John Middlesworth wrote:
>
> 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
Hi John,
In advance, sorry for the dissertation!
There is a lot that needs to be determined before you decide how much to
shave off the head. Are you going after increased compression or just
"cleaning things up"? What else has been done to the car (cam, overbore
pistons/cylinders, exhaust header/free flow system, carbs, porting,
oversized valves, distributor, etc etc)??? You may not get full
advantage of increased compression without other modifications. On the
other hand, a mild increase of compression can help the stock original
systems perform a bit better, and you can add other stuff later.
Most importantly, you can take a mild cut off a head and come back and
take more later. However, if you take off a lot, then don't like the
results, you can't put it back!
You must also consider what you have to start with. There were at least
3 different heads put on the TR3 & TR4. The earliest (which you would
probably have) was cast "thinner" and won't tolerate as much trimming,
or you might cut into one of the water galleries when reworking the
combustion chamber. On the "early" head, there is also some work needed
around the thermostat housing, I believe, and may be some trimming
needed to accomodate the manifold gaskets.
You are probably aware that trimming off some of the head will also
require reworking the combustion chambers.
With those disclaimers in place, I'll say that you can select from
"mild", "moderate" and "wild". .060 to .090 are relatively "mild" cuts.
You should end up with compression ration around 9.5 to 1 or slightly
higher. Any more than that on an "early" style head is getting into
dangerous territory. My car with a later style head has .120 trimmed off
and should be about 10 or 10.5 to 1 compression, which I would call
"moderate". It is possible to take off .150 with the later style heads,
pushing compression up to about 11 or 11.5 to 1, along with all the
benefits and problems that come with it (high octane fuel, not terribly
"street" friendly, etc... but producing the most power with reasonable
reliability). I'd call that "wild". Higher compression ratios have been
tried with only moderate increase in power, but a significant drop in
reliability.
Maybe you've already done so, but I would suggest crack-detecting a head
before getting any work done. Make sure it's a good usable unit before
spending anything on it! With any of the increased compressions, it
might be a good idea to use an upgraded steel head gasket (although this
is probably optional).
I strongly suggest you order the Competition Preparation Manual from one
of the vendors (less than $10, if I recall). Either the TR3 (might be
hard to find) or the TR4/4A would give you and your machinist a better
guide how to proceed. I have the TR4 manual and it devotes more space,
with lots of good illustrations, to reworking the head than any other
topic! It has lots of details, too many to include here.
I hope this helps!
Alan Myers
San Jose, Calif.
'62 TR4 CT17602L
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MZ
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