Thanks for all your comments Chaps!

Stuart.
I used Bluecol top up mix, in my Robinson engine. It rusted, immediately!

The O ring idea, didn't work. Leaked, again........

So, I filled the groove with Hylomar. Then wrapped around with thick cotton. To fill the groove a little proud.
Then applied more Hylomar, once it was through the first bore, before entering it into the rear one.
It worked........

Now, using coolant mix, at 10/1. No rust, so far........
Well........ It's
finally finished!

The sun was shining too brightly for the camera. The actual colour is Ford burgundy......
Scott Vacuum Engine......
Scott Vacuum Engine......
Scott Vacuum Engine......
Scott Vacuum Engine......The duck, is in there, somewhere. The wind was a nuisance, too.......
Not a valid youtube URL
Thanks for following along!

David D