gas cap questions galore..

[jocko]

Let me tell you what I THINK I know, but since I'm also thinkin I might be wrong, I'm hoping someone can square me away...

1/2 ton:
67-71 caps were unvented (or, actually, what I mean is that they were for "unvented" fuel tanks in the cab)
72 caps were vented

3/4 ton:
67-72 caps vented

a) were the caps for different dia opening filler necks as well? (I seem to have 1.5" and 2" floating around in my head) - and I think the vented one was the larger dia??

b) So, will a 71 1/2 ton cap fit on a 72 3/4 ton truck?

c) Is there a way to tell if a cap is for a vented tank (evap emission 72 thing) just by looking at the cap only (or vice versa, a way to tell a non-vented cap just by looking at the cap only).

d) I believe all aux tank setups, factory NL2 and dealer-installed dual aux tanks) were all unvented. So, which cap would a 72 with an NL2 aux tank use on the aux tank? A 67-71 non-vented cap? Or what id the truck is a 3/4 tonner? A 67-72 3/4 ton cap? I THINK the aux tanks all had the smaller dia filler neck (and associated cap to go with it..?), all years.

Appreciate your help.

[volksworld]

iirc the different size filler necks used the same size caps...the rubber grommet where they go thru the cab is different....the vented cap (for the non vented tank) has a raised area inside the cap with a small hole in the center...that's the check valve....as far as what specifically fits what truck I cant help you...and they could have been swapped by now

[cdowns]

67-70 gascaps vented
71-72 gascaps non vented // venting goes thru charcoal cannisters

that is for 1/2 tons

[Spartan]

Iirc my 69 3/4 is non-vented. At least...the cap isn't vented, but I know it's not OG either.

[cdowns]

pretty much a foolproff way to ttell is take the gascap off// if you hear a release of pressure you have a problem either a plugged vent from the 71-72 tank or a nonvented cap in the others

[jocko]

thanks all, I appreciate the info - cleared up some stuff for me.

cdowns, I was thinking that 72 only was non-vented, thought EEC canisters didn't come out til 72?

As for what I have, you all are right - in fact, I know it's non-original. My 71 had a tank swap, and haven't stuck my nose back there to make sure what kind of thank the po put in.

Any i/p on the aux tanks caps (for a 3/4 ton)? I'm betting the vented early caps will fit it. For example, the cab tank cap on my 72 (which does have the original tank) does NOT fit the aux tank filler neck.

Thanks again all, appreciate the help.

Oh - and cdowns, thanks for your correction, I was using the terms vented and non-vented backwards (i.e. early were vented//ater non-vented because it went thru the can - thanks!)