I own a 1980 Fiat Spider which I turbocharged. The main issue regarding the turbo system has been the exhaust nuts staying torqued. I think this is also why the car had been blowing exhaust gaskets.
The typical split style lock washers were not holding up. I finally found something that seems to work.
I've been using these things called Nord-Locks. They are made in Sweden and are not sold everywhere in the United States so I have to get them through mail order but are so good they should make the split style obsolete. I'm sure you can find them in the UK.
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They have these little steps inside that lock in place when torqued. The interesting thing is if you tighten a wingnut with your fingers it takes more pressure to unscrew the nut.
So far I've put about 1000 miles on my turbo system since I tore it down last winter to make some repairs. I've had to tighten a few nuts a second time just a hair but so far not a third time. So they seem to be doing great.
Also I don't know how many people would recommend this but I did it. On the opposite end of the stud that doesn't have a nut (such as the end that screws into the cylidner head) I put antiseize compound on that end but the other end that the nut contacts I deleted the antiseize compound. My theory is there are situations where you actually want the nut to seize. Like on problematic exhaust nuts. I figure if I have to remove the nut, at least I can back the stud out if the nut is seized.
Also this debate will go on forever but I chose grade 8 steel on all my exhaust fasteners. Some turbo guys recommend stainless steel but they mention nothing about it being bad to gaul. They say grade 8 isn't strong enough to use on turbo systems. Well I work in the chemical industry and my company has been in business for 80+ years and we never use SS on anything that get's hot for this reason. Also Chrysler used grade 8 on their turbocharged engines so I don't know why people think SS is better.
As far as using brass nuts. Well I can't say. I've never heard of using brass exhaust nuts. But would think they wouldn't be good on anything that gets hot.
I would have the surface of the exhaust manifold checked to make sure it's not warped. This will cause the gasket to blow.
Can't help you on the torque specs. Although there was a Talbot built during those years that has a copy of the Plymouth Horizon/Dodge Omni. The body shell is the same but the grill, taillights and dashboard are different. I don't know if they use Chrysler engines or not.
Hope this helps.