I've never done an overbore on a 350, but I've done 4mm oversize pistons on two 450s with the stock sleeves over the years and they are reliable, so 3mm over on the 350 seems like a safe bet. Even if the pistons aren't designed to be high-compression ratio, your compression ratio will still go up a bit because of a larger piston compressing the air/fuel mix into the stock-sized combustion chamber. Since it's an air-cooled engine, you'll have to be more aware of idling for any significant period because it will overheat more easily
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