There are three types of exhaust stud nuts in common usage the cheap ones from LAS, the standard ones from Lycoming and the self locking type from Lycoming.
The LAS type are OK if access is not a problem.
The standard Lycoming nuts are usually the most economic as these are slightly smaller than the LAS nuts and if the space is confined they are less trouble to fit and the extra cost of the nut ( over the LAS nut ) is offset by the Labour costs of fitting.
The Lycoming self locking type I have only seen fitted in locations that are tight and probably preclude the fitting of lock washers.
I would recommend all Exhaust nuts/ washers / lock washers are changed at each annual check. This is because if used for more than a year corrosion sets in and they can become corroded to the stud and this will require stud replacement. He cost of stud replacement can be many times the cost of renewing the nuts & washers at annually.
That is why one uses locknuts, and for some reason the high temperature ones are expensive.
Exhaust leaking is potentially a real problem.
That’s probably not the nuts backing off though, it’s probably because most of the take-up of the torque was just overcoming friction when the nuts were first put on. If you mark the nut and stud you’ll probably find it hasn’t moved.
On continentals, it is recommended to retorque after a few hours post exhaust gasket replacement. I have had them back off.
The engine that started this thread though is an O-360, these use STD1410 nuts on the exhaust which cost 20p each. According to the Lycoming parts catalogue you can use the STD1410 on the IO-540 as well. One thing I’ve never seen ever try to back off is an exhaust nut so I’ll stick with the 20p ones myself rather than have to contort myself into funny angles to do maintenance jobs that are far easier to do with the exhaust taken off!
Hmmm, not the special hi-temp locking ones – STD-2043.
I am N-reg; paperwork is not a concern.
Peter wrote:
The Lyco exhaust stud nuts are very expensive and arguably should not be re-used.
They are 20p each from LAS! Even the studs themselves are under a fiver. (Or does every nut require an EASA form 1 if you’re EASA regulated?)
From @pilot_dar ’s post it sounds like removing the exhaust is key to getting the valves back in. So the “rope trick” is really there to avoid removing the exhaust. The Lyco exhaust stud nuts are very expensive and arguably should not be re-used.
I wonder if there is a tool which can pick up a valve stem through the spark plug hole, and push it back up into the freshly reamed guide?
An overhaul can be done with or without new cylinders, depending on the condition of the existing cylinders and the cost to overhaul existing cylinders within the specific circumstances and resources of the owner. NDT can be cheap, depending on the specific circumstances and resources of the owner. Parts cost money, the amount depending on where you buy them. 
It cost $15K for parts and specialized work to overhaul the AEIO-540 I mentioned. As an aside, that engine and the aircraft it was installed in was bought for $80K from an insurance company and sold for IIRC $170K, albeit with about $25K spent and at least a year of screwing around with the local FSDO. I wasn’t financially involved but it was interesting to see. For the owners it was just a way to make a little money and have some fun using their skills and the hangar they have anyway.
If I had the feeling that owning an aircraft required me to submit myself to a totally preordained and regulated business process, I’d sell mine. That’s actually one reason I like owning aircraft now, and have limited my involvement with new cars and motorcycles. Contrary to popular perception, at least for N-registered planes and particularly in the US, there is more scope for flexibility and creativity when dealing with aircraft, and a whole network of resources.
Doesn’t an overhaul require
An IO540 is about $30k at a top shop in the USA.
