That's a common problem area and it often takes a couple of tries to get it properly sealed. The key is to have the face plate of the sender unit perfectly flush with the rubber collar gasket before threading on the lock ring and to keep it from moving while tightening. If needed, use nylon fishing line or a very stout thread looped down through one of the hose fittings to keep the barrel pulled back a bit while you bump it into initial alignment and thread the lock ring on -pull one end of the loop to remove it when done. Be sure to use a large SS screw clamp lightly tightened around the outside of the locking ring to help keep the plastic threads from jumping and stripping. Tighten the locking ring as hand tight as you can get it. Snug up the screw clamp a little bit more then use a hammer/drift or other tool to get a further approx. 1/8-1/4 turn out of the locking ring (depending on how hand tight you got it). Fully tighten the screw clamp. Make sure you start with all threads and surfaces clean. You can use a thin wipe of petroleum jelly on the threads if that helps things along.


--
Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now