Engine advice

[beamdream]

Thanks, I checked a couple other heads and no plug. Never would have known of them if I didn't clean the block water passage.
Cyl Head Quest | SAOCA Forum (sunbeamalpine.org)
What is it's purpose and why is a plug called a jet?

As far as Im aware all heads originally had a "thimble " inserted into the that centre hole in the head, has to do with added cooling around the middle exhaust valves.

Almost never see them used on refurbished heads but they are a recognised spare part; check the WSM they rate a mention.

 

[Tim R]

My understanding is that the insert restricts the coolant flow and thereby increases its speed. This improves the cooling in the head. If I'm wrong no doubt someone will correct me.

Tim R

 

[Knightowl61]

Ordered the part and now I see why it's called a jet.

 

[JSLoeffl]

For what it's worth, a post from jumpinjan from May 2010 states. The slit faces toward the middle of the head.