I realize it's an old thread but wanted to mention. My fuel send sender for my HZ Sandman Ute was non functional so mechanic pulled the tank out and installed a new aftermarket Fuel sender unit. On filling tank the O Ring seal supplied with aftermarket sender unit failed to seal and leaked on a full fill of the tank. Upon tank removal I checked the O Ring and it appeared slightly to big and of a very hard duro hardness, with average quality nitrile. I purchased a new replacement O Ring (part:7421765) and it was exactly the same? I am a seal guy so checked application position and size; and the correct O Ring for the job should at the very least these days be a quality Viton material, to deal with our current fuels. If anyone else does this job or is planning to do the job use P50A Viton - 49.6 x 5.7 V75. see picture below the brown one is the Viton material the black is the replacement (7421765) I was supplied.
IMG_1575_small.jpg
I'm not looking to sell them to other Sandman owners, as you should be able to get from Bearing Companies; but if you get stuck send me a message I'm happy to help.
Geoff