The sleeve has different timing. The boost port is much larger on the 353. The exhaust port is a different shape slightly..Similar to the new batch of +12 engines. The transfers have also been reshaped slightly similar to the new batch of +12's. The crankshaft is basically the same as the +12, but the crankpin is smaller. You must use the new connecting rods with the new engines. 353, and 359. The piston is cast in the 353. The case of the 353 no longer has the wing shaped transfer port seperators. The backplate of the 353 has a steel insert to avoid scratches and extend engine life. The stock engine is smoother, faster, and cooler running than the past +12 engines from Novarossi. The modified version has similar characteristics, just more overall power. I will be pounding the mod 353 at 301 this weekend. Stay tuned to see how it runs over the long haul.