Itinerant Air-Cooled

Plug #4 is stuck.
Plug #3 was very hard, I think oil did penetrate and did help.

Stuck #4 is weird, I measured the resistance between electrodes......Found 4800 ohm where it should be infinity ( a perfect isolation ).
( No, I am not confusing with other resistances. I found they are ok: Rotor checks at 5000 and plug wires RF Supressors check at 1000 at #3 and #1 and 5000 at #4 and #2 )
How come the engine runs quite ok ?...Ignition at #4 cannot be right with this straigh 4800 ohm fooling its electrodes. Engine starts rough, sort of on 3 legs though, but idles smoothly when warmed up.
Presently, I run on three new plugs and the stuck #4 that I keep soaking with penetrating oil.

Stuck stuck, or backs out slightly and tightens as you back out?

Stuck stuck.
Doesn't move any bit either way.

Hm. Have you ever had it out?

Who knows what mecanics have done on my bus, may be that plug never came out, since I changed the engine some 30 years ago.
Plug #4 doesn' t move a bit either way.
I use a 60cm extension tube and I guess put some 30 kg.

How much can it take before breaking the plug or tearing the thread ?

Not sure what you're soaking it with but I suggest using Kroil. I've had incredible luck with it on a variety of things. Hit it with that for a couple days and I bet it'll come out.

What is the penetrating oil that you are using? PB Blaster, or Liquid Wrench, soaking overnight, ought to loosen that plug up. I know it's a difficult reach, but can you get a flat blade screwdriver down there, and tap on the metal part of the plug, hitting the butt of the screwdriver with a light hammer? If you can get some vibration going, it may help get the penetrant down into the threads. That helped me get some plugs out of a '46 DeSoto one time, that otherwise were not budging. Of course, they were much more accessible on that flathead 6.

Best penetrating oil I've come across is a product called Moovit by Lloyds Laboratories. It beats PB Blaster and Liquid Wrench hands-down. Can't recall where I bought it, though. Probably brought it with me from Canada, as it is a Canadian-made product.

Now it is possible that if the aluminum threads in your head have corroded, generating aluminum oxide, then no sort or quantity of penetrant is going to help. The aluminum oxide takes up a greater volume than the pure aluminum it was spawned from, so it basically wedges the threads of the spark plug in place. By this exact mechanism I have a beautiful Cinelli stem stuck in the head tube of one of my bicycles that I haven't been able to budge: I just haven't gotten up the nerve yet to hack it off and bore it out.

Not sure how this helps with your spark plug. You might be at the point where you need to pull the head to work on it effectively.

Slight heat from a torch may help, but be careful!

Whats French equivalent for 'PB Blaster ? 'Blast du PB'? Aux de Blastaux '?

Dégrippant PB.

Indeed,
I am aware of aluminum oxyde welding the plug
and penetrating oil being effective on steel rust, not effective on aluminum rust.
I think this terrible oxyde is from a long rest in a damp place.
I have tryed vibrations but cannot get blows stiff enought. There is a quite good access at plug 4 once carb horns and air filter can are out.

I am out of "liquid wrench" ( my favorite while I was in the US ).
I use some french equivallent " degrip oil" ( not as good ).
and WD40, the regular.
and lastly I found, another WD40 product tuting it is super penetrating.
I' ll see wether PB Blaster is available.

A quick Google for "best penetrating liquid" turned up some hits on hot-rodding forums which touted the amazing effectiveness of a homebrew mixture of acetone and ATF. Presumably the acetone acts as a carrier for the ATF with capillary action superior to the commercial products. Try at your own risk: acetone is nasty stuff.

If you are in Canada, try and find the Moovit. It's worth having, even if it is also ineffective against aluminum oxide.

I know weirdos use acetone to make home made powerfull explosives. Extremely dangerous, because that stuff is unstable.

What is ATF ?

BTW: I am not in Canada, I am in France.

Automatic transmission fluid. I mix it 50/50 with mineral spirits, and that does a pretty good job, too. I like the PB Blaster because of the spray can convenience, as well as the effectiveness.