I suspect you already know that you'll probably do better with flat top
pistons to begin with, eh? (Reading too many posts about Canada lately)
Too bad about the lousy work. It's very hard to find good machine shops,
and when you do, the guys running the place tend to be two years from
retirement (or worse). I don't know much about GT6 heads (okay, make that
nothing) but I'd be worried about blowing head gaskets. When you take a
TR3/4 that far you get really skinny near the water passages and the head
can dish. Most folks either pin them or weld up the chambers and remachine
them rather than milling that far. It's probably too late to pin--you'd
have to skim off even more after you screw in the pins.
Just another ray of sunshine.
-----Original Message-----
From: Michael D. Porter [mailto:mporter@zianet.com]
Sent: Wednesday, July 02, 2003 4:20 AM
To: triumphs@autox.team.net; spitfires@autox.team.net; fot@autox.team.net
Subject: Minor tale of woe....
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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