Drill hole in ss tank

[southernman 45]

A sight glass is a good way if you don't want a gauge. You can put calibrated fill marks on it as you fill the tank, easy to make out of clear PVC and fittings.  

[raz88 56]

We've moved from a calibrated old sail batten to an electronic gauge with our most recent change in boat and I'd trust the dipping the batten option more. Now use the assumptions Jon describes.

[Island Time 780]

I fit a few gauge systems. Symmetrical tanks are easy. If you use a pressure sensor with a probe, top mounted is easiest. Fill the tank. Drill/cut the hole. Tap the bolt holes. Its best if you can remove the unit in case of failure. Then insert the tank spear into the tank, right to the bottom. Mark the spear at the top of the tank. Withdraw it and cut to the right length, 10mm shorter than the marked length. Now mark 1/2 way, then  1/4 and 3/4. Put the sensor on the spear, insert 1/4 of the way in, calibrate 1/4 on the gauge. Then 1/2 way, then 3/4 and right installed. Done. Easy.

 

If it is not a symmetrical tank, best to know the capacity, start with an empty tank, calibrate as you fill. Harder.

 

I usually use smartswitch tank monitors. 

[wheels 375]

If I understand the original question correctly, the problem is the thin metal of the tank. There are several ways to do this.

 

Simplest is using self tapping screws and simply screw the sender unit directly into the tank. Use a sealant suitable for Diesel, between the flange and tank. Because the Fuel will mainly be splash rather than submerged, self tappers should provide plenty of grip

The best way is a lot more work and best if the tank is removed to do this. A bit of fiddly and exacting work is required.
Use Rivet Nuts. These are a Nut that the head "forms" and is rolled over the tank metal. Then you simply bolt down the sender using either a gasket or a sealant between.

The third way is not really used in this situation. It is used when some pressure is involved, such as the fitting being mounted or permanently submerged at the bottom of a large tank. I won't go into in great detail, but it uses a split bolt flange. The Bolt flange is cut and spread slightly to make it look like a spring washer. It is fed into the tank and then held in place. Sometimes a couple of small counter sunk screws pull it up into place and holds it till the main bolts are torqued up.

[vesper1 2]

I put a mechanical fuel gauge on my tank same issue re fastening to a tank wall about 1.5mm. I used self tappers pan head with a fibre washer under each head and plenty of sealant under the housing. It's been over 15years and no issues. The key is a washer or o-ring under the screw head.

[marinheiro 152]

what has been found to be the best sealant in diesel applications that is available in NZ?

My reading suggests the silicon RTV products should not be used with diesel long term. 

[wheels 375]

No do not use RTV products. Although there is a Black RTV gasket sealant that is Oil resistant. But it should not be used for contact with fuel.
There are many specialist products available for use in Fuel contact situations. For instance, Permatex has a large range of products. Although one could argue the range is too large. It gets confusing trying to work out which is the best choice.
https://www.permatex.com/product-category/gasketing/?fwp_per_page=48