When I got my first alloy engined vehicle in the '70s I also got the
factory workshop manual. And it insisted you used a special thread sealer
anywhere a bolt or stud went into the ally. This stuff has four basic
functions.

1) It prevents corrosion in the threads.

2) Seals the threads - useful where they go into the water jacket.

3) Locks the thread once it sets.

4) Lubricates the thread thus allowing the correct torque setting where
applicable.

It's called Scotch-Clad 776 and is made by 3M. It's an industrial product
really for the aircraft industry so not available retail in small
quantities. The smallest size is a 1 litre tin. Which costs about 15 quid.
And will last a DIYer many years - although it's rather like a paint in
that it will go off eventually as it's used up and there's a lot of air in
the tin. Decanting it into a smaller ones so there's no air would probably
slow this down. I'm only on my second tin.

Later factory workshop manuals for the same engine don't mention it. But
I wonder why since steel to aluminium threads remain a problem under some
conditions.

It could be there is a similar Locktite product. But this stuff is rather
different than most of those in that it's a thick clearish goo rather like
cellulose varnish. And indeed cellulose thinners is used to clean it off
used components before re-use.  

I found the recent supplier by Googling on 3M. A place near Southampton
which would do mail order but had a fairly high minimum charge of IIRC 50
quid. Luckily they had other products I could make use of to make that up.

The water pump bolts go because a couple of them corrode where water
goes past them (or mine seem to have done). There are a couple of the v8
inlet manifold/head bolts that are similar..
Always use a torque wrench or a small ratchet so you can feel what you
are doing and not abuse it. Other than that it is sods law that the hard
to reach ones break :)

Derek - drilling and tapping/helicoiling stuff yourself isnt too tricky
- though since its off anyhow you might as well get it done while he's
polishing it (its a set rate for helicoiling stuff at the place i use)