Ever since buying a set of the mini pliers (previously named midget pliers from snap-op), stuck bolts are a non issue. This lil bugger allows a solid grip to muscle the needed torque w/o slippage. they say it tightens as you pull, but i just use the biceps-o-squeeze power.
Knipex Tools Mini Pliers is the brand I own. I actually used it on this exact scenario. A new cap is required and a few yells-aloud at the darn PO who decided to torque the ring down to 10,000 lb-ft.
jeez darn cliche PO's!!
If it comes down to it, you can use a chisel (or sharp screwdriver! or nail driver!) to chip away at the very outside of the cover. Similar to cutting a notch with a chisel (if they are really stuck this hasn't always worked for me) but making a few notches, if you do it on the outside of the o-ring you won't risk getting chips in your engine.
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