As I just recently learned there is Lycoming SB 388 C requiring a valve wobble test for any engine where not all of the cylinders are fitted with “Hi-Chrome” valves.
Two questions about that:
Can it come any close to being reasonable to exchange a valve guide by a Hi-Chrome type, e.g., if it only was one Cylinder? Or is it just crazy compared to the fact that the wobble test is rather simple and quick doing?
There seems to be the “rope trick” to get out the valve, as Achimha wrote here:
https://www.euroga.org/forums/maintenance-avionics/2755-lycoming-360-stuck-exhaust-valve-lesson-learned/post/43395#43395
Could this do the trick in order to get rid of the required wobble test? According to SB 388 C the valve guides have to be exchanged, not the valve itself, so by means of the rope trick one could let the valve fall into the cylinder and exchange the valve guides. Correct?
And if not, does anybody have that valve wobble test tool and would dare to lend it away (or sell a used one if, e.g., not needed anymore)?
I know that the meaningfulness of that test is questionable and that there are other means to better identify a sticky valve, but as there is an EASA-AD about that Lycoming SB and the specific test procedure, there is no other way.
Edit: Third question: Is a mechanic here around or does anybody know a mechanic who knows that rope trick and could exchange the valve guide on one cylinder, which is, without pulling the cylinder and the corresponding rat-tail of other work involved with pulling a cylinder (and who can sign off the work)?
I’ve never had anything to do with changing valve guides on a Lycoming cylinder specifically but valve guides are typically pressed out from the inside. I can’t see how you’d do it more easily with the cylinder in place.
If it were approved, you could install a K-line valve guide insert, and apparently they work well, but I’ve never seen it done or discussed for aircraft engines. This repair method was originally developed to repair truck engines that nobody wants to remove from service for long.
For whatever it’s worth, when I decided to check the valve/guide fit on my (then) 1000 hr in operation O-320 they were all so free in the guides that there was little point in running any test. My only concern was with guide wear (i.e. loose valve stem clearance) as opposed to contamination and sticky valves.
The cylinder needs to be removed to replace the guide. It would certainly need a oversize o/d guide to maintain correct interference between the guide and cylinder head. The valve seat would need refacing after to ensure concentricity of the valve and seat
There is a vast amount of debate around this topic 
Chrome guides (“C” cylinders) do seem to greatly reduce stuck valve problems. I still got one, 2 years ago – here.
The old rope trick is just a way to stop the valve falling into the cylinder, while someone is poking a reamer into the guide, while the plane is stuck somewhere in [insert your favourite 3rd World spot].
The rather more crude method is to remove the springs, hold the valve in the chuck of an electric drill, and rotating it while moving it in and out, while applying grinding paste to it. Then cleaning up the grinding paste as best as you possibly can, for obvious reasons 
This really needs the exhaust pipe removed so you can reach the “live” end of the valve, and clean up the paste there. Again, a real 3rd World method to get back home.
The proper way is to remove the cylinders and ream, or replace, the guides, but that can take a lot of work even if you have the tools. You need the torque wrench adaptors to reach all the studs, and it has to be done the right way.
Good shops will just ream the guide out unless it is worn out of spec. And of course fit a new valve; never re-use the old one. And re-grind the seat.
Unfortunately, there is very little consensus on how to avoid sticky valves happening. They get stuck due to lead. There is some anecdotal evidence that 15W50 shows the problem more often than W80. I have changed totally to W80, due to this
OK, ok, understood, I got it.
So I’ll stick to that wobble test.
One interesting thing is, though, why there is still that cylinder without “C” on an engine which had an overhaul post 2000. Shouldn’t there only remain cylinders with “C”?
Could it be that the cylinder, during overhaul, got the “right” valve guides, but it only lacks that marking?
The cylinder may have been kicking around on some shelf, for many years. This does happen, and it is likely to be sold to a customer who looks like he is not going to look too closely.
Not even the examiner noted it for many years. Until I bought that thing.
But in the end it’s only yet another cumbersomeness. The wobble test seems to be not too time consuming. And with these silicone valve cover gaskets (or for the British readers: rocker box gaskets) there are not even spare parts to throw away any more
