In reply to a message from MarekH sent Tue 21 Jul 2015:
At the risk of putting my 2 cents in where it’s not needed
or wanted, Just to recap, the discussion is on the lines
of raising the compression ratio on a pre-HE V12.
I hang out at the local Jaguar shop quite a bit, and what
I’ve learned from watching is. Milling the head(s) is 1 a
point of no return, and 2 some valve / piston work will be
required. I don’t know this first hand, but from what I’ve
read, and witnessed, Jag engines (all of them) are
interference engines. (just in case you don’t know what
that means - the tolerences between valves (timing and
clearances) and pistons are, well lets just say tight.
I witnessed a XJS come in after a ‘‘valve job’’ on a flat bed
because the owner wound the engine up (under redline) and
the valves kissed the pistons. The owner reported the car
went like stink, until the unfortunate event that occurred
which ultimately landed him and the Local Jag Service Cntr.
That being said,
I would think long and hard before I’d mill a head. If it’s
too late, I would definitely revisit the valve and piston
tolerances and most importantly factor in enough tolerance
for valve float.
In my previous life, I built high performance small block
chevy engines for some local racer wanna be’s. I’ve
addressed the tolerance issues with flat top pistons a
couple of ways.
- change the cam to a cam with less lift, or
- machine space in the top of the piston for the valve.
See the manufacturers specs as to how much can be hog’d out
of the piston top without jeopardizing the structural
integrity of the piston.
- Two head gaskets. Not recommended; almost guaranteed
head gasket leak.
If it were me, just to save money (in the long run) and
countless hours of frustration, start saving for a
head/valve and piston assembly that already been tested.
As we all know, these engines aren’t cheap, and (in the US)
hard to come by. So breaking an internal engine part, can
be catastrophically expensive.
The milled head guy mentioned above, his total bill was
about $8,000.00 to fix and bring back to stock.
For what ever it’s worth
2002 XJR-100, 1988 XJS Coupe, 1985 XJ6, 1974 XJ6
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