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Q & A: Gas Tank Venting

Dear Mr. Twist:
I own 3 MGBs: a 1965; a 1970 (GT); and a 1975. The 65 and 70 have had
their original gas tanks replaced, and the 75 still retains its factory
installed tank. Every summer I encounter a situation that has been a real
puzzle. On hot days, gasoline in the fuel tank backs up the filler
connection, pushes out the vented cap and spills all over the back bumper.
I've talked with several people in an effort to find out what is going on.
Everyone has theories on why it happens, but the only working solution is
"don't fill up the tanks when the weather is warm, or going to be warm."
Only recently has it occurred to me that the 75 (with the original tank
still in place) doesn't suffer from the overspilling. Additionally, I
reread, more carefully, several of my MG books and came across the statement
that factory installed gas tanks have some sort of "buffer" inside to absorb
expansion that gasoline undergoes when the ambient temperature gets very
warm. Apparently, the after-market replacement tanks do not contain this
"expansion" buffer, and this is why such tanks overflow when the gasoline
inside expands. What do you think?
Sincely yours.
Kevin J. Holland

Kevin!

Gas tanks, from the T series through the 1969 MGB were vented
directly to the atmosphere. In this "old fashioned" method, gasoline
expansion pushed air/fuel mixture out of the vent hole in the... read more