I'm glad I asked, I hadn't thought about the difference in thermal expansion. I'm pretty sure these manifolds are plain grey iron, so figure 5.7 ppm/F. 304&308 are 9.3, 309 is 8.2*
410, on the other hand, is 6.1. too bad no one makes V-band flanges in 410.

mild I'm seeing listed at 7.3, and I'm seeing 5.5 for 410 on another reference.

http://cp.literature.agilent.com/lit...5517-90143.pdf
http://www.engineeringtoolbox.com/li...ents-d_95.html

I guess I'll use mild flanges. my other option is to fab a new downpipe and move the v-band under the bellypan.


*this explains why 309 does well welding 304/308 to mild.