I think the gasket makes a difference
Duke,
I have a 3952708 gas cap with the neoprene gasket in front of me.
For vacuum in the tank, the cap runs free and clear, and vents the tank with no resistance. One can easily suck air through the hole in the bottom of the cap.
For pressure in the tank, it is the clamping force of the spring in the cap against the gasket which holds the pressure. The hole in the bottom of the cap seems to have a check valve in it. You can't blow air through it.
The neoprene gasket on my fairly new one is thick. About 1/8". I notice that it really clamps on hard and the spring seems fairly stiff. The diameter of the bottom of the cap exposed to the tank is just under 3" and the area exposed to pressure is about 5 square inches. This means to me that for every 5 lbs of clamping force one would have 1 psi in the tank.
Just feeling the spring compression to move it 1/8" feels to me to be a lot more than just 5 lbs but there is some compression of the gasket. The pressure buildup to release would therefore be the total clamping force of the spring against the gasket, divided by 5 for psi, but then a little more to account for the resiliency of the gasket.
Total clamping force is really a matter of the geometry of the cap as well as the lip on the tank and gasket, so only testing with the cap on the tank will give you a true pressure relief figure. There probably is a spec on that.
I'd really have to be crazy to test the relief pressure of this cap on my 67 gas tank. I think I'll just get one of those fiber gaskets for it, or continue using my early 70's loose fitting locking gas cap, that leaks with a full tank.
However, I think the gasket makes a difference in the relief pressure, a newer cap with the neoprene gasket would seem to hold more pressure, maybe as they wear, the pressure relief figure will be lower. I'd give the design an F, it is probably one of those later emission control changes.
But to those who get that drip, drip, drip in the carb after shutdown following a long hot run, I'd advise to check the gas cap first.
Jerry Fuccillo
#42179
Duke,
I have a 3952708 gas cap with the neoprene gasket in front of me.
For vacuum in the tank, the cap runs free and clear, and vents the tank with no resistance. One can easily suck air through the hole in the bottom of the cap.
For pressure in the tank, it is the clamping force of the spring in the cap against the gasket which holds the pressure. The hole in the bottom of the cap seems to have a check valve in it. You can't blow air through it.
The neoprene gasket on my fairly new one is thick. About 1/8". I notice that it really clamps on hard and the spring seems fairly stiff. The diameter of the bottom of the cap exposed to the tank is just under 3" and the area exposed to pressure is about 5 square inches. This means to me that for every 5 lbs of clamping force one would have 1 psi in the tank.
Just feeling the spring compression to move it 1/8" feels to me to be a lot more than just 5 lbs but there is some compression of the gasket. The pressure buildup to release would therefore be the total clamping force of the spring against the gasket, divided by 5 for psi, but then a little more to account for the resiliency of the gasket.
Total clamping force is really a matter of the geometry of the cap as well as the lip on the tank and gasket, so only testing with the cap on the tank will give you a true pressure relief figure. There probably is a spec on that.
I'd really have to be crazy to test the relief pressure of this cap on my 67 gas tank. I think I'll just get one of those fiber gaskets for it, or continue using my early 70's loose fitting locking gas cap, that leaks with a full tank.
However, I think the gasket makes a difference in the relief pressure, a newer cap with the neoprene gasket would seem to hold more pressure, maybe as they wear, the pressure relief figure will be lower. I'd give the design an F, it is probably one of those later emission control changes.
But to those who get that drip, drip, drip in the carb after shutdown following a long hot run, I'd advise to check the gas cap first.
Jerry Fuccillo
#42179
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