the 81.5 mm headgasket is the stock headgasket. the bore of the stock headgasket is much larger than 81.5 mm. You have plenty of clearance. I'm running the same bore and using a stock headgasket. There's nothing warping. This individual has posted 3 times on this site with deliberate incorrect guesses to mislead everyone. He's outa here. We don't need people who deliberately irresponsibly post wrong info to steer you in the wrong direction. We had so many people who were pulling things out of their asses at the other website where I used to mod at that the whole info provided was useless and helped no-one. This also dropped the credibility and reputation of the source of info (i.e. the members posting). We try to keep guessing down to minimum here, if we can.