technical - Dizzy Question

How do you know when your distributoer needs replaced or rebuilt?
JOHN
JW Colson

Often the top bushing wears out and allows the point gap to go all over the place. When that happens, the timing also get sloppy. Hard to verify without pulling the dizzy apart. Installing a pertronix or other pointless system will make this problem go away. The other thing that happens if the advance pot's diaphragm gets a hole in it and no longer will advance the timing. Applying a vac source to the connector while the cap is off will alolow you to see if the points plate moves with the vac.
gerry masterman

John. You physically measure what the wear is, using a pair of dial calipers to determine what the slop is at the points cam while the distributor is assembled. Simply pick a point on the side of the distributor body, push the points cam towards it, then measure the distance between the body and the points cam. Use the bottom of a pair of dial calipers against the outside of the distributor body and the bottom of the movable arm against the points cam. Then, with pressure against the top of the points cam (where the rotor goes), push it backwards and measure again. Subtract the first measurement from the second measurement to determine bushing wear. Anything more than .010" is bad. Ideal is .001" to .002". Rate of wear seems to be about .003" per year of use with a daily driver. There are two types of bushing systems. The earlier 25Ds had a one piece bronze bushing. World Wide Auto Parts, up in Wisconson, has drop in replacement bushings of this type. Later model 25D distributors had two steel bushings, one at the bottom of the hole in the body and one at the top. These distributors tend to wear less, but the shaft will wear in addition to the steel bushings. With the bronze bushings, the shaft seems to hold up indefinitely, but the bushings wear faster.

After that, it is a general cleaning and lubrication. Most of the distributor shafts show about .001" of wobble, however, I have seen up to about .042" of wobble. This is best checked on a lathe, with the spindle mounted between centers and read using a dial indicator. A quick check of shaft straightness is to install a new set of points, gap them, then measure the gap on each of the four points cam lobes. If there is more than .002" difference between the size of the points gap, you need to replace the distributor shaft, points cam or both.

Springs for the centrifugal advance are a problem. Anglo Parts in Belgium sells a five pack of generic 25D springs and people have used the spring pack for one of the GM distributors to approximate the factory springs. Info in the archives under distributor springs or dizzy springs.

Easiest and best way to rebuild a distributor is to ship it to John Twist at University Motors Ltd and let John do it correctly, then "tweek" it until it is exactly right. AutoZone sells a rebuilt distributor for the MGB, but the one I tested was so far off as to be un-usable. Les
Les Bengtson

Any decent machinist can do all of what Les describes. I turn my own bronze bushings and then ream them to fit the shafts. Hot rodders used to replace the top bearing with a roller bearing as well. The replacement springs are availiable from any auto parts place that sells Elderbrock or MR Gasket accesseries. Finding a advance unit is considerbly harder
gerry masterman

Gerry-
Vacuum advance units are available through Moss Motors:

Lucas 25D4 Units:
Lucas Part#'s 41155, #41220 Moss Part# 163-630 $48.95
Lucas Part#'s 41156, #41290 Moss Part# 560-150 $38.95
Lucas Part# #41339, Moss Part# 163-660 $64.95
Lucas Part# #41370, Moss Part# 560-530 $69.95
Lucas Part# #41491, Moss Part# 163-670 $82.65
Lucas 45D4 units:
Lucas Part# #41427, Moss Part# 163-695 $69.95
Lucas Part# #41599, Moss Part# 163-680 $28.95
Steve S.

I've seen this claim about magnetic/optical triggers 'fixing' shaft wear. I can see that if a shaft moves closer to and farther away from the points it will cause a timing variation, at least, that wouldn't happen with an optical trigger. But if the wobble is across the face of the trigger then the timing variation will affect the optical unit as well as points. And doesn't the magnetic trigger operate when the magnet gets within a certain range? And doesn't that vary with 'closer to and farther away from' wobble as well as 'across the face' wobble?

Timing variations can also be caused by wear in the timing chain and gears, but only shaft wobble will affect dwell.

You can test the vacuum capsule using the Mk1 Mouth as a suitable vacuum source.
Paul Hunt

This thread was discussed between 13/07/2003 and 14/07/2003

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