Help to ID C2 gas cap

[Stuart F.]

Re: Help to ID C2 gas cap

Harry;

That's very interesting. The cap I bought is p/no.#3952708, and it was described to me in one of the threads as being 2-way and it was also described by the parts counter man as such (not that he'd know much). But, like you, there's no substitute for running your own test which I will do as you've tweeked my interest. I am presently using it on my car as I figured it was a safer piece to run than the original.

Funny how these little things can grow big.

Stu Fox

[Stuart F.]

Re: Help to ID C2 gas cap

Harry (and others);

Further to my last post, I checked in Noland Adam's Corvette Restoration book (Page 93) where he shows the drawing for Cap # 3820865 and #3843576 and has some discussion in the text. The drawing of the latter shows a cross section of the cap with both a Pressure Valve and a Vacuum Valve, i.e. 2-way. The earlier cap just has the vent hole. They say the first indicated use of the second design was around s/n# 17,000.

Stu Fox

[Joe L.]

Re: Help to ID C2 gas cap

Harry;

That's very interesting. The cap I bought is p/no.#3952708, and it was described to me in one of the threads as being 2-way and it was also described by the parts counter man as such (not that he'd know much). But, like you, there's no substitute for running your own test which I will do as you've tweeked my interest. I am presently using it on my car as I figured it was a safer piece to run than the original.

Funny how these little things can grow big.

Stu Fox

Stu-----

The GM #3952708 is virtually identical to the 3843576 EXCEPT it uses a neoprene gasket whereas the 3843576 uses a cellulosic-type (paper) gasket. Both caps have an internal pressure/vacuum valve. There are a few nuances of other configurational differences between the caps, but functionally they are the same.

[Joe L.]

Re: Help to ID C2 gas cap

Should I be able to find a part# on the cap? When I blow through the hole I can hear air escape and when I shake it something rattles. Does that mean it is not functional anymore? I haven't tried pushing over the hole as someone has suggested but will tonight. Thank you for all the replies to date.

Wayne

Wayne-----


NO part numbers will be found on any of the 63-74 Corvette fuel caps.

[Stuart F.]

Re: Help to ID C2 gas cap

Joe;

Thanks for your input. I'm certain we can all say this matter has been put to bed.

Stu Fox

[Wayne K.]

Re: Help to ID C2 gas cap

Should I be able to find a part# on the cap? When I blow through the hole I can hear air escape and when I shake it something rattles. Does that mean it is not functional anymore? I haven't tried pushing over the hole as someone has suggested but will tonight. Thank you for all the replies to date.

Wayne

Updated info,

Last night I tried pushing the inside against the outside and there definately is a spring as suggested by John Armour. When I blow into the hole lightly I hear air escape but as I increase the air pressure a valve must close and I can no longer get air into the cap. As I said before when I shake it it makes a rattle sound as Michael Hanson indicated. Am I safe to assume this is a 65-74 correct cap?

Wayne

[Joe L.]

Re: Help to ID C2 gas cap

Updated info,

Last night I tried pushing the inside against the outside and there definately is a spring as suggested by John Armour. When I blow into the hole lightly I hear air escape but as I increase the air pressure a valve must close and I can no longer get air into the cap. As I said before when I shake it it makes a rattle sound as Michael Hanson indicated. Am I safe to assume this is a 65-74 correct cap?

Wayne

Wayne----

It's a 64-68 cap which was also used for late 1963. It's virtually the same as the 69 cap, except for ther seal. The one you have looks like it's missing the seal so that's a moot point. It's NOT correct, with respect to configuration or function, for 70-74 Corvettes. Those applications used SEALED caps of 2 basically different configurations.

[Wayne K.]

Re: Help to ID C2 gas cap

Wayne----

It's a 64-68 cap which was also used for late 1963. It's virtually the same as the 69 cap, except for ther seal. The one you have looks like it's missing the seal so that's a moot point. It's NOT correct, with respect to configuration or function, for 70-74 Corvettes. Those applications used SEALED caps of 2 basically different configurations.

Joe,

If you look at the first of the two pictures included in my original post you can see a clear gasket ( yellowed with age ) that has a split at 10 oclock.
It is hard to see as it looks like the color of the aged cap. Should there be a gasket in addition to this? The gasket that is there seems like it is made of some kind of plastic/hard vinyl ???

Wayne

[Joe L.]

Re: Help to ID C2 gas cap

Joe,

If you look at the first of the two pictures included in my original post you can see a clear gasket ( yellowed with age ) that has a split at 10 oclock.
It is hard to see as it looks like the color of the aged cap. Should there be a gasket in addition to this? The gasket that is there seems like it is made of some kind of plastic/hard vinyl ???

Wayne

Wayne-----


Yes, I see what you are referring to now. It does look like some sort of plastic gasket. However, I am unfamiliar with any of the 63-74 caps using a plastic gasket or, even, a plastic "under-gasket". As far as I know, the 63-68 caps used the cellulosic material gasket which was a "tan" cardboard-like piece. I suppose it's possible there was a plastic "under-gasket" that I've never noticed. I've never really looked at these with extreme care and study. Does anyone have an original 64-68 cap they can check for the presence of a plastic "under-gasket"?

The 1969 cap (as well as the 70-74 caps) definitely did not use any sort of plastic "under-gasket". For those applications, the neoprene rubber gasket was in direct contact with the metal surface of the cap.

There is one other cap I mentioned earlier that I have little information regarding. That's the GM #3830983. It was supposed to have been used for 1963 Z-06 applications. It was a vented cap with, I believe, an internal pressure/vacuum valve. I have no idea what material the gasket on that cap was manufactured of, assuming it was any different than the cellulosic material used for the other cap gaskets.

[Michael H.]

Re: Help to ID C2 gas cap

Stu, I've done a pressure test on that valve and it only lets air go in one direction, into the tank. I imagine this is to allow air to take the place of the gas as it used. It does not allow excess pressure in the tank out.

I still have never had one of these apart but I'm pretty sure I have the valve figured out.
Air WILL pass in both directions IF the cap is held in it's installed position. (handle up) That allows air to pass in both directions.
However, if you blow hard, the check valve seats and stops the flow of air.
That means there's a free floating check ball, or some other check mechanism (flapper, as mentioned previously?) that operates only by gravity (inhale) and higher volume air movement/pressure.
The spring that separates the outer cover and the internal plate doesn't have anything to do with venting, at least until the volume/pressure are greater than the spring pressure. The spring forces the internal/lower plate and gasket tightly against the rim of the fuel tank filler neck.

Inspect a cap and press the two pieces together. The outer housing and inner/lower plate can be squeezed together against the spring pressure that is forcing the two pieces apart.

Harry, I don't see any type of flapper or check device in your picture though??

[Stuart F.]

Re: Help to ID C2 gas cap

Guys;

If you look at the cross section drawings in Noland Adams book P. 93 you will get a lot better idea of the valve arrangements.

Stu Fox

[Michael H.]

Re: Help to ID C2 gas cap

Guys;

If you look at the cross section drawings in Noland Adams book P. 93 you will get a lot better idea of the valve arrangements.

Stu Fox

Yeah, except fer I don't have an Adams book. Someone borrowed it about 15 years ago. Guess they ain't done with it yet.

[Stuart F.]

Re: Help to ID C2 gas cap

Michael;

I know, you're waiting for the next edition. So was I and I got fooled into buying a second one. If you're going to be at the winter meet next week, I'll truck it along and you can have it. I find it useful once in a while, but darn heavy!

Stu Fox

[Michael H.]

Re: Help to ID C2 gas cap

Michael;

I know, you're waiting for the next edition. So was I and I got fooled into buying a second one. If you're going to be at the winter meet next week, I'll truck it along and you can have it. I find it useful once in a while, but darn heavy!

Stu Fox

Wow, thanks Stu. I'd be more than happy to pay you for it though. I hear they are a little expensive.

I never even had much of a chance to go through that book before it was borrowed. (permanently) I hear my name is in it somewhere too?

See ya at Old Tow next week.