Bicycle Mechanics - Question about Sun Tour rear Derailleur

I am working on a bike with a Suntour Vx rear derailleur (see pic below). My question is about the upper pivot bolt. Unlike other derailleurs, this the upper pivot bolt does not pivot; the bolt holds the derailleur in place and does not move. Is this typical of this kind derailleur?

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I think you have this wrong, unless your bolt is corroded in place. The bolt is free-spinning in the upper body, and once it's bolted onto the hanger, the derailleur will hang free. When you add the chain, the idler cage tension pulls the RD forward until the B-screw (visible on top in your photo) engages the stop tab on the hanger.

That's what I would have assumed also (no corrosion, bolt and washers wire brushed with a Dremel tool with a drop of oil) but the unthreaded portion of the bolt does not extend beyond the derailleur, as it normally would to stop the bolt from locking up the derailleur. The second photo may not show it but the unthreaded portion is about .5mm short of extending out of the hole.

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That's what I would have assumed also (no corrosion, bolt and washers wire brushed with a Dremel tool with a drop of oil) but the unthreaded portion of the bolt does not extend beyond the derailleur, as it normally would to stop the bolt from locking up the derailleur.

Something is wrong. The correct bolt is a "shoulder bolt" and the shoulder should end just beyond (barely) the inner face of the body. It's possible that you have the wrong bolt, or someone stuck a washer under the head, or the shoulder is worn/rolled back a bit from prior over tightening. There is no stop cam, the RD butts directly against the dropout.

What may be an issue is that hangers of the era had a raised seat for the RD (similar to many modern crank arms) with the rest of the hanger recessed a bit. Without that seat, the body may be rubbing elsewhere on the hanger. You might try a thin 10mm washer between the RD and hanger, and if the shoulder is still recessed too far, a small shim washer with an OD small enough to pocket into the body to prevent the bolt from going in as far. Otherwise, if you only need a few thousandths of an inch, you can file the inner face of the upper body back to establish the correct relationship to the shoulder bolt.

If still not right, post a photo of the RD taken from the back showing the inner face of the bole and upper body together.

The bolt I was describing is a shoulder bolt. This is a schematic of a similar Suntour rear derailleur from http://www.disraeligears.co.uk You will see that the top pivot (shoulder bolt) has both a thin washer between the top of the upper pivot bolt and another thicker washer below. This is identical to my setup except the upper pivot bolt threads into the dropout bracket instead of being held on by a nut in the back. With a washer on the end of the upper pivot bolt, it seems to me that the upper pivot won't be able to move the same as mine. I have also read that the Suntour Mountain Bike derailleurs had the upper pivot stationary for rigidity. Do you thing it would harm anything to file down about .75 mm so the shoulder bolt will extend out farther?

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Before you go filing on anything, is this causing a problem? It appears to be designed that way.

What frame do you have ? Early Cannondales had a similar problem because the dropout face was all 1 plane, with no raised section. We had to scramble to get special washers to simulate the raised section when used with certain rear derailleurs.

I don't know if you have both washers, but they're simply to provide for good fit so there's near zero clearance for the body to slide in or out on the bolt. Remember that the RD never moves during use, the pivot is only so you can pull it back to remove the wheel. So it doesn't have to be truly free, only free enough for you to pull it back when necessary, though it would be nice if you didn't have to remember to push it back forward.

If it's still too tight, is it possible that you have the 2 washers reversed? Or maybe you can try it without the outside washer. If all else fails, here's a trick I've used in emergency field repairs -- wind a turn or two of dental floss around the base of the bolt, which will jam and effectively extend the shoulder depth when it's threaded. You might have to do a bit of trial and error to dial in the exact clearance you need.

I don't know if you have both washers, but they're simply to provide for good fit so there's near zero clearance for the body to slide in or out on the bolt. Remember that the RD never moves during use, the pivot is only so you can pull it back to remove the wheel. So it doesn't have to be truly free, only free enough for you to pull it back when necessary, though it would be nice if you didn't have to remember to push it back forward.

If you reverse the washers it would make no difference (the bottom washer is the same size as the top only thicker); the top one shortens the amount of shoulder that extends out the bottom and the bottom one widens the width of the derailleur essentially doing the same thing. I'm thinking it was simply designed this way as there is little problem getting the wheel axle past the derailleur.