Squirrel Power: Making a Minibear Part 2
FA50 50cc vs 60cc Cylinder & Piston Comparison
We picked up a 60cc cylinder kit from Treatland, our favorite quirky moped store. The new kit includes all gaskets as well as a new piston (with rings preinstalled), and a large bore cylinder. The kit makes use of a brand new cylinder casting that is nearly identical to the original (though with a bigger bore obviously). You can see the difference in size between the two kits here.
FA50 Cylinder Porting
Before we installed the new cylinder we spent some time cleaning up the ports with a die grinder. This is an old trick we learned from building nitro R/C engines in high school. The as-cast runners are very rough and restrict airflow. We cleaned up the runners as best we could to improve airflow. The real key is to not modify the ports in the barrel itself. This affects cylinder timing and is not something we want to do without experience or dyno tuning. We also didn’t want to enlarge the ports too much to keep the air speed adequate. We performed this hand porting on the intake, exhaust, and boost ports.
FA50 Piston, Piston Pin, & Malossi Needle Bearing
Along with the 60cc piston we replaced the piston pin and the needle bearing. Since these engines do not have a pressurized oil feed, needle bearings are used on both ends of the connecting rods. We went with a Malossi needle bearing as it is a bit stronger than the Suzuki bearing. The piston pin is a factory replacement and comes with the 60cc kit.
FA50 Milled vs Stock Cylinder Heads
Above the cylinder, we are going to raise the compression by decking the head. The FA50 has a pathetically low compression ratio somewhere around 6.1:1. The easiest way of raising the compression is to mill the mating surface of the head flush with the cooling fins. This removes around 1.0mm of material for a small bump in compression ratio (around 0.2 points). We picked up a spare head on eBay and bribed the machinists at work with a box of donuts to deck it. You can see the difference between the milled head on the left and a stock head on the right.

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