Making and adjustable vacuum advance unit

[raygreenwood]

Sorry…it’s kind of a busy time right now work wise so I don’t really have time to illustrate this completely.

But someone has asked about this so....

Bosch made several adjustable vacuum advance cans. The most common one I have seen came on Porsche 914…both 1.7L and 2.0L with D-jet used on distributor # 022 905 205 P mostly….and some 1.7L 411 and 412 with the same distributor. Since it was not on all of them, I am inclined to believe it was a dealer optional tuning part. It’s very helpful.

The number stamped on the arm of the advance unit was #917. This is the adjustable factory model. Notice the very fine thread of the screw. I “think” it is a 10mm 1.0 pitch super fine thread.

The thread insert is pressed in. I can take better pictures later. These are all I had on my drive at this time.

First…..select a thread insert. These are rivet nuts. Mcmaster Carr has them in metric and standard

http://www.mcmaster.com/#94020A383

This is a metric insert that uses a 6mm screw. Be sure to look carefully at the OD and length…and the installed length….and the thickness/thinness of metal it can be used on.

All of this is in the specification list for each part 3 on Macmaster carrs site. You can adjust the insert length a little shorter if need be with a Dremel tool. Make sure you get the open ended nuts. Also these come with a seal around the top. When installing the nut I also added a little silicone.

Or you can use standard thread like I did (its what I had on hand last time I
did this)
http://www.mcmaster.com/#94020A343

After thinking about this and remembering what the issues were….try to select a light duty rivet nut.
The issue is trying to keep the nut from spinning in the light sheet metal while you are turning the screw to crimp or deform the nut to clamp it. In retrospect…this would have been a better nut…and is similar to what the factory insert was. It has teeth to help keep it from spinning.

http://www.mcmaster.com/#94430A442

As I noted …when I get a little more time I will do another one and update to what the exact part number is that works best.

The metal in these advance units is really thin. So….I started with a 1/4-20 drill and a drill stop and drilled a hole in the dimple on the vacuum unit body.

Then I widened it with a larger bit. You just do not want the get the drill wound up in the sheet Metal and damage it. The hole must be smooth and round.

So I finished the hole with a Dremel and a thin grinding bit and a small sanding drum. The hole must be dead on or slightly tight to the OD of the insert you choose. be sure to vacuum out all debris and grit.

These nuts are installed by installing a screw that bottoms out at the top of the insert and then deforms it to squeeze the sheet metal and compress the seal…then remove the screw.

This is the factory #917 adjustable vacuum advance can….just so you can see what we are trying to duplicate.

This is what the rivet nut looks like. Notice the top flange, the sealing gasket and the ridges or splines on the OD to help it not twist around while you are compressing it by turning a bolt in it. This one is a little long and a little too heavy duty…requiring a lot of torque to the bolt to deform it….but it worked well.

This is what the finished hole looked like

The rivet nut being inserted. It took light taking to get it down to seat flush

After finishing the thread insert install.

The finished part next to the factory part.

I have also made the retard section adjustable but I have not photographed it yet. More to come.
Ray

[j-dub]

Awesome Ray, thank you for the details.
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