C3 Gasker sealer for gas cap gasket

[Jim T.]

Re: Mo 1970-72 gas cap details

My original 70 gas cap does not have a hole to vent vapor. It is spring loaded. I have not used it for over 33 years but reinstalled it because the GM Corvette locking gas cap is not sealing like it did when new.
My original 68 gas cap does have the vent hole and it is also spring loaded. One difference between the two caps other than one having Vented and the other Sealed and one with a vent hole is the size of the rubber gasket. The 68 cap's gasket is not as wide as the 70. The 70 cap's gasket is the full width of the grove it fits in but there is clear gap in how the 68's gasket fits in the groove.

[Joe L.]

Re: Mo 1970-72 gas cap details

Jim-----

Yes, you have identified another difference in the gas caps which I noticed but did not mention. The width of the gasket on the 68-69 vented cap is narrower than on the 71-74 cap. I did not know how it compared to the 70-early 71 cap, but now I do. The 70-early 71 caps (with and without label, but otherwise the same) are, apparently, kind of a "transitional" cap; they have features of both the 63-69 caps and the late 71 through 74 cap.

The GM locking gas caps never sealed really very well. I expect the reason that yours got worse is wear/deterioration of the gasket. There is no cam lock feature on these locking gas caps, so as the gasket wears the seal becomes "looser". Even when new they don't work all that well. I have several of both the vented and sealed variety [none for sale], but I've pretty much given up on using them when the car's on the road. I carry one in the car, though, and if I happen to be parking over-night in an un-secured area (like a hotel/motel parking lot), I install it. When I get on the road, though, I switch back to the regular cap.

Given the "uniqueness" of the 70-early 71 gas cap, you might want to keep yours stored away for judging and just use a 3994339 for day-to-day use.