Setting up a TR3-4A head for hot street

> I want a good street-driving TR3A. What should I tell the machinist about
> taking metal off the bottom of the cylinder head? He already has to clean
> up that surface a little: do I tell him to keep cutting down to a certain
> level using the same machine?
> John Middlesworth

John :

IMO you should only clean up the surface, not remove any extra. It's already difficult to buy gas with the factory recommended octane (at least in the U.S.), and any extra compression ratio will only aggravate the problem. If you want to try a higher CR, use one of the thinner head gaskets. That way, you can go back later, if you want.

Before I'm deluged with mail, let me point out that the 92 octane 'Premium' commonly available in the U.S. is rated by the "R+M/2" method, which gives a reading 3-5% higher than the "motor" method that was commonly in use in the 50's, when Triumph wrote the recommendation for 90 octane gas. Thus, it is actually only 87-89 octane. The federally mandated addition of MTBE only aggravates the problem.

However, be sure to rebuild your head with the later 5/16" exhaust valve stems instead of the earlier 3/8" ones. IMO this will make a bigger difference than milling the head. Get the hardened exhaust valve seats, too.

If you do decide to mill the head, be sure to start by measuring for any previous milling. According to Ken Gillanders, the stock head measures between 3.325 and 3.330 from the valve cover gasket surface to the head gasket surface. Ken also suggests .060 off the head and a thin head gasket as being the maximum for the street.

Randall Young

Note if you are racing and using racing fuel, Uncle Jac considered 0.130 to 0.150 "normal" to mill a TR4 head (TR3 heads with rounded lower theromostat housing has less metal at the bottom ahd should not be milled beyond 0.06).   0.190 to be where it started to get dangerous being too close to the water jacket.  That said, there is some variability from head to head on where the internal water passages are. The tinner the metal between the water jacket and bottom of the head the more likely the head will crack under use.

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