Vacuum Advance MS 360 12

[Joe L.]

Jeff,

With 93000 miles I would inspect carefully the area where the tach gear rides against the inside housing for wear. If the gear gets out of line with the main shaft gear you will be replacing both parts.

Timothy-----


I totally agree. I'd be amazed if there were not considerable wear and the result will be just as you mention.

[Jeff B.]

Well gentlemen, you are correct and I had been thinking about how to address it. There is a wear pattern with a depth of .008”. This yields a total end play of the cross gear shaft of .02”. I believe the repair buttons are about .03” thick, so I don’t have enough end play to install a brass button unless I mill a relief about 1/2 the thickness of the button. I have a Bridgeport mill so I can do that, I was just trying to decide if it’s necessary at this point.

[Timothy B.]

Jeff,
If you decide to install the brass button, what I do is install the screw in bushing and get a drill bit the correct diameter and drill to knock off the nub in the housing where the tach gear rides. After that, get a small drill bit small enough to have a slight interference fit with the brass button stem then drill through the housing.

When you finish that, mill the distributor inside housing so the brass button will sit flat with the surrounding housing surface when you tap in the button. You can trim the button stem and fill the back of the distributor with some J&B weld then paint and all good.

[Jeff B.]

Timothy
That sounds like a good plan, I was thinking I would do that exact procedure with the nylon button and hopefully not have to drill all the way through. My car is right on the border of have the later distributor with the button, so I think it would be a natural modification. I have also used the brass in the past with good results, but thought I might try the nylon this time.

[Jeff B.]

I thought I would close the loop on this thread.

After comparing a couple different models, I installed the B-26 vacuum advance, NAPA VC 1765. Duke has this right. However, the advance unit I pulled off the car had a small bushing on the actuating rod that looked as though it would limit the travel a bit, as well as keep the rod centered in the travel slot. That bushing was broken, and the new B-26 did not come with one, so I initially installed it without one. This yielded 20 degrees of advance at 15”, and while off-idle acceleration was smooth, I did not like the idle quality with that much advance. So I pulled the B-26 back off and made a nylon bushing that would limit the travel by about .06”. Math predicted this would limit the total advance to 12 degrees, which it did exactly.

Once complete, I ended up with the factory timing specs: 4 deg initial + 32 centrifugal for a total of 36 deg, plus 12 degrees max vacuum advance. It sits perfectly at 16 degrees at Idle (4 initial + 12 vacuum).

I did also rebuild the distributor, and machined the bearing surface for the cross gear to a depth that would set the new nylon button at the original depth of the bearing face. I did use the thread-in drill guide to center the receiver hole for the button. It was not necessary to drill all the way through the distributor to install the button. I have not used the nylon button before, so I’ll see how it holds up. At least it’s easy to replace, just snap in a new one.