|Jensen Healey wrote:
Apparently the o-rings reduce heat transfer from the manifold as well as dampening the vibration from the engine. Obviously they need to be air tight.The O-ring's primary function is vibration damping, and any thermal barrier benefit is incidental. Of course, it's important to use the right O-ring. But given that, the tighter it's screwed down (the more narrow the gap), the less effective the damping is. If it's too loose, there can be air leak issues, and too tight they're no longer effective as dampers. The goal should be looser rather than tighter, and the specified 0.070" gap was the loose-optimum without tempting air leaks.
Jensen Healey wrote:
This is not a critical tuning parameter, it's adjusting the slop in the system. There's more to it than you give it credit for.
.070 or .040 or anything close will work just fine. If you have older O-rings, make sure they seal and have an even gap.
I doubt the various "anti popping spacers" and o-rings are manufactured to any specific spec. If we knew the thickness and durometer rating of the original o-rings the gap could be set to .070 with confidence.
Use common sense and snug them up 'till they feel right.