I have a used 6.5 gallon /6 tank that I want to use on my old /5 for travelling (the smaller 4.5 gal Toaster Tank is stylish, but more gas is better).
It ought to fit but isn't. It just barely
doesn't. The two rearmost lower inside flanges of the tank contact the two sides of the frame rearward of the backbone and won't let it drop down onto the M6 mounting studs. Same width exactly at that point as the Toaster Tank.
At the front of the tank, the top lip above the front tank mount contacts the head bearing dust cap. Again, just barely (maybe less than 1/8") but enough to not let it drop down onto the front mount and slip forward to the rear can drop down. Easy to see why-- the bigger /6 tank is p'haps an inch of more higher in the front. These tanks have always fitted, though it's a tight fit. I guess it's our old foe, Tolerance Stacking.
What to do? My first thought is to get socket and use it as a dolly and gently hammer the front curved area / lip back a little for more clearance.
But before resorting to blacksmithery I thought toss it out to the collected wisdom...