: Talked to our machine shop yesterday, and left
: off the block and piston/rods. Head guy took
: one look at the #6 piston and said
: "detonation!". All 6 pistons have
: their skirts distorted, and #4 piston is
: missing a small chunk above the top ring
: near the valve pocket. #6 broke not only the
: top ring but the middle ring too. All 6 rod
: bearings show fret marks from the stress.
: The shop called today and the block is OK
: with the #6 cylinder just 0.0005"+
: larger than the other 5. Have reordered the
: KB268 pistons, and expect to order the rings
: next. Will also try to get DI top rings and
: set ring gap at 0.030". Shop boss says
: the only way we will be able to get max
: performance out of this combination will be
: to use 100 octane gas. He expects that we
: were loosing up to 75 hp to poor combustion
: using the wrong gas.
:
: I can't believe that others have not had this
: problem with this combination and the high
: compression that comes with it. We are kind
: of locked into using the KB pistons now, and
: the only other solution may be to assemble a
: new head that has a stock chamber volume and
: hope that we can get the compression ratio
: down to a more managable level. Hate to go
: this route, as the 11:1 ratio (35cc chamber)
: makes great power, and the head is all
: ported and has big (for slant 6) valves. Are
: there any other pistons, short of a special
: order forged slug that would work? Even a
: special forged piston would need a huge dish
: to lower the compression ratio by 1 to 1.5
: points. KB's already have an 18 cc dish
: volume.
: Thanks
: Smith

: Smith,
: Have you considered adding a second head
: gasket? I did this on my daily driver '68
: Dart since it was a bit testy on premium (it
: has 30 cc chambers, but stock Slant bottom
: end, static comp is about 10.6:1). I used
: two FelPro composites (about 0.075"
: total). It has held up fine for about 25k
: miles.
:
: Another route might be a longer duration cam. I
: have 11:1 in my '64 Dart that I run mostly
: on premium, but have a 250 @ 0.050"
: cam. No detonation and no damage in 25k
: miles with that head on it.
:
: Sorry, I have no firsthand experience with
: those pistons, except that I saw Doug's
: motor a while back with the same broken
: piston skirt.