Just for basic quick metric identification... as these are often confused.

Rear 2 are for 1/4" BSP gas taps with approx 13mm O.D measurement. 

Midway are for 3/8" NPT with approx 17mm O.D. measurement.

Both measured off the outer tops of the thread on gas tap we have in stock.

Or our Brass 1/4" BSP option, or blanks.

 Mustang Gas Tank 3.3 U.S. Gallon Single Cap

Can also be used on any custom Bike, Trike, Chopper or Bobber 
Raw Steel Low Tunnel Same universal mount at front as you see on the rear
Requires 1 Cam Lock Style Plain Vented Gas Cap 

Tunnel Measures approximately 2 1/2" High x 2 3/4" Wide & 20" mounting tab to tab end
The Tank itself measures around 17" Long x 12" Wide ( plus 11/2" of length tab front & back ) excepts either two 1/4" BSP Petrol taps at the rear or a 3/8 NPT midway on the left hand side as your sat on the bike

Now here comes the disclaimer part....as the manufactures states... pressure test and use a sealer/liner before use. As always it`s best practice to do your own preventative measures, for a trouble free life.