One of the best ways to increase horsepower is to increase displacement by overboring the cylinder. This can be ideal for play or Vet Class riders, where the increased displacement won't be illegal for your race class. When done right, a big bore kit can give you more power everywhere rather than an increase in only the top or the bottom of the powerband. Such increases are typically more usable and give you more power where you need it.

Piston manufacturers such as Wiseco make oversize piston kits for popular model bikes. These kits boost the displacement of the cylinder to the limit of a racing class or to a larger displacement class, for example: 80cc to 100cc, 125cc to 145cc, 250cc to 265cc or 300cc, and 495cc to 550cc.

The AMA has a limit of overboring any cylinder used in amateur modified classes. The limit is 2 millimeters. Wiseco makes a line of Pro-Lite pistons for this purpose. Normally no head modifications are needed, but cylinders with exhaust valves that operate close to the cylinder bore will need to be trimmed for clearance. Cylinders that use steel head gaskets will require oversize gaskets. Cometic makes 2 millimeter oversize and big bore gasket kits. The process of overboring and electro-plating a cylinder can be a cost effective way to save a cylinder that suffered a top end failure and scored the cylinder wall.

Riders competing in the AMA veteran class can ride a bike with any displacement. Riders competing in hare scrambles and enduro can race the 200cc class with a 125 converted to any displacement. AMA motocross and enduro racers can make the 250cc bikes legal for open class by increasing the displacement a minimum of 15 percent (to 286cc). Wiseco makes 74-millimeter piston kits to convert the popular 250s to 300cc. Be careful if you decide to go with a big bore kit, though. If the overbore is not performed properly, though, it can result in the wrong kind of power or, at worst, a ruined cylinder. When you change the displacement of the cylinder, there are so many factors to consider, such as port time-area, compression ratio, exhaust valves, carb jetting, silencer, and ignition timing. Here is an explanation of what you need to do when planning to overbore a cylinder.

Also, you should at least consult with an expert before tackling a big bore kit. To get the most from an overbored engine, you need to make sure the carburetion, exhaust, porting, and timing are all adjusted to suit the larger bore.

Port-Time Area

The term port-time area refers to the size and flow range of the intake and exhaust ports, relative to rpm. The ports enter the cylinder bore at angles. When the cylinder is over-bored the transfer ports become lower and wider. The same thing happens to the exhaust port. This effectively retards the port timing and reduces the total degrees of duration. When the displacement of the engine increases, so does the demand for more port-time-area.

If you just overbored and plated a cylinder, it would have much more low-end power than stock but the top-end power would suffer. Normally tuners have to adjust the ports to suit the demands of the larger engine displacement. The proper dimensions for the ports can be calculated using a computer program from Two-Stroke Racing (TSR) www.tsrsoftware.comThe program "PORTTIME" enables tuners with limited math skills to run strings of formulas for determining the optimum dimensions of the ports. Generally speaking, if the ports in the overbored cylinder were raised to the same heights as the stock cylinder, that would make the port timing sufficient to run with stock or aftermarket exhaust systems.

Cylinder Head

After overboring the cylinder, the head's dimensions must be changed to suit the larger piston. First, the head's bore must be enlarged to the finished bore size. Then, the squish-band deck height must be set to the proper installed squish clearance. The larger bore size will increase the squish turbulence, so the head's squish band may have to be narrowed. The volume of the head must be increased to suit the change in cylinder displacement. Otherwise, the engine will run flat at high rpm or ping in the midrange from detonation.

Exhaust Valves

When the bore size is increased, the exhaust valve-to-piston clearance must be checked and adjusted. This pertains to the types of exhaust valves that operate within close proximity of the piston. If the exhaust valves aren't modified, the piston could strike the valves and cause serious engine damage. The normal clearance between the exhaust valves and the piston should be at least .030 inches or .75 millimeters

Carburetor

The larger the ratio between the piston's diameter and the carb's size, the higher the intake velocity. Overbored cylinders produce higher intake velocity which draws more fuel through the carb. Of course a larger engine will need more fuel. Normally when you overbore an engine 15-20%, the slow jet will need to be richened and the main jet will need to be leaned. Start with the stock jetting and make adjustments after you ride the bike.

Ignition Timing

The ignition timing has a minimal affect on the poweband. Retarding the timing has the affect of reducing the hit of the powerband in the midrange and extending the top end over rev. "Overrev" is a slang term that describes the useable length of the powerband at high rpm.

The scientific reason for the shift of the powerband to extremely high rpm, is because the temperature in the pipe increases with the retarded timing, and that enables the pipe's tuned length to be more synchronous with the piston speed and port timing of the cylinder.

Advancing the timing has the affect of increasing the midrange hit of the powerband, but makes the power flatten out at high rpm. The reason is that the relatively long spark lead time enables for a greater pressure rise in the cylinder before the piston reaches TDC. This produces more torque in the midrange but the high pressure contributes to pumping losses at extremly high rpm.

Pipe and Silencer

Because only the bore size is changed, you won't need a longer pipe—only one with a larger center section. FMF's line of Fatty pipes work great on engines that have been overbored.

Head Gasket

The head gasket will need to have the bore diameter increased to the dimension of the new piston. If the head gasket overlaps into the cylinder bore more than one millimeters on each side, it could contact the piston or be susceptible to pressure blowouts.

10 TIPS FOR REBUILDING A TWO-STROKE TOP END

1) Before you disassemble your engine, power-wash the engine and the rest of the vehicle. That will reduce the risk of dirt and debris falling into the engine. Once you remove the cylinder, stuff a clean rag down into the crankcases.

2) The cylinder and head use alignment pins to hold them straight in position from the crankcases on up. The pins make it difficult to remove the cylinder from the cases and the head from the cylinder. Sometimes the steel alignment pins corrode into the aluminum engine components. Try spraying penetrating-oil down the mounting studs before attempting to remove the cylinder and head. Never use a flat-blade screwdriver, chisel, or metal hammer to remove the cylinder. Instead use this technique; buy a lead-shot plastic mallet, swing it at a 45-degree angle upwards against the sides of the cylinder. Alternate from left to right, hitting the sides of the cylinder to separate it from the cases evenly. Clean the steel alignment pins with steel wool and penetrating-oil. Examine the pins closely. If they are deformed in shape, they won't allow the engine parts to bolt together tightly. This can cause a dangerous air leak or a coolant leak. The pins are cheap at about $2 each. Replace them if they're rusty or deformed.

3) Never re-use old gaskets. Remove them with a razor blade or gasket scraper. Don't use a drill-driven steel wool type pad to remove old gaskets because they can remove aluminum from the cylinder and head. That will cause a gasket to leak.

4) Always check the ring end gap on a new ring by placing it in the cylinder between the head gasket surface and the exhaust port. The gap should be between .012 to .024 inches.

5) Always install the circlips with the opening facing straight up or down, that way inertia will hold it tight into the clip groove. Place one clip in the groove before installing the piston on the connecting rod. Its easier to install a clip with the piston in your hand rather than on the rod. There also less chance that you'll drop the circlip in the crankcases.

5) Always install the rings on the piston with the markings facing up. Coat the rings with pre-mix oil so they can slide in the groove when trying to install the piston in the cylinder.

6) Always install the piston on the connecting rod with the arrow on the piston crown facing towards the exhaust port.

7) The traditional way to assemble the top end is to install the piston assembly on the connecting rod, compress the rings, and slide the cylinder over the piston. That can be difficult with larger bore cylinders, or if you're working by yourself. Try this method instead. Install one circlip in the piston, install the piston into the cylinder with the pin hole exposed, install the piston pin through one side of the piston, position the cylinder over the connecting rod and push the piston pin through until it bottoms against the circlip, install the other circlip. It only takes two hands to install the top end using this manor and there is less chance that you'll damage the rings by twisting the cylinder upon installation.

8) On cylinders with reed valves and large oval intake ports, take care when installing the piston assembly in the cylinder because the rings are likely to squeeze out of the ring grooves. Use a flat-blade screwdriver to gently push the rings back in the grooves so the piston assembly can pass by the intake port.

9) For steel head gaskets, place the round side of the "bump" facing up. Don't use liquid gasket sealer; use aerosol spray adhesive types instead. For hybrid fiber/steel ring head gaskets, place the wide side of the steel rings facing down.

10) When you initially start the engine after a rebuild, manipulate the choke to keep the engine rpm relatively low. Once the engine is warm enough to take it off choke, drive the vehicle around on flat hard ground. Keep it under 2/3 throttle for the first 30 minutes. Two common myths for proper engine break-in are; 1) Set the engine at a fast idle, stationary on a stand. 2) Add extra pre-mix oil to the fuel. When the engine is on a stand it doesn't have any air passing through the radiator and it is in danger of running too hot. When you add extra oil to the fuel you are effectively leaning the carb jetting. This can make the engine run hotter and seize.