Ask The Editors: Undoing a Big Bore Kit

The big bore state of mind.

This is a very good question. Unfortunately, the answer isn’t quite so simple. Why? Because there are many ways of increasing an ATV’s cylinder bore. We would suspect that in this case, whoever performed the modification would have simply swapped the stock head (and cylinder) for an oversize unit. If this is actually the case, going back to the stock dimension would be as simple as purchasing a stock head and swapping it for the unit on the machine now. Note that the above photo is an actual Honda 300EX big bore kit – to give you a rough idea of everything you would need to swap back.

Conversely, having the stock cylinder bored to greater diameter is another means of achieving an oversize displacement. However, since this entails drilling away cylinder material, the only means of undoing such a modification would be to have the cylinder re-sleeved to stock dimensions or to, like above, replace the entire head. With how common 300EX engine parts are (both new and especially used on sites like eBay), this would be your easiest and most affordable option.

In answer to the last part of your question – this is where things can get tricky. Unless you know precisely which modifications the prior owner performed, swapping back to the stock head may or may not be the extent of what you need to do for the machine to run right. Namely you will want to study the carb on your machine. Oftentimes a larger bore carb (or radically changed jetting to the stocker) are also done to maximize the performance of the big bore kit. If this proves to be the case, you may end up having to dial in the jetting to coincide with with the stock cylinder’s output.

Other than that – any modifications to the bottom end or the final gearing (number of teeth on the sprockets) should transfer over just fine regardless of which head you run.

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