3mm and 4mm Stroker Crankshafts for the GY6 150cc engines made in Taiwan by Shengee. 

3mm is the easiest to install. Will require a spacer. (Available in our ebay store) 

3mm uses the stock timing chain and studs. 

4mm is more difficult to install and is more for people that are experienced. 

4mm requires a spacer, uses stock chain and may require you to shave down the piston. 

AGAIN... 4mm IS A LOT HARDER TO INSTALL. If you are not experienced... choose 3mm