Well I finally have enough hours on my bike with the Wiseco big bore/high compression kit to give an opinion of its worth. I think I paid $350, all included. It was definitely worth every penny. If this mod doesn’t cause any future reliability problems that is. I have already blown a head gasket but that can probably be attributed to not retorquing the head bolts. I have about 8 hours on the second gasket. So I recommend retorquing the head bolts after the first few hours on a new gasket. At any rate here is my impression of the kit. Keep in mind that I went oversize AND higher compression (13.5:1) at the same time. I have no idea which mattered more but I’d certainly recommend a higher compression piston to anyone replacing their stocker that rides at elevation. My bike has been ridden at 4,000 ft. and above on plain pump gas with not one sign of pinging. For some perspective I don’t ever race motocross but I will probably race 5 or 6 off road things in a year. Extra punch is always welcome out here in the desert, the trails I ride are deep sand between the cacti type of stuff. I have a PC T4, notable because Mr. Gorr mentioned that the extra displacement works well with large diameter headers. Currently I’m geared 14/47 but I think a 46 or 45 may be in the cards (read on). The best analogy I can offer is the 400 vs. 426 difference. Anybody who has ridden both these bikes can relate to the big bore kit by simply adding the extra OOMPH the 426 enjoys over the 400 to a stock 426. It is a one gear difference. On two occasions I slowed mid-trail and went back down through the gears to make sure I was in fifth. The motor was responding like I was in fourth. This on a favorite trail I ride often that never saw fifth gear before. You might as well just head for fourth or fifth as soon as you get under way and leave it there. Unbelievable torque down low with a scary mid range to accompany it. Shifting and any use of the clutch is simply not needed. Bring on that 520, I wanna drag race! There is a very steep, long and sandy hill with a short approach that I use to struggle up on my KX 250, third gear fanning the clutch. When I got a KX 500 that was the first thing I tried - fourth gear, no clutch. My 426 required a tad more work but also would pull fourth gear, no clutch with the stock gearing. I can now catch a full second of air time after blasting up the entire thing in fifth, and I’m down to about half throttle at the top (I’m crazy but not stupid). This with gearing that is two teeth taller than stock. That should give you an idea of the difference. Now for the downside: It seems to vibrate a bit more but this isn’t readily apparent. I only recently noticed this during a long ride. It is definitely easier to stall. It may be time for me to finally bite the bullet and install a flywheel weight. It doesn’t rev quite as readily. This also struck me when riding my (brand new) 426 after getting off a 400. Part of this may be attributed to the new top end but it is pretty well broken in by now. Mind you it will still find the rev limiter, but it bounces off it like a tennis ball instead of a crazy ball. It did not make the bike easier to start Comments? By the way I would recommend the services of Mr. Gorr based solely on his telephone manners. He almost always answers the phone and is happy to answer any and all questions. (920-356-0741) Here’s hisWebsite too.