DMBZ.6A distributor micrometer adjustment

Can anyone advise how many turns (or clicks) to degrees please for the fine timing adjustment screw.

Welcome to the fun, Michael.
I measure the adjuster screw thread as about 3/16" diameter and without taking the thing apart it looks to be 32 threads per inch, so the thread is either a #2BA or 3/16" BSF or #10-32 UNF.
It acts on a radius of about 1-1/4" and there are 36 notches in the clicker.
So if I do my geometry right, one rotation or 36 clicks on the threads is .03125" and the circumference is 7.854"
(.03125/7.854)(360) = 1.43
So 36 clicks is 1.43 degrees of advance or retard, and one click is .040 degrees.

1 Like

Not sure if this will help or hinder this was from an MG site;

From what i read in the owners manual, degrees are irrelevant! You set the static timing, run the car up a grade, measure the time it takes to complete, adjust the micrometer and try again, leaving it set to whatever click gives the fastest time… don’t over think it!

I think that is a very close calculation. Mine tends to be around 3-4 clicks per degree. The total range on mine is about 90 so it gets installed set to 45’ish to be centered.

Here is a from a very well respected Morris Minor guy , no longer with us . His famous saying was “it’s never the coil” . This is what he said about timing .
“A strobe is useless - what will you set it to ??? The Ign settings from 40/50 years ago are no use now with our modern fuels. Just twiddle the dizzy by hand to get the best idle - then road test to make sure it’s not pinking. Ideally - you then advance until it DOES pink, and then you back it off very slightly so it’s ‘not quite’ pinking. Assuming it still idles ok - that’s your best setting. Wise to check the vacuum advance is working - suck the pipe and watch the baseplate move”


I think the book says 6 clicks per degree IIRC. Paul.

Quite a variation. The MG world have it sussed, no problem. Even Ken Jenkins, the JEC Technical expert didn’t know.

Does each increment marked on the adjusting sleeve indicate 1 degree.
I wonder?

No, Rob above is right I think. I just want to be able to advance 2/3 degrees easily subject to fuel availability, etc. I set timing dynamically but as Ken Jenkins advises if when cold at low revs if she spits back advance a couple of degrees. I always try and use premium fuel.

If you mean “no” to my post , what do they indicate then?

Mike has an XK150 and his question was concerning the DMBZ.6A distributor, like this.

The DVXH.6A found in XK120 and DVX6A in XK140 are quite different.

Yes I’m aware of the difference Rob, it’s just that the DVXH came to hand first.
Here’s the DMBZ version, does your calculation correspond with one or more of the incremental marks?

Without disassembling the vacuum unit, mine has no incremental marks visible. That’s probably the reasoning behind Mike’s original question, as there is nothing to go by but the clicks.

1 Like

I just realized that this may be misinterpreted. This calculation is the movement of the distributor. The crankshaft turns twice for every turn of the distributor, so if you are looking at advance/retard on crankshaft damper marks or flywheel teeth:
36 clicks is 2.86 degrees of advance or retard on the crankshaft.

1 Like