I don't know whether this has been covered before (I searched «"spark plug tube seal" "O-ring"» and didn't find anything), but in buying my million-dollar pile of tune-up parts, I discovered that slant-6 spark plug seals are now unavailable.
Some places list them, but show them Out of Stock, while others no longer show them at all.
None of this is terribly conducive to actually laying hands on a set.

With the need for an alternative as I clean up the new dart, I gave it a thought and decided that there is very little difference between the spark plug tube seals and regular O-rings, aside from the square vs round cross-section.

The seals on my motor are 1.5" OD / 1.25" ID, making them 1/8" thick.
Coincidentally, the Size-218 O-ring has these exact dimensions.
(Available at your local hardware store as Hillman 56051-B for 69¢ each, which adds up to about 4 bucks, less than the "real" seals cost).

On installation, they seem to fit the same (I have no photos because they are indistinguishable visually), and the sleeve rests in the same spot against the head when the plug is tightened.
There is salt on the roads here now, so the car won't be driven until the Spring, but I'll post again if they leak or otherwise don't work.

In the photos, the factory seal looks thicker and tapered, but, as far as I know, that's just the set it took after 60 years in the head.
– Eric

i seem to remember they are the same as corvair push rod tubes.

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Hyper_pak

Guess the Corvair guys are having trouble getting them, too, then...

– Eric

Badvert (forum member here) also has them on his ebay store:

https://www.ebay.com/itm/334535917602?m ... media=COPY

Greg

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http://www.youtube.com/hyperpack

I would not trust the Dorman shelf-stock items to be made of rubber good enough for sealing more than water at household temperature ranges—why would they?

The spark plug tube seal starts out as a square-cross-section O-ring. Which means if a № 218 is the correct size, the actual-real OE-duplicate item is this in Buna-N rubber (good in most oils, good to 212 °F), or this in Viton rubber (good in a wide variety of oils and chemicals, up to 400 °F).

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

I searched, but I didn't find those.

$9.65 a set, shipped. That's a fine price.

Order placed.

– Eric

Good point, Dan. And admittedly not one I'd thought of.

I may do an experiment, now that I've ordered Badvert's set, and install one of the "regular" O-rings, and see how it fares.
Thank you for those McMaster numbers, Dan.
For those watching at home,
the Buna-N 212°F seals are 22¢ a piece, but you have to buy 50, while
the Viton 400°F seals are $3.83 each [coincidence???], sold singly.
The Viton ones are clearly superior, and are likely better than original or aftermarket "auto parts" seals.

Those McMaster numbers should stay active for quite some time, hopefully achieving the original intent of this thread, which was to provide a valid alternative for a necessary part which is becoming harder and harder to find.

Thanks, Dan!

– Eric

…and if you wanna get extra super de luxe, there's this Viton X-section № 218.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Very nice. Not sure whether the X-cross-section shape would be an advantage in the \6's tapered holes and conical sealing edge, but would be interesting to try... at some point.

Thanks!

– Eric