ATV Tech: The Ins and Outs of Big Boring Your Engine

May 25th, 2012 by Jason Giacchino



A common misconception in the world of engine performance is that going bigger bore on the cylinder is a shortcut to limitless power gains. The editors of ATV Connection have done their share of boring read… erm, cylinders, throughout the years.  Let’s clear up some of the common myths and facts to the weird and wonderful world of engine bores.

All cylinders regardless of number of cycles, type of cooling, or size are measured in terms of bore (diameter of the cylinder/ piston that sits within it) and stroke (length of cylinder/ distance the piston travels up and down).

Both bore and stroke can be altered to influence the power characteristics of the engine. Big bore kits are very common among ATV enthusiasts on account of their ability to make more power fairly inexpensively and perhaps more importantly, can be used to replaced a scorn or damaged stock cylinder.

When you increase cylinder’s bore you are simultaneously increasing the piston’s surface area. What this means is that since you are spreading out the area responsible for dealing with the process of fuel & air igniting, performance gains can be experienced without any alterations to the compression ratio. This is important because once you start increasing compression; factors such as decreased reliability and having to run race gas enter the equation. As a general rule, making power increases without altering cylinder compression can make life a lot easier in the long run.

While even small changes in bore can make noticeable differences to your machine’s powerband, sky is not the limit when it comes to going bigger. The fact of the matter is the relationship between bore and stroke is taken into careful consideration by the engineers in designing the motor. It is entirely possible to increase the bore past optimal in terms of the stroke thereby losing gains in usable power.

Additionally going too far with bore can weaken the walls of the cylinder. There is unimaginable force being exerted on the cylinder wall with each and every stroke of the piston and compromised strength can lead to catastrophic failures.

Many big bore kits forgo the entire process of boring out & honing the stock cylinder to increase its diameter and simply include a brand new oversize cast cylinder & piston that bolts up to the stock engine’s lower end.

Jetting/ fuel map changes are not always necessary to maximize power changes associated with going bigger bore. In fact generally speaking, the smaller the engine, the more sensitive it will be to changes in fuel delivery to compensate (ie: the more likely you will have to make jetting changes).

Additional Disadvantages to over-boring include:

· Too large a bore can result in incomplete combustion/ excessive un-burnt fuel
· A larger piston in motion adds additional stress to the rod and crank
· Possibility of increased engine vibration due to the heavier piston
· Inability to run typically much more common OEM piston

However, when done correctly and to the optimum spec, a larger bore is a fairly simple means of increasing an engine’s power output and a lifesaver for those individuals who have damaged or scratched their ATV’s stock cylinder.