bolt spacing differences mean that heads/jugs from one won't swap to the other. All the hi-po parts out there are for the 4-bolt heads. 2-bolts are orphans as far as upgrades go. no upgrade heads, cams, springs etc. However, the SYM brand 2-bolt engines made to carter's specifications are said to be one of the best designed/built gy6 150s. It's a shame, as I think not only is it in the gearing, it's also in the head/valve/chamber design (this particular SYM has a 4-valve head).
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