This filler neck seal is like an adapter. It will allow you to use a newer style filler neck that is plastic and a smaller diameter, with a gas tank that is made for the metal filler neck. The sizes are 38mm inner hole that goes into a 70mm outer hole. So if you want to retrofit a newer style plastic filler neck onto your older Vanagon, you can use this seal to do that. The only thing you want to watch out for is that if you try to do this with a diesel Vanagon. The hole where you put the diesel pump nozzle is too large and you will have to modify the hole in the plastic filler neck opening if you want it to work at the fuel station. It can be done, but if you don't do it, then you can't fill the van because the nozzle will not fit.