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My 66 AIM lists the part number for the vented gas cap as 3843576. As recent auction on e-bay listed for an NOS cap with original GM box listed the part number as 3952708. Does anyone know the difference between these two part numbers? Does the one in the AIM include the gasket and the 3952708 is perhaps w/o the gasket? One was the 3952708 a service replacement part that differs in some way from the original produciton part?
Gary - 3843576 is listed in the Oct 1966 Chevrolet Parts and Accessories Catalog so the AIM part number was available as a SERVICE part number. Not sure what the changes were but Parts History shows 3843576 was replaced with 3921689 on 7/68 and 3921689 was replaced with 3952708 on 5/69.
I know the newer caps have a rubber gasket instead of the hard gasket material used in the 66 caps but don't know if that's enought to merit a part number change. Perhaps it's something internal.
The GM #3843576 was the cap used for 1964 thru very early 1968. It is a vented cap with an internal pressure-vacuum valve. It uses a brown, cellulosic fiber gasket. It has "scalloped" perimeter and it's zinc or cadmium plated. It is embossed with the word "vented". It has a welded-on "tab" for a hand-hold to facilitate its removal in the Corvette filler cavity.
The GM #3952708 is the 1969-only cap which became SERVICE for all 63-69 in 1969. The configuration of this cap is virtually the same as the 64-E68 cap EXCEPT it uses a neoprene rubber gasket.
As Pete mentioned, "in between" was the 1968 cap, GM #3921689. I've forgotten which gasket it used but, vaguely, I recall that it's the neoprene rubber type. Might be cellulosic fiber, though. In any event, whatever the gasket material of the 3921689, the external configuration of the cap is the same as the 3843576 and the 1969-only 3952708.
One of the things that the judges look for on an original cas cap is small spot welds on the handle. The current reproductions have much larger spot welds. I saw one of the '708 caps listed on ebay, but the photo quality was too poor to determine the size of the spot welds. This is one of those things that could possibly have changed on the service replacement part without otherwise affecting the appearance of the cap.
Joe, Nice to see that you know that the 63 gas cap is all on its own. My 63 has the original gas cap although it was buried in all the junk for many years. I had the usual service replacement on. I was impressed that when my car was judged at Marlborough last Fall the judge knew that my cap was original. I believe there was an article in "the Restorer" about these caps. JD
I don't know about the originals, but the repro one with the neoprene gasket I have on my 67 S/B doesn't seem to vent very well. (even though it says Vented)
On a long trip on a hot day, you stop for gas and notice the pressure buildup in the gas tank when you remove the cap. I think this is probably the cause of most of the "percolation" problems one has with a Holley, after you shut it off after a long hot run, and you get that drip, drip, drip into the air horn.
Maybe the cork gasket would help, need to try that.
Jerry Fuccillo
#42179
Jerry Fuccillo
1967 327/300 Convertible since 1968
Re: Operationally, they don't seem to vent very we
My understanding of the '64-up design is that even though the cap is "vented" it actually has pressure and vacuum relief valves which allow slight pressure or vacuum before they open. I am curious what the actual specs are, but I doubt if they allow more than about one psi of pressure or vacuum.
IMO such a cap is a good deal for cars that don't get driven much as it may reduce evaporative loss, and the HC species that have the highest vapor pressure are also high in octane rating, so "old gas" can loose octane rating. Once cars got evaporative emission systems they all have some sort of pressure and vacuum relief built into the system - typically the cap, with relief pressure/vacuum somewhere in the range of about 1-3 psi.
Traditional "vented" fuel tank caps just have a vent hole with a labyrinth to act as a liquid/vapor separator to prevent liquid fuel from splashing out. This is the configuration of the '63 fuel tank cap.
In addition to the smaller spot welds, the lip on the relief valve is more pronounced on originals. Reproduction is on the left and a restored original is on the right.
Re: Operationally, they don't seem to vent very we
jerry,take your gas cap and run 1 or 2 cans of spray brake clean through the small vent hole on the tank side along with some air pressure to clean out the brake clean---you should notice a new rattle and improved venting---i blew through it--not recommended due to toxins---also in the archives there is a detailed article on how to rebuild the cap---but if the brake clean gets R done you will save some time---quick check is to take the gas cap off and shake it --- it should rattle---good luck,phil
Re: Operationally, they don't seem to vent very we
jerry---brain dead on this end---this was covered a couple of weeks ago---the caps are vented towards the fuel tank---i had talked about a 68 bb camaro that i had drilled a hole in the filler neck to relieve pressure, and clem, i think, said he drilled holes in a vet cap to do the same---phil
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