The cap for the coolant recovery tank (or "radiator cap" as most sites list) appears to come in two sizes (types). I've seen references to a 'short neck' and 'long neck', or 3/4" and 1" versions. For the purpose of this discussion I am only referring to X1/9 applications, with factory tanks, and only the size (dimension/fitment) of the cap...NOT the quality, brand name, pressure rating, color, price, origin of manufacture, personal preference for, etc. Most of the part suppliers I've checked only offer the shorter (3/4") cap, except on Vick's site he only lists the taller (1") one. Many of the other catalog listings don't specify which one they are offering but only have one. Also the application charts from cap makers seem to only list one, but again they don't specify any dimensions (that I've found). On Obert's site he states the original metal/stainless tanks use the 'long neck' cap, while the plastic tanks use the 'short neck' cap (however he only seems to offer the shorter one). Yet when I measure my S/S and plastic tanks' necks, they both are the same dimensions (there is no cap on the S/S tank to compare). Furthermore, the shorter (3/4") version is readily available from most aftermarket cap makers (with commonly found part numbers) however I do not find any aftermarket part numbers for the long (1") version. So what's the story on cap sizes?