Changed mine a few months ago. Cut the old pipe out, as close as possible as it leaves an oval hole. I then used the new cap gasket as a guide to enlarge the sides of the whole, and drill for the bolt fixing. Used a chunk of Ultraflex fuel pipe (as the old pipe was straight on my 4 seater). Slight issue - the filler cap needed a 50mm hose fitting and the tank was a 47mm but I used some petrol resistant rubber as a filler piece and it is fuel tight.

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I used Newton mounting stuff - I think it's what MMC use on the M3W and other stuff. It has a locking vented cap inside, although the Aston cap itself was a £30 eBay special. Polished Alloy and a fraction of the price of others.

"Y mae dafad ddu ym mhob praidd"
1972 4/4, 1981 MGBGT, 1984 FLHT1340, 1980 XLH1000, 1990 ZX10