They are tapered. I use the 43t’s and 43ts’s alot andvthey are tapered. 5/8 hex. Same with the r41t and r42t used in the 525sc’s.

Ok I must have really been tired last night. Looks like the plugs changed between 1969 and 70 The earlier ones used a R43XLS which is definitely a gasketed plug. Then it switched to the R44T And if I remember correctly some of those heads could use either one. The only problem I see and its hard to tell for sure without having the plug and the head in front of me. The R43XLS looks like it is a long reach plug and I dont think that is what those heads use.

I think you need to look real close at the plug holes and see how deep the threads are. Also see if you can tell if there is a spot for the taper seat plug to seat ? A real good close up picture of the spark plug hole might tell for sure.

Too bad you dont have the original plugs :(

Hi BillK

yea having the old plugs would definitely make this easier. I can take a pic of the spark plug hole tonight and post. The plug I bought to test is a t44xls I believe and it threads in. Not sure if it’s long reach or not. I will have to measure and will also figure out a way to measure the length of threads in the spark plug hole. Hope to figure this out soon as I’m planning to install engine tomorrow and as you all know much easier to do the plugs out of the boat. Lol

Maybe XL’s and not XLS ? I’ve made mistakes like that before. Lol.

As far as bummed plugs in head are gone, i disagree. I see wrong plugs in engines a lot. Ecsmple: People install wrong length spark plugs in the sbc vortec heads all the time. Uggghhh.

Heads off engine much easier to see if length is correct. :)

Not sure if you have the valve covers still off the heads but there will also be a date code on them which would help narrow it down. Didnt think to ask that before.

If you have a small right angle "pick" you can probably use it to count how many threads there are in the head.

Here you go. Great info in this thread:
https://www.corvetteforum.com/forums...g-for-l36.html

https://cimg3.ibsrv.net/gimg/www.off...0f8fc6851.jpeg
1st photo of spark plug hole
https://cimg7.ibsrv.net/gimg/www.off...5b96115dc.jpeg
2nd pic of spark plug hole
https://cimg1.ibsrv.net/gimg/www.off...fe0a5a33a.jpeg
Plug that screws into head. Gasket type
https://cimg5.ibsrv.net/gimg/www.off...36b36c0be.jpeg
Same as 3rd pic

Not sure if pics will help or not. I can probably use the small pic idea to get a measurement of threads if needed. Also can look for date code with inspection camera. That’s how I got the casting #’s. Pretty frustrating how difficult it can be to find correct plugs. Too many options. I really appreciate everyone’s help and ideas

Info is on that thread I linked. So you have proper plug, although most of us use heat range 43 for the marine bbc’s.
So, R43XL or NGK equivalent will be gine. I’d guess NGK heat range 6 is equivalent heat range.