For those who may want to use the thermostat blanking sleeve and a modern thermostat together, there are a couple of issues to pay attention to. First, very recent issue blanking sleeves may be a bit oversize on diameter of the top flange. This requires grinding a bit off the edge of the flange to get the sleeve to fit down into the counterbored recess in the cylinder head. That may be enough to assemble this combination properly if you use a soft cork gasket. --- Placing the thermostat on top of the blanking sleeve flange makes double thickness, so the thermostat flange protrudes slightly above the cylinder head surface. A modern Klingersil hard paper gasket may not squeeze down enough, leaving a small gap under the bottom of the the thermostat cover.

On 10/19/2023, Terry Holden wrote:
I did notice that the studs went through in to the water jacket and have already sealed them but very good point. I wonder how many people forget. --- My sleeve only needed a very small adjustment to the flange and now sits perfectly flush with the casting.

The problem with the sandwich stack you suggest is that it creates a gap the thickness of the thermostat flange I.e. 0.045 inch. I guess either a cork or Klingersil gasket might just squeeze up enough to seal with a thick coating of sealant but it did not look right to me.

Will have a search through my box of gasket materials to find something thick enough to fill that gap. It will need a slightly larger hole than the standard gasket to suit the OD of the thermostat gasket. The standard gasket can then sit flat on that and should seal better. --- Found I had this gasket material - just the right thickness.

Made up a gasket /spacer from it. -- The thermostat sits inside the green gasket/space, not on top of it, and closes the gap in the sandwich. All went together well so should not leak. Waiting for some blue antifreeze to arrive can then fill and run her up to see what is happening.