Well, I've done the rounds of suppliers and not come up with anything. Every product I looked at, including Loctite 515 and Prmatex #2 (which is just another silicone) says the parts should be clean and dry so here's what I now thought I'd do.
After draining the oil and flushing, I can easily clean the bolts one at a time as I remove them. It is the female thread that causes the problem and for that, I thought I could use some carburettor cleaner squirted in through the hole and let it drain out the bottom. I could maybe also make up something like an oversized pipe cleaner to help. I don't
really want to do this again so maybe I use some string or yellow teflon tape as well as Loctite 243 to seal things up. I'd also put the aluminium washers I made under the bolt heads. That is 3 different methods - surely that will shut it up!
