I won it...but what is it?

[regularjoe2]

I went through about 3k on-line images looking for fuel tank/fuel filler caps .

Came up with a few , none exact to your item , hosman321 .

I'm of the opinion that so many 'vintage' oil tanks (& their caps) wound up in the junkyards .

It was common for these types of caps to be slightly convex on top , as well as having a chain or ring attached .
There were normally tangs (or one tang) cast into them , for gripping the slippery devils , when wet from weather or fuel .

Below are images of a modern cap w/chain , a cap assembly from a horse-drawn fuel tanker , and an iron cap from an old John Deere tractor .

Perhaps searching through old iron & brass gate valve collections might turn up results (have not done so yet) .

[talesofthesevenseas]

You know, it sure looks like it could have been a fuel cap for a tractor or something, but the fact that the numbers would be upside down when it was in use really bothers me. It goes against the "Three F's" of Form, Fit and Function. However, I wonder if it could be a cap for draining oil from the BOTTOM of an engine?

[regularjoe2]

Talesof , although I'm not 100% certain what the iron item was used on , I am 100% sure that no such fitting would be used on a low point (oil , diesel , or water) on any engine .

All drain plugs/petcocks are threaded & have a seal , o-ring or gasket - in order to withstand temps , pressures & vibration encountered with working (internal combustion) engines .

My first thought , when I saw this item , was a post-cap for a tennis net (yup , still have some questions about the iron thingie) .

I will say that I think either the iron item - or the item it mates to - has got an archimedian "self-tensioning' fit .

[talesofthesevenseas]

Gotcha Regular Joe! Thanks for clue'ing me in!

[hosman321]

Ok...this thing is driving me crazy. Got it today. Looks turn of the century to me. Circa 1900-1915? The hole isn't even big enough for a finger to fit partially through. So I have a feeling it hooked onto something or had a chain on it?
I just gotta know what this cap thing was for. Gives me kind of an automotive vibe. Some kind of radiator cap for an early car? Thanks for any ideas! I sure don't have a use for it since it's not very old. Who knows, maybe it's some kind of part somebody out there has been searching for! It'd be nice to be able to put it back up on ebay under the correct description. Not a skillet the size of a Coke can. Maybe I'll make my $15 back.

[hosman321]

I guess I should have read regjoe's post better, I think it is that kind of cap. Wonder what exactly it was used on?

[waywardangler]

I do not know either...I was thinking more along the lines of a hit 'n miss engine but have not had time to look into that.  Maybe for a tractor?  I do not think it is automotive but I have no experience with old engines so who knows?  It fits in two slots and then turns to lock to keep it from falling off or blowing off.  Maybe a steam engine?  Any old machinery or steam engine meets in your area?  There are a few here in the Midwest and I am sure some antique machine aficionado could look at that and tell you what it was for, what it went to, and what year it was.  I do not think it was for any pressure device.  Could be the bottom (or top) of an oil cup? 

I do not think it was for a tennis post cap.  Those were threaded so kids like me years ago would not have been able to do a half-turn on it and take it home.

[cogar]

It could have been a "cap" for an oil drum or for a 'fill spout' on an oil tank(er).

I wouldn't worry about that number being right-side-up or wrong side down, one often finds such old items like that. Oftentimes it was the fault of the "mold maker".

Those numbers have to be engraved in reverse ...... and sometimes upsidedown ..... and unless one is careful its easy to get "started out" the wrong way. And if you do, you probably won't realize it until the 1st casting is made ..... and by then its too late. 

[sapphire]

Came up with another idea while searching for oil tank/drum caps............wha t about a gas tank cap from an old vehicle ( truck/tractor, maybe)?  Unlikely to be in cast iron ....but who knows!   
 

[danny1979]

well just to put my 10 pence worth in i think it could be the water out let cap off a horse drawn fire tender I'm into vintage tractors and have seen things like this before then the hose would connect onto where this cap came off

[sapphire]

Great input danny!! Somehow I was picturing this attached to some sort of large, old vehicle.  Never thought of the old fire 'trucks' 

Definitely makes more sense as containing water is much less of an issue then something flammable.

[sapphire]

Did find a pic showing an oil tank cover like regularjoe posted.......

[hosman321]

Thanks for all the input guys! That would be pretty cool if it was a water cap for a horse drawn fire tender. At least now I know where to start, I never would have figured that out. Maybe I'll email a few collector's clubs and see if they have seen one like it. Thanks!