What Are You Doing To/With Your Car Today?

Replacing a fuel tank on my 72 valiant . Always easier taking sh** apart then putting it back together . Is there a certain way to put in the filler to tank grommet ? Old one was shot so I bought a new one from classic industries but I'm having trouble sliding the tank in to the filler neck . I put the grommet on the tank and tried sliding it on to the filler neck but that sucker is real tight. I compared the original on and the new one is a bit smaller in diameter which is making it difficult. Was wondering if the original one was bigger due to age and heat? It expanded ?