The original seals that came with the OS header were wide and flat, maybe "D" shaped. They're a bit pricey to replace so I used 2 of the Viton o-rings as replacements. Sometimes it helped to clean up the slot on the header with a needle file as the casting left some flash. The fit with the o-rings can be really tight, depending on the pipe, so you may have to lube the o-rings a little to get the pipe on.