Hi, Arnold-
Just got in from "trip" to read this message on suspected detonation
"mechanical" damage.
I doubt detonation is the problem. It
sounds like "mechanical" interference.
The compression figures are lower than
acceptable; way down, oil or not.
Let's start at the beginning.
1. You must ascertain the height of the
standard un-milled head to see "where
you are", here, combustion chamber
capacity notwithstanding.
2. Remember, you have "modified" the
combustion chamber but not necessarily
improved the flow. You didn't modify to
any "known" spec. Whether flow has been
improved or reduced is anyone's guess.
"Flowing" cylinder heads is an art based on a SCIENCE; in your case,
there was no science! (No criticism intended here
at all, my friend; just objective fact.) You
simply "dug" them out to lower static
compression ratio.
So, the head is virtually, an "unknown" in
these respects. However, we know slower
flow equals increased heat to the head.
Now, we find we may have other unknowns which complicate the issue.
I had always thought you were using stock
pistons. Stock pistons "over here" for the
1275's were 6cc dished which, when combined with the stock head (21cc),
head gasket and crank yielded a static
compression ratio of 8.8-1. There was
(as I understand it) a European version
which used all the above "stock" stuff as
mentioned, except for the pistons which
were "flattops", yielding a "stock" static
CR of 11.7-1.
Assuming all else is stock for the moment
(except for your "modified" head, of course!) which pistons have you
got;
"stock" 6cc "dished" or "flattops"?
A .020 overbore (1293cc's) on an otherwise "stock" original 8.8-1 1275
should raise your static CR only very
slightly; I'll have a better "ballpark"
guestimate on this when I talk to my
"pro" mechanic tomorrow AM and get
back to you on this. Too tired to work
out the formula myself right now (time
to go have a beer or two with "the guys";
after flying to 6 days straight I'm bushed!)
There is also no way on a "stock" 1275
you should be "hammering" spark plugs
unless the head has been drastically cut
or you have a "different" stroke, especially
with "flattops". What do the top of the
pistons look like? (Yes, look! The head
has got to come off at this point, anyway!)
Any lister out there have a known to be
stock, uncut, 1275 head they can measure
height on and help us out here, please?!
Cap'n. Bob
'61 :{)
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