Re: [xj-s] Vacuum advance module repair

["Kirbert" <palmk@mailer.gadcomm.net>]

> From:          "Lee Opausky" <Opausky@netcom.ca>
> I have completed a John Napoli described repair of a spare
> advance module.  The  flat link assembly  from a module bought for a Chevy
> 350 did not fit the internal Lucas spring, also the size of the Chevy
> assembly inside the Lucas dome did not allow for the full 5/16 " travel
> required. ( 88 - 89 non Marelli).

Arrrgh.  Will add that tidbit to the book.

> Therefore I fabd a new actuating rod out of a brass rod, treaded at one
> end, using the old Lucas washers to hold the Chevy diaphragm compressed by
> a nut .

I started with one of those long bolts used to hold electric motors 
together; they are threaded only at one end, so I cut the head off 
the other end and fabbed the shank into a suitable link.

> Only one concern: even using the smallest bolts ( 3-48 I believe) , the
> holes required in the rim of the dome left barely 1 mm metal to compress
> and hold the edge of the diaphragm. This is less than 1/4 of the original
> compression area...

With compression area, the smaller the better; the same amount of 
bolt tension divided over a smaller surface area provides a better 
seal up to higher pressures.  Pipe flanges are commonly designed 
with all but a small area recessed to concentrate the compression 
force to secure the joint at high pressures.

> and with the continuos workings of the diaphragm the
> rubber is likely to tear...

Why?  Would the configuration result in localized flexing or stress 
concentrations?

> Any suggestion to clamp the old Lucas dome together other than drill away
> the surface of the mating rim?

How about this: two metal rings, both exactly the same, with an ID 
just inside of the clamping area and an OD just enough LARGER than 
the OD of the module itself to fit an array of screws just outside 
the module and against the outside edge of it.

Anybody wanna machine up a few dozen and give us a price?

> Kirby indicated recently that he did a vac adv module repair years ago, I
> wonder what clamping method he used?

I very carefully uncrimped the original assembly and crimped it back 
together when done.  Only works once, and doesn't work well then -- 
but using the relatively thick and soft inner tube material makes it 
seal OK.

> He also mentioned that he used a
> piece of INNER TUBE for diaphragm, this really grabbed my interest!  How
> did it stand up in the high heat?

Inner tubes apparently handle heat fairly well, in that hot truck 
tires are not supposed to fail.  Mine sure seems to be hanging in 
there OK.

I was more concerned about its flexibility; would the vacuum merely 
flex the diaphram without moving the advance?  Considering the 
diameter of the disks on either side of the diaphram (which are there 
to provide a definite piston area), I decided it would at least work 
well enough.

> ( the Chevy diaph is really
> fragile looking...

How it LOOKS may not be critical.  In general, this membrane doesn't 
see much stress -- engine vacuum is not a large force.  Having it 
THIN may allow it to flex with a minimum of stress.

> - maybe Lucas' idea of having a double membrane and fiber
> or canvas reinforced has a merit).

The fiber or canvas reinforcing is intended to allow the material to 
flex but prohibit it from stretching -- which is what you want for a 
diaphram.  This is what I worried about with the inner tube, but the 
material was so thick it was hard to stretch and normal engine vacuum 
won't stretch it significantly.

I don't recall mine being double, but whether or not, doubling 
generally does little for life.  When the first one fails, the second 
fails shortly thereafter because it has withstood the same age and 
wear.  I'd rather they provide ONE layer that lasts than two that 
don't.

It may be that they provided two layers to achieve a desired 
stiffness, though.

> Medical rubber cut from hot water bottle
> or a piece of acid proof rubber gloves which has a fiber mesh backing could
> be more heat resistant.

In my experience, medical materials suck.  I'm sure there are good 
ones and bad ones, but several I have tried on different projects 
couldn't handle heat, exposure to hydrocarbons, or age well at all.  
And they are always overpriced.

There's always the possibility of finding a suitable diaphram in a 
small engine shop in a diaphram-type carburetor overhaul kit.  I 
doubt if it'll be more cost-effective than the Chevy advance module, 
though.

 -- Kirbert      |     If anything is to be accomplished,
                 |     some rules must be broken.
                 |          - Palm's Postulate

---

Follow-Ups:

• Re: [xj-s] Vacuum advance module repair
• From: dmeadow@juno.com

References:

• [xj-s] Vacuum advance module repair
• From: "Lee Opausky" <Opausky@netcom.ca>

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