The TR6 came with 4 different heads over the years. Ranked in order of
disirability they are:
1. Circle B marked by a B inside a circle. (Early)
2. Block N marked by a N (Early)
3. Nothing head. No designation, usally a "BIL" on one end and a 3 on the
other
4. Diamond W marked by a W inside a diamond.
All these casting marks are under the valve cover nearest the firewall.
Manifolds must match to the ports also. Only two manifolds were made.
Also there was one year where the valves had single springs which should be
convert to doubles.
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