Those were cup style core/freeze plugs which are pretty simple to change.

Volvo used disc type core/freeze plugs and the installation procedure is different.

FYI, Volvo doesn't describe in any repair manual that I have found, that describes the proper installation technique and they don't offer a special tool either. I have never found a proper written installation procedure for the disc plugs anywhere.

The opening needs to be scrupulously clean, especially the area on the sides of the opening as that is the sealing surface.

Then put some sealant on the perimeter of the disc. We use Permatex #2 non hardening sealant for all of the disc plugs except for the plug on the back of the motor for the camshaft which gets oil resistant silicone glue.

Install the disc in the hole make sure is seated all the way to the inner stop. I use a socket and hammer and gently tap all the way around the outside of the disc to make sure it is seated.

Then using a socket and a hammer, deform the center of the center of the disc. For a 35mm disc, I use a 9/16" socket. This will cause the disc to flatten out some and the outer edge will seal against the side of the hole.

The tricky part is to know when to stop and I suggest you use the other plug in the front of the head as a guide.
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Eric
Hi Performance Automotive Service (formerly OVO or Old Volvos Only)
Torrance, CA 90502
hiperformanceautoservice.com or oldvolvosonly.com