"STOP!! Here's the Certif
"STOP!! Here's the Certified, Fastjeff Marine and RV manifold removal procedure:
1. DO NOT try to unscrew the bolts! If rusty, they will snap off in the head.
2. Take a mini-grinder (4 1/2 inch angle grinder--Harbor Freight 40 bucks) and grind the heads off the bolts. (Alternative: Drill the heads off by centerpunching and progressively drilling larger holes until the head falls off.)
3. With the heads off, slide the manifolds off. That might require using wedges.
4. Blast the studs with penetrating oil and let them sit overnight.
5. Clamp Vise-Grips to the studs and work them out. Sometimes that requires back and forth turning until they co-operate.
Do all of the above and you'll be fine, but try to unscrew the bolts, snap one off, and it's heads to the machine shop time.
Jeff"