In the old days of cast iron cylinders, when a piston was worn, a new one would be installed and the cylinder bored out. Typically, the increments were 0.010” or 0.25 mm larger than stock. The engine would feel fresh and new again, but any power gains would be minimal.
Fast forward to today’s aluminum cylinders and fancy cylinder wall plating. When a piston is worn, it is usually replaced with a standard size and no machining of the cylinder is necessary if its in good shape.
A Big Bore Kit, however, is an oversized piston and cylinder. Not just 0.010” or 0.25 mm larger, but significantly bigger. Like 20 times that or more. So, for example, if a 450 cc engine is punched out by 7 mm, that takes a 449 cc engine to a 518 cc monster! That’s a 13% increase. So, if you start with 60 hp, +13% equates to 68 hp. Now that is something that you will notice.
Big bore kits on snow bikes typically mean that you will be pulling a higher gear. This is great when you want to go faster, have more track speed, or simply be able to ride with greater control in the mid-range of the engine’s RPM.
Are their downsides? Possibly, but minor in my opinion. You will have more reciprocating mass so you may notice a bit more vibration – particularly on motocross bikes which have lighter flywheels.
Cost / Horsepower
Big Bore Kit
- $1,299 for the big bore kit (after cylinder return). Add $250 for an installation so let’s call it $1,550.
- 1,550 / 8 hp = $194 / hp
- Nab an FMF Factory exhaust system for $629 and you can expect about 1.5 hp.
- $629 / 1.5 hp = $420 / hp
The big bore kit is the clear winner. Sure, the pipe will look cool, shave some weight, and sound awesome, but if you want more power, a big bore kit is the way to go. If you are flush with cash, you can always do both!
~ Ian McKill, Riders Edge Suspension, Vernon, B.C.