Okay, I got bit by the local machine shop (the owner is a low-rent circle-track
racer--I should have known better--but there are only two shops in town and I
didn't know anything about the other one
at the time).
I'm just now getting back to putting together the engine for the street `70
GT6+, after too many years of 80-90 hours a week on the day job. I had the
machine work done years ago, just before I
started putting in crazy hours at work.
I'd asked for a 3-angle valve job, hardened exhaust valve seats installed and a
total of 0.020" milled from the head (0.003 to clean up warpage and the rest to
get compression up to about 10.7:1 with
domed pistons, the limit I thought I could manage on premium pump gas and
well-polished chambers, valves and piston tops). I simply picked up the head,
paid the bill, took it home, and put it on a
shelf in the garage without paying much attention--by that time, I didn't have
any time to examine it or finish it up.
Some of the screw-ups I have managed to fix, or will. The hardened exhaust
seats (probably taken from a diesel catalog) which made the exhaust ports 0.75"
in diameter, I've fixed with a couple of
hours boring on the milling machine. The single-angle, badly-cut, valve seats I
can fix once I get the valve cutter set I need.
But, the real horror I just discovered this past weekend. I'd checked head
warpage early on, and it was about 0.003" diagonally. A cursory examination of
the head height before I had it milled told me
it was very nearly stock. I'd bought a set of early domed pistons at +0.030",
thinking that even if those didn't provide the best power, it was a better way
to get compression up to 10.7:1 without
over-milling the head, and worrying about collapsing the relieved area around
the intake ports.
So, this past weekend, finally with more time available, I cc'ed a sample
chamber. 32cc. Did the calculations without including the dome on the piston
and came up with 10.62:1. Huh? So, tonight I ran
the calculations with the volume of the dome on the piston included. 14.4:1.
Did a couple more calculations and it seems likely that the yo-yos at the
machine shop took off about 0.100"-0.120" from
the head, instead of 0.020". And, after a closer look at the rocker side flange
of the head, it looks pretty thin.
So, I have two questions. First, what's the list's experience with regard to
the longevity on the street of a GT6 head milled to that degree, and second,
does anyone have a new set of +0.030" GT6
flat-top pistons they'd like to swap for a new set of +0.030" domed-head
pistons? The pistons are pretty much what was available at the time from a few
vendors--AE, with the usual aftermarket ring set
(not gapped yet), bought new from BPNW quite a few years ago. I paid $260 for
the set on closeout. Not concerned about an advantage on the relative swap
value; I just don't want to have to convert it
to diesel to make it run with those pistons. (!)
Yeah, I know... drat.
Cheers.
--
Michael D. Porter
Roswell, NM
[mailto:mporter@zianet.com]
Never let anyone drive you crazy when you know it's within walking distance.
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