cylinder head probs

[mawsy]

Hi there I have a rather strange problem with a type 3 engine, I was given this and informed it was a runner. I fitted it up for a test run and found it to be blowing from the top of the barrels and cyl heads, i thought it may have been cracked barrels so i stripped and inspected all barrels and found no damage, after much head scratching i located the problem. the recess in the cyl heads was 13.5 mm deep, the spigot on the barrels is 9.5 mm deep. what would be the best remedy for this problem. what effect on compression would there be if i installed a 4 mm spacer ring to the cyl heads. stranger yet it has 3 pistons and barrels 86.54 mm, and 1 cyl and barrel of 86.92. grateful of any solutions Mawsy