You can, sort of. But the stock 5 speed box design is also a split case system. Meaning they can twist with high pressure and cause problems. The 6 speed is a much better design that is solid. The gear unit slips out vertically instead of the 5 speed spliting lengthways.

The actual 5 speed design just isnt as good compared to the 6 speed.

Google some pics of the two boxes to see what i mean.

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