I have 12 Scepter MFC's (Gas, Red strap, viton gaskets) and have broken 4 flanges, all are used for gas . I  get the cap as tight as I can by hand .  Am I over tightening them.? Why do these break?
Anyone know who has the best deal on replacement flanges?

Thanks

Are you sure you have viton gaskets?

MFC Parts

they are grey not black

Color has nothing to do with material.  Viton comes in a rainbow of colors as does buna rubber.

Your flanges break beacuse of 2 reasons, 1. you have a buna gasket and the additives in the fuel swell it and break the thin flange.  Or 2. you tighten them too much and destroy the flange.

You can replace the flange with a solid viton disk, or buy new caps and viton seals.  

go over to the survival forum and read the MFC oracle.

All my Scepters came with grey gaskets.
They were diesel cans and had buna gaskets.

I bought viton gaskets from vatopa and replaced the gaskets on the cans I use for gas
and have not had a broken flange (yet) and it has been well over a year.

The red strap should mean they are for gas, but I don't know where you purchased them.

The DSTactical link above, has about the best price on replacement flanges and other parts that I have seen.
(except for their viton gaskets)

ETA: If i was replacing flanges, I would replace the gaskets with something I know is viton, just to be sure.
I also tighten my caps pretty tight.