There is really no upgrade for the stock 2mm apex seal springs.  Those people running aftermarket 3mm apex seals that only originally use one apex seal spring per apex seal can make a their own double apex seal spring system.  You can shave down another set of long apex seal springs so you can run a double apex seal spring system as with the stock, three-piece, 2mm apex seals.  Greater spring force translates to better apex sealing, especially for high boost/high power engines.

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