Anit Siphon Valve Size
 

The anti siphon valve on our 2003 Gibson houseboat needs replacement but unable to find the correct size. Original is 1/2" compression but the only ones I can find are 1/4"NPT (whatever that means). Also will the hose end work with a barbed fitting even though the original was not barbed. Help please

Do you have any pictures? Is it hose or tubing? I do not think compression fittings are USCG approved. Could it be JIC? You can also use a fuel shut off valve. It sounds like a rubber type hose with a push lock type tubing fitting to a tube to National Pipe Thread into the tank. Pics would help.

Looks like you found your issue by the suggestions I posted.

A great tool to have is a remote fuel tank - fuel line and primer bulb. Like I have always said.

Also a great tool to have for boat lay ups and winterizations

https://www.offshoreonly.com/forums/...-1-vortec.html