When I first got my ATR headers 15 years ago, I was peeved to find out that they had mild steel flanges even though the rest was stainless. Then I did some reading and found that maybe that wasn't such a bad thing. The stainless steel expands 50% more than cast iron, so if the flanges were stainless the headers would have to slide on the head every heat cycle. Course, all this assumes that the flanges were not cut between ports. Some numbers:
Material thermal expansion, microinches/degF/in
carbon steel 6.3
cast iron 6.6
304 stainless 9.6
356 aluminum 11.9
The end exhaust ports are about 12" apart, so going from 70 F to 350 F (my guess at a "max" head temperature) they will move (350 F - 70 F) * 12 in * 6.6 uin/F/in=22176 microinches = 0.022" further apart on a cast iron head, and 0.040" on an aluminum head. I haven't measured the header flange temperature but my guess is that it will be a little hotter so I'm going to use 500 F, and the end ports will move 0.033" on mild steel flanges and 0.050" on stainless steel flanges. So, steel flanges on cast iron heads will expand a difference of 0.033"-0.022"=0.011", and steel on aluminum 0.007", a pretty good match for both. The stainless flange on cast iron will expand a difference of 0.028", and on aluminum 0.010". So, the stainless steel flange on cast iron head is the worst mismatch, and will try to slide back and forth 0.028" every heat cycle (that's a little more than 10% of the header bolt diameter, for reference). Stainless on aluminum and steel on either cast iron or aluminum are all about the same match.
My headers needed surfacing after about 50-60,000 miles, and got it at 75,000
, and needed it again after another 75,000 or so and will get it "real soon now" after about 100,000. Anyone else got 176,000 miles on a set of ATR tubular headers, including year round driving through Maryland salt and winters
?
