kas kastner wrote:
> Those heads are quite reliable with .100" taken off them Michael.
It turns out that it's more than that. See below. I'm hoping you can say the
same thing about the additional amount. (!)
> Also take
> a look at the pistons you might be able to just machine off the dome if the
> top of the piston is reasonable. (you didn't say how much of a pop-up) The
> 6 cylinder heads are a lot better about milling than the TR-4 stuff.
Yup, Kas, I thought about that, until I had a look at the pistons (these are
the standard domed pistons used in the early (I think) 9.5:1 GT6 engines)--the
dome height is 0.298" and the piston is only
about 0.180" thick at the center... wouldn't even have to wait for detonation
to set in to make holes in them--milling would do that just fine. *g*
Thanks to Scott Barr, I think I have figured out what's going on. He sent me
Mark B.'s list of six-cylinder head specs (which I'd gotten once before and
lost in a computer change).
When I first calculated the chamber volume, before having it milled, I came up
with somewhere around 41.5-42cc. With the domed pistons and 0.020" shaved off
it, minus the little nibbles here and there
to unshroud the valves and smooth the chambers, it should have come out to
close to 10.7:1, since dome volume is, by calculation, 10.37cc.
I looked at the head specs, and nothing made much sense at first. Didn't seem
as if the head height and chamber volumes matched anything, with the amount I
thought had been milled off of it by the
shop. But, thinking back, the engine had come out of a car with the odometer
stuck at 91K. There were a few maintenance receipts with it that showed the car
had run at least three years after 90K
miles. So, it likely had over 100K on it, and probably more than that. And yet,
the head and valves didn't look at all bad. From that and specs, it seems that
a late MkIII head had been installed at
some time, and was probably milled about 0.035" undersize then or before. That
would have taken it down from the stock 44cc to the 42cc or so I calculated
Then, the shop I used milled it 0.120" more, instead of the 0.020" I asked for.
That makes sense from the standpoint of current chamber depth and head height.
Take 0.155" off of the stock late MkIII
head and one gets 3.245" for height, and that's what the head mikes to now.
Take 0.155" off of the stock chamber depth for the late MkIII head, and one
gets 0.395" (mine varies a little from place to
place from 0.390-0.405"). It cannot have been the original GT6+ head milled
properly to 0.020" undersize, because the head height then would have been
Unfortunately, no one has wanted a trade on the domed pistons, so I'll probably
end up having to buy a new set of flat-tops, and put the others on eBay.
Thanks, all, for the commiseration and the suggestions.
Michael D. Porter
Never let anyone drive you crazy when you know it's within walking distance.
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