Sealing off cylinders without screwing spark plugs back in?

At some point I need to do a piston soak, and I want to check the level of the solvent in each cylinder periodically using some sort of dipstick. Normally the cylinder is (mostly) sealed off by screwing in the spark plugs a few turns, but I'm looking for a sealing method that doesn't require taking them in and out frequently.

Teflon is not affected by the organic chemicals, like those in a product like Berryman's B12 Chemtool.

So the simplest method would be to get some teflon disks the same size as the upper part of the coil pack, and just set those down were the coil packs normally go. Maybe with a little arm so that the pack retaining bolt can hold it down. This would be cheap and easy. Unfortunately it might be a problem because the edge of the valve cover gasket would be constantly exposed to fumes from the solvent for a period which could be a couple of days. The vapor pressure for the Acetone component could be as high as 500 Torr on a hot day (if the compartment is fully sealed), and that might chew up the valve cover gasket, eventually leading to oil dripping down onto the plugs. (The edge of the head gasket is subjected to the same chemical environment, but whatever it is made of is pretty resistant to gasoline. Plus I have never heard of a head gasket failure following a piston soak in a Prius.) The valve cover gasket is exposed to the solvent briefly when it is poured in or blown out, but the stuff is so volatile the contact is likely only for a minute or so.

Jamming a rag tightly into the top of the hole would be easy, but potentially have the same issue with fumes damaging the valve cover gasket, plus it might introduce other issues like not sealing as well or stray lint or threads getting into the cylinder.

To avoid that potential valve cover gasket issue, it would have to seal where the spark plug normally is placed. For the sake of argument let's say the diameter of the washers on the plug is 1/2". My first thought was to obtain a teflon ball a bit bigger than that, drill it half way in, tap it, and put some threaded rod in the hole. Picture an olive on the end of a big toothpick. It would be possible to feel when the ball is "in the hole" and just lean the rod over onto the edge of the hole in the valve cover. The problem here is that the threads for the spark plug probably come right up to the surface of the metal (anybody know for sure ?), so there is some chance the ball might bend the lead in thread, which would really suck if the plug could not then be threaded back in!

A more complicated plug would use a bit of teflon cylinder on the same sort of metal threaded rod. There would be no way to tell when it was centered on the hole though, at least by feel. Anybody know the inner diameter of the cylindrical area around the top of the plug? If the teflon cylinder is a bit smaller than that, and it has an upper piece of some sort that centers it on the upper hole, the cylinder should sit square over the spark plug hole sealing it, and stopping fumes from reaching the valve cover gasket - which would also be well ventilated to the atmosphere if any did get by the plug. I don't know how smooth the area is past where the plug sits, possibly the plug could not sit flush. It would have to be thoroughly blown clean to keep any bits of dirt from preventing a good seal.

Any other ideas?

 

Nothing, once, or even twice. However, I wanted to get some numbers on how much solvent remains in each cylinder over time, probably every 4 hours or so for two days. That's a lot of de- and re- installations.

 

Head down, threaded shank up? Could work if I can find some bolts of the right size with perfectly flat heads and no raised letters.

It does make me wonder if the washers on the OEM spark plugs are steel or aluminum?