Paul,
This sounds like a case of rust out. Check the area carefully
for corrosion. Unfortunately, this is not totally avoidable with
chrome-plated headers. The difference in thermal expansion
coefficients between chrome and steel becomes significant enough to cause
separation between the plating and the parent metal, and this directly
invites water vapour to condense in that layer.
The reason it would fail in certain spots and not others is due
to the issue of radiant heat. The outward facing surfaces of the
header are cooler since they are allowed to "shine" a good percentage
of their heat away to their surroundings. In the case where two or
more tubes are next to eachother, the nearest surfaces will be hotter
since they will radiate heat between themselves, resulting in very
little net heat loss. A six-to-one header collector is probably the
best example of this. In repairing (welding up) cracked headers, this
is one of the most troublesome areas. You're right that this type of
failure doesn't have to do with excess weight. Wrapping the header
will eliminate most of the heat loss from all surfaces, but on a plated
header, it can certainly cause separation to occur earlier.
I suppose my advice to you is to try to find a header which is
either a three-to-one (twice) or go with something that's not plated,
then get it coated. TRF sells the six-to-two headers and they look
real nice.
Greg
Greg Meboe meboe@wsunix.wsu.edu
Dept. of Mechanical and Materials Engineering
Washington State University, Pullman, Wa.
'85 XJ-12 H.E. (daily) '67 Spit-6 '74 TR-6
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