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These washers are fairly common in automotive use, or at least used to be in certain cars. Ask the shop where you got the cooler and seals. I'll bet that they will have them in assorted sizes. You need 10mm ones.I wouldn't be surprised if nylon washers would also work.
I was wondering if one of those fiber washers from the carb rebuild kit kit might work, at least temporarily.
Daniel ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ List info at http://www.vwtype3.org/list | mailto:gregm@vwtype3.org </x-flowed>