Bongo Fury

I have a van I imported to the USA , and have been fighting the air con since day one.i repaired a leak on a rear line that was going into the rear matrix it is the smaller line with the 10mm bolt , the issue is the thread is stripped , on the matrix side , I am either going to tap it it out to a 1/4" as its 6mm now , or use a threaded m6/7 insert , my question is on the bolt the last 1/8" has no threads and the bolt end is a narrower diameter . how far can I thread with a tap does the thread go all the way through the aluminum
1995 diesel bongo thanks I have enclosed a photo of the area

I some how doubt that anyone will have an answer for that! The mech diagrams aren't that detailed either