I’ve e-mailed Bjorn but got no reply.

Alutight is just a brand name, the same stuff can be had in lots of places. I think Durafix is the easiest version to get hold of in the UK.

Would this stuff be OK for chopping aluminium sumps.
Would it be good enough to get an oil tight seal.

I've used a Technoweld kit (Aluminium brazing/soldering) to make several Aluminium parts for my Locost.

I'd recommend it - sounds like a very similar product to Alutight/Durafix.

You need a good torch to use this stuff on big aluminium parts. They just conduct the heat away so quickly. I would certainly try it on a similar large test piece before I did any chopping.

I've watched a couple of YouTube videos and it's quite impressive stuff.

Totally agree with Luke2152.

It's a real bugger to get big parts hot enough!

I built a stand to put the part on and put a second torch under the part being brazed. It still takes ages if the part you're working on is anything over 300 grams or so