Tony M Oct 30, 2017:

@ Asker Like Abel, I read this as slightly telegraphed (or possible not even native-speaker EN?) — but my interpretation here would be rather "...Install Regulator Assembly .. onto nipple [in such a way that it is / as to be] leak tight."

I suspect a non-native speaker, who was struggling to find a way of saying "Install Regulator Assembly ... leak-tightly onto nipple." — but in EN, creating ad hoc adverbs in this way has to be done with great care!

HERBET Abel Oct 30, 2017:

Je le lis comme ça e'faisant court
Install LEAK TIGHT reg. ass. Into nipple
Bien fixer sans fuite sur le raccord