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the orings that go around the cam thrust plates and seal against the cam holes in the chaincases mentioned in the above post are made of silicone and silicone does stay soft and resiliant for a very long time.
those orings usually don't leak unless they are installed incorrectly or pinched and cut while installing with no silicone oring lubricant, curil T, light coating of hylomar, or grease on them.
if the heads were flycut or spun in a lathe and milled down and the chaincase is not machined down the same amount as the heads where it mounts to the case then the cam thrust plate will be of slightly off center to where it bolts to the cam tower.
moving the thrust plate a little sideways to compensate and bolt to the cam tower with it's paper gasket will squish one side of the oring and the other side will be loose and the loose side may leak oil.
i think supertech makes different thickness shims you can put in between the heads and cam tower to space the cam tower back out to where it was before the heads were resurfaced or machined. That will keep the chain length the same and make it so the chain cases don't have be machined down the same amount as the heads where they attach to the case.
depending on how well those are installed with 574 or threebond they can leak oil too.
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