Two people I would believe on this... TNC and Bill Blue. I'm thinking they're about as knowledgeable as you will find. I'm thinking Bill's kit is a decent buy with a proven track record. TNC has just screwed around with the 300 and 250 with more than adequate knowledge to be a very good source from what I've learned from talking with him and reading his posts. Far more than I for sure.

 

My friends tell me bore kits are bad for your engine in the long run.. shortens the life of your engine, is this true?

 

Yep, your friend is right. Also riding is bad for your bike as it shortens the life. You should send the bike to me so i can put it away where its safe and won't wear out.

 

I dunno, lemme put another 30,000 on my big bore 650 and I'll get back with you.


Your friend is wrong. Simply larger displacement doesn't cause any issues as long as you aren't making the cylinder walls paper thin. Maybe if one goes high compression or bigger cams and stuff, that might change reliability, but that's true with stock displacement as well.

 

You hit on the point that probably causes a bigger bore effort to get the occasional bad reviews. Unless the bigger bore approach was done for no other reason than to replace or repair a worn engine component, people are usually doing other performance mods that "can" but not necessarily guarantee reduced engine life. Adding a lot of compression, more aggressive cams, heavier valve springs, bigger carbs, etc. "might" indeed reduce engine life by some factor, especially when you consider that most folks are doing this to wring out the bike's power on a more frequent basis.

The KLX is an older design that is somewhat overbuilt in most of its component design. The reduced weight and higher horsepower engines of today made compromises to achieve that power and overall power-to-weight goal. Intake and exhaust mods do little if anything to reliability and durability when done properly. So adding the bigger bore kit only adds a point to a relatively low compression ratio in most cases...11:1 to 12:1. In most cases and especially in the case of the KLX, a properly built big bore for a KLX will do almost nothing to the KLX's longevity. The wear factor will probably come down more to how much and how hard you whack on the throttle.

 

Yep, it's pretty well accepted that the KLX250/300 family of engines is quite stout and capable of handling any additional power that we can reasonably produce via modifications. The stock clutch is the first thing to go with more power, but that's easily solved with stiffer springs. Cooling capacity is probably the next concern.

There is a very real argument to be made that a big bore kit in the KLX could make the bike more reliable: that argument is the potential to run the motor at lower RPM to do the same work. But like TNC and KLX678 point out, if the big bore is run to it's power/RPM limit, it will be more stressed than stock.

There are other engine families that can and do reach the point of diminishing durability with additional power (torque really), for example, the XR400 family of engines was very stout, but with power modifications much beyond the 440 big bore engine size, it would start to have transmission issues.

The thing that concerns me with extreme end of big bores for the KLX is exactly the point made above: thin cylinder walls. Let me lead off by saying both the 351 sleeved and 331 plated combinations have been proven to be reasonably reliable, and have been great for most people. However, though infrequent, there have been cracked cylinders walls in both the 351 steel sleeved and the 331 plated aluminum cylinders. There are other contributing factors to the failures of course, but the relatively thin cylinder walls do play a role. Under the right conditions these big bores will run without issue for as long as a stocker, but under the wrong conditions they'll be less forgiving before failure.

But that's a classic hot rodding dilemma: how far are you willing to take the benefit/risk ratio in search of more power? One could feasibly build a KLX385 with a max level bore/stroke combination in these motors. I'd love to do that (if $$ wasn't an option). But such a combination is also well beyond what I'd consider the threshhold of a 'reasonable' modification for this motorcycle.

Edit: is this thread still about using local machine shops for big bore work? My answer didn't contain anything about that. So just to stay on topic, I'll say it now. Other than the largest sizes that use custom order sleeves/pistons/gaskets, there's nothing about a typical big bore that is particularly specialized. I would be willing to use a local shop if I were confident they know what they're doing. There are other 'mail order' shops that are plenty capable of this type of work too; I won't plug any, but it's certainly not just a two horse race. Now would having another shop duplicate Bill Blue's development work on the 351 be cost effective? Not likely. Do you want to pay for and wait for someone to retread ground that has been covered already?