I have got a Mallory unilite distributor in my 390 right now. I need to pull it out and get a new one, because i keep on breking the roll pin that holds the gear on to the shaft. It is the only yhting that i need to change i have canged everything else that i think col be causing thid. I was wondering what the diference in a mechanical advance and a vacuum advance distibtir was, adn which of them is better. How can i tell which one i alrady have.

 

[updated:LAST EDITED ON 16-May-02 AT 09:53 PM (EST)]A vacuum advance has a vacuum hose going to it, and a mechanical does not. I don't really think the distributor is causing the roll pin to break. They are turned by the gear, and do so quite easily. The cam turns the dizzy by the gear, then the dizzy turns the oil pump by way of the shaft in the bottom of the dizzy. The problem is most likely related to the oil pump some how, or the weight of the oil you are using. Of course if you still think it's the dizzy, you can pull it off the engine, and see how hard or easy it rotates. My friend had the same problem. It turned out to be trash in the oil pan. Old pieces of valve stem seal and plastic from old timing gears. Tiny bits would get sucked through the oil pickup somehow once in awhile. This would cause the oil pump to lock up momentarily, and break the roll pin. Once the pan was cleaned out, it didn't happen any more. I still don't see how anything got through the pickup screen though.
Oh yeah. Vacuum advance is better for the street.

 

Thanks for the reply, i figured it was the oil pump causing the roll pin to break so i changed it, and it hasnt broke again yet. Bu twhat worries me is that the shaft on the dizzy is worn down where the gear goes on andthe gear will spin freely by handwhen you get it lined up to put the roll pin in. It has the tight fit all the way up until you get it to where the roll pin is at and the when you get it there it will spin without the shaft spinning. This is probaly from the pin breaking a couple of times and the gear kept on spinning and ground away the shaft. SO the only strength holding the gear on is the pin itself because the shaft is worn down, so m thinking I need a new dizzy. I was told i cant get just the shaft.

 

Yeah you need a new dizzy. Autozone sells them for $35.

 

>I was wondering what the difference in a mechanical advance and a vacuum advance distributor was and which of them is better.

Eightysix, most all distributors have a basic mechanical advance built in. This is a speed sensitive mechanism that advances the timing as the engine speed increases. A vacuum advance is something that is added on in addition to the basic mechanical advance. Sometimes these are called dual advance dizzys meaning both mechanical and vacuum advance. Vacuum advance is a load sensitive mechanism that advances the timing even more under a light load and decreases the timing under a heavy load. For a standard passenger car, a dual advance dizzy with a mechanical and vacuum advance is usually the best way to go and will give you more mileage. For a truck it's debatable, you could go dual advance or mechanical only depending on how much of the time the truck is loaded and the engine lugged.

>How can I tell which one I already have?

You can take for granted that you have at least a basic mechanical advance dizzy. If the outside of the dizzy has a round bellows-looking thing with a tube connected to it, that’s the vacuum canister for the additional vacuum advance. Then you would have a dual-advance dizzy with both mechanical and vacuum advance.

>I was told I can’t get just the shaft.

…too…many…jokes…must…control…myself…:-X23

Barry

 

set myself up for that didnt I:-X11