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My original distributor vacuum advance has a ruptured diaphragm and I'm looking for a replacement. Orielly Autoparts searches return Standard vc25, vc31 and sometimes vc156. How do I know which one I need?
 

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This doesn't answer your question directly, but, I'll offer it in the spirit as a suggestion. Since it's our original, mostly likely can benefit for a total rebuild and advance curve upgrade. Dan Nolan (Mustang Barn) can do this and provide a vacuum that's correct for your distributor. I've sought Dan's services, as have many others here. He does great work and the distributors are happier for it. It'll be one of the best outlays (bang for the buck), you'll ever spend.
 
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Do you have a hose or the OE hard line?
Pick one that has the original type screw fitting, not just a hose port. They should be adjustable.
 

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65 Fastback 289 4 spd, 65 convertible 5.0L 5 spd. 3.73 8.8
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My original distributor vacuum advance has a ruptured diaphragm and I'm looking for a replacement. Orielly Autoparts searches return Standard vc25, vc31 and sometimes vc156. How do I know which one I need?
I don't know what year it was, but at work I had a mustang that would suck air, not leak gas, but would starve and it was at the left front shock where it went through from inside frame rail to outside by shock and back through to fuel pump. the steel line had a pin hole in it.
 

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The original-style ones for a hard line are very difficult to come by, and honestly they aren't as good (more places to leak, have to adjust with shims, etc). The modern replacement style ones are adjustable with an allen wrench inside the hose port. Generally, the farther in (clockwise) you turn the screw, the more travel the vac. advance has, so: in=more advance, out=less advance. The VC 31 is the correct replacement for a 66 289 with original distributor. It comes with a reference sheet for how many turns is specified for each application by casting number on the distributor. You can, of course, tweak it from there if need be.
 
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