C3 Gasker sealer for gas cap gasket

[Terry M.]

Mo 1970 gas cap details

Joe,
The edges are scalloped. Four depressions between each latching notch, for a total of 12 depressions.

If one holds the cap with the word SEALED upright, the mounting tabs for the handle face toward the observer. It looks exactly like the drawing in the third edition 1970-72 TIM&JG, page 39, Fig. E 18.2.

[Joe L.]

Re: Mo 1970 gas cap details

Terry-----

What you've described is pretty much what I expected. Apparently, the 1970 gas cap is, basically, the same in configuration as the 63-69 EXCEPT for a change in the internal valving and the change from the "VENTED" embossment to the "SEALED" embossment.

For 71, the the "OPEN SLOWLY" embossment was added, the "scalloped" perimeter was eliminated, and the irridite overplate was specified. I don't know if the internal valving was changed, but I wouldn't expect so since the 71-74 caps became the SERVICE caps for 1970. By the way, these "sealed" caps DO have internal valving for pressure and/or vacuum; they are NOT 100% sealed.

As a guess, I would expect that the "scalloped" perimeter was eliminated because someone finally figured out that the "scalloped" edge was unnecessary for a "finger hold". The top tab would be the way that most folks would remove and install the cap. Trying to get one's hand down into the "well" to grip the perimeter of the cap might even have been considered a safety problem since cut wrists/fingers could result. So, perhaps the "scallops" were eliminated in order to discourage any attempt to remove the cap that way or make it seem intuitive that the cap was to be removed by gripping the perimeter. It's just a guess, though.

[Terry M.]

Re: Mo 1970 gas cap details

Joe,
There is no valve in the 1970 cap. One can easily take the cap apart by bending the three lugs that are the attaching points to the gas tank neck. When those lugs are bent back to vertical, the internal plate that has the Stant logo and the groove for the rubber seal comes right out. Under that is a large diameter spring which is what provides compression to the rubber seal when the cap is installed on the tank. One can feel the spring by depressing the bottom center of the cap even when it is assembled. That is all there is to the 1970 unit.
On most 1970 Corvettes there is a check valve on the top surface of the fuel tank that seals when liquid (fuel) contacts it and allows air to be drawn into the tank when there is no liquid contact. Thus while the cap is sealed (no doubt what the lettering refers to), the tank is not. I don't know how long that system remained in effect, but I suspect the 1970 NA9 and the universal vapor recovery application to 1971 and later Corvettes may have resulted in removal of that check valve from the tank.
I have no doubt your conjecture about the ribs on the outside diameter of the cap is correct. They certainly serve no useful purpose on a C3.
The early 1971 fuel caps had a plastic caution warning (OPEN SLOWLY CAUTION) attached to what otherwise appeared to be a 1970 cap. Later in 1971 production, and through 1972 production the "open slowly caution" admonition was embossed into the cap, and at that time the outer ribs went away. I am not so sure about when, if ever, the production caps were dichromate dipped. I would be interested in hearing from some folks with later 1971 and 1972 Corvettes, who believe they have the original gas cap, with respect to their color. One of the problems is that the gas cap is easily replaced, and once the car changes hands the provenance is lost. Since most of my judging has been at the other end of the car, gas cap details are not my strong suit.

[Roberto L.]

Re: Mo 1970 gas cap details

Hello, I got from my Chapter some time ago a cap which is identical to the one described by Terry. 12 notches, cad plated, etc. But it has a red plastic OPEN SLOWLY CAUTION tag. We had a doubt at the time about the correctness of the plastic tag, as my car is NA9 optioned (California). It is an early february car.

We supposed there is a chance that NA9 cars could use the red plastic tag, but I'm not sure.

Roberto, NCRS #30019, RMC

[Patrick H.]

Re: Mo 1970 gas cap details

Terry,

Assuming original to the car, my 1971 #19518 had a dichromate dipped gas cap. To be honest, I can't tell you from memory on the 72. Owning 10 years vs. 15 months makes a difference.

Patrick

[Joe L.]

Re: Mo 1970 gas cap details

Terry-----

Yes, the pressure/vacuum valve was added to the fuel tank in late 1969, about August, 1969. My September-built 69 was built with such a tank. The in-tank pressure/vacuum valve was then used right through 1974 for all Corvette fuel tanks. As I mentioned in my previous post, these late 69s could have been fitted with the 70 style cap, but they were not. Even though my car has the fuel tank with the pressure/vacuum valve, it was originally equipped with the "VENTED" gas cap.

Incidentally, all GM SERVICE fuel tanks for 63-69 Corvettes for the past 25 years, or so, have included the integral pressure/vacuum valve. GM provides the SERVICE instruction that these tanks are to be used with the "SEALED" gas caps and NOT the original style "VENTED" caps.

Usually, the post-1969 "sealed" fuel caps for all GM applications have some sort of pressure and/or vacuum relief function built into them. It is my understanding that this is a "safety" feature in case the in-tank valve fails. The pressure and/or vacuum relief in the caps is usually set "higher" than the in-tank valve relief parameters. The Corvette cap may not, though; I've never taken one apart to see. However, isn't there a small hole in the center of the bottom of the cap on the 70 cap? If so, this implies the existence of some sort of relief function. Otherwise, I don't know why a hole would be there.

[Terry M.]

Re: Mo 1970-72 gas cap details

Joe,
I've had some of the 1970 style apart and there is nothing inside beyond what I described. No hole in mine either.

Thinking about the use of a vented cap with pressure/vacuum valve: if the pressure/vacuum valve malfunctioned and allowed liquid to escape (parked on a hill or fuel expanding while parked in the heat with a full tank) a vented cap would allow gravity flow, but a sealed cap would minimize the spill. That is the only reason I can think of quickly why they would recommend the use of a sealed cap with the pressure/vacuum valve style tank.

Dr. Pat,
I'm sure (there goes the memory again) that I have seen some caps with the plastic caution label that were not dichromate dipped. That said, I'll let the exterior judges deal with gas caps and I'll go back and duck for cover under the hood.

[Warren F.]

'71 gas cap details

Terry:

My 1971 gas cap is as you described. It is silver, not gold irridite. It has the red tag that says "CAUTION OPEN SLOWLY" in the two line format, also embossed is the word SEALED on the opposite side. It too has the black rubber gasket. The car build date is Apr 20

[Warren F.]

Addendum

Forgot to include, it also is scalloped around the edge.

[Joe L.]

Re: Mo 1970-72 gas cap details

Terry-----

I just checked one of the 71-74 caps that I have and that cap doesn't have a hole on the bottom, either. However, I think that the "spring loaded" feature built into the cap serves as a pressure relief function, but relieving only at tank pressures considerably higher than the in-tank vacuum relief valve. The "spring loaded" feature was not built into earlier cam lock-style fuel caps (i.e. those used on many passenger cars and also C1 Corvettes), so I don't think that this feature is necessary just for the cam lock to function. The earlier 63-69 caps have both pressure and vacuum relief functions built into them. As I vaguely recall, the "sealed" caps have only one or the other function built into them as a safety feature, so I guess the one function may be pressure relief.

As far as the SERVICE instruction for use of the "sealed" caps with the SERVICE 63-69 fuel tanks, I think that there is another reason for this instruction. The original-style tanks vented through the cap. In pressure relief mode, this could create fuel vapors in the area of the cap which vented out through the fuel door, especially if the fuel filler SEAL (often called a "boot") sealed tightly against the underside of the rear deck as it was designed to.

Especially with respect to 1968+ convertibles with "kammback" design rear deck, fuel vapors generated in the fuel filler area can "waft around" and enter the passenger compartment through the side windows or when the top is down. Although 68+ coupes also have the "kammback" rear deck, the fuel vapor problem is, apparently, much less of a problem with coupes for wahtever reason. This fuel vapor odor was a MAJOR problem with my car, especially after I added the convertible top to my original hardtop-only car in 1986. I struggled with this problem for quite awhile after verifying that there were absolutely no leaks in the fuel tank, whatsoever. The vapors were so strong at times that the car was practically undriveable.

So, how did I solve the problem? Well, I first replaced the fuel tank to ensure a properly operating integral vapor/vacuum valve (the valve is not available seperately). That didn't solve the problem, though.

Next, I replaced the filler neck with the 72-74 style "baffled" unit and replaced the original "VENTED" cap with the 71+ style "SEALED" cap. That solved my fuel vapor odor problem COMPLETELY and PERMANENTLY. The integral pressure/vacuum valve vents in the area OUTSIDE of the fuel filler seal and fuel vapor is, consequently, vented out the bottom of the car.

So, I think that the SERVICE instruction to use the "SEALED" fuel caps with all 1963-69 SERVICE fuel tanks is precisely for the reason of minimizing fuel vapor intrusion into the passenger compartment. After all, GM didn't add the pressure/vacuum valve to the 63-69 SERVICE fuel tanks because the original design worked just as well. I think that the addition of the pressure/vacuum valve AND the related SERVICE instruction to use the "SEALED" type fuel cap with these tanks was done to help alleviate fuel vapor odor problems.

[Joe L.]

Re: Addendum

Warren-----

I think that the 1971 Corvette AIM pretty well answers this question as far as 1971 Corvette fuel caps were concerned. Also, there was a cap that I did not previously know about in this whole scheme of things.

The cap specified in the AIM is the GM #3994339, which is the "non-scalloped", "sealed", "open slowly"-embossed, gold-irridite plated cap. However, there are TWO revision notes:

(1) "4-7-70---was 3974743 cap"

(5) "7-7-70---was 3983384 cap"

It's usually several months after a revision date before the particular revision actually makes it to the PRODUCTION line. So, it's very likely that early 1971 Corvettes received the GM #3974743 cap, the same cap as 1970 models.

Sometime later, cap GM #3983384 was used. That's the one that I didn't know about. I'll bet that this cap is the 1970 cap with the "open slowly--caution" label added to it. As a matter of fact, this cap part number was once specified as the SERVICE cap for 1971 Corvettes. It was discontinued from SERVICE in February, 1972 when SERVICE inventory of it was exhausted and replaced by cap GM #3994339.

Sometime later yet in the 1971 model year, the GM #3994339 cap was, apparently, used in PRODUCTION. Given the revision date of 7-7-70, it would seem that this cap made it to PRODUCTION before 1971 production ended. However, if your April, 1971 car still, apparently, received the 3983384, then it may have been that the time lapse between revision and PRODUCTION utilization of the 3994339 cap was an unusually long period.

But, I think that we've "pieced this whole thing together" for the 1971 gas cap situation. Waht we don't know, and likely never will, is when the actual changeover occurred between the 3 caps.

[Warren F.]

Thanks, Joe.....very interesting! *NM*

[Terry M.]

Makes sense Joe *NM*

[Terry M.]

1970-72 gas caps are becoming clearer

Now we are getting somewhere. The ECR (Engineering Change Request) might tell the disposition of the old parts - that is the disposition of the 3983384 caps that were on hand when the 3994339 caps arrived in St. Louis. They may have used up existing stock delaying the change point, or put the old parts into the service supply chain once the 3994339 cap was on hand and thus speeded up the change point. Of course they may have used either style interchangeably for a while. Even with an indication of the disposition of the old parts we may never get it right down to the exact change point. At least we have a better indication at this point than we did a couple days ago.

[Bob D.]

Re: Mo 1970 gas cap details

Forgive me for getting into this discussion a little late, but I have an early '71--Nov '70 build date-Corvette. I'm the second owner and bought it when it was 8 months old, so the likelyhood of someone changing the gas cap is probably pretty small. My '71 gas cap is as Terry described, it is identical to the '70 with the exception of the red plastic caution warning(OPEN SLOWLY CAUTION). The cap was not dichromate dipped.