The Stovebolt Site Forums The Shop Area Engine Shop Picture of Vacuum Advance diaphragm

For no other reason than to have pictures for reference, here is what the inside of a vacuum advance looks like.

This is the diaphragm that is a common cause for the unit's failure to hold 'vacuum.'

As these vacuum advance units are available for $30, it's not worth the time to re-build the original ones. I took these pictures last year just to see what all would be involved with replacing the diaphragm, and thought some of y'all might be curious. I couldn’t find my pictures of the dissected spring end of the unit.

Brad

I have always planned on cutting one open. Thanks for saving me the time.

Same feeling here Brad. One more thing I can scratch
off the list.

That rubberized fabric material that's used in the
diaphragms,,, I've been looking for a source for some
of that for a long while. Got some ideas for using it
else where but can't find any thin enough for a diaphragm.
Anyone ever run across any, please let me know about it.
dg

Denny;

McMaster-Carr has fabric reinforced Buna rubber sheets. The thinnest I saw in their catalog is 1/16". Might be something you can use(?).

SimS

Yep, lots of that around the net SimS. I deal with McMaster about
once a week, they're my mainstay when it comes to hardware.
The sheet goods used for diaphragms is a lot thinner, and that's
the problem. Buna-N is the correct material for fuel but it has
to be very thin, less than a 1/64.
Thanks anyway,
Denny G

I have some small Kevlar sheets that are about 1/128" thick (0.0105" exactly) and my idea was to rubberize or seal it with a fuel-resistant coating, sandwich it together, and see how it worked. Brad's Kevlar-reinforced vacuum advances, or something like that...

Like Denny says, it was difficult to find anything thin enough for the diaphragm material, BUT- if I would have known about this website this time last year, I would have a working rebuilt vacuum advance right now. And that's just from getting the "Buna-N" material validation.

Brad

Just how had you planned on sealing up that vacuum chamber after you've cut the crimp off to get it open?????

dg

The oldtime method for checking a vacuum advance unit was to try to blow cigarette smoke through the vacuum nipple. If the smoke came out the other side, we trashed it and installed another one. Occasionally a new one right out of the box would fail the smoke test.

I'd try splitting a piece of 3/16" steel brake line tubing with a cutoff saw in a Dremel tool, wrapping it around the pieces of the advance unit, and sealing it with 5-minute epoxy.
Jerry