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Message: [QUOTE="EricGorr, post: 382693, member: 19611"] CR450 Top End Maintenance Update The Honda CR450 that I’ve been maintaining for District 16 expert Tyler Smulders, has required a bit more top end care because of usage and typical abuse that can’t be avoided in racing. If you read this forum you may remember an article that I wrote in the spring regarding servicing the piston and rings. This is what’s happened since then. Tyler accidentally over-revved the engine by downshifting one too many gears in an effort to let the engine absorb the G-force of a hard landing from over jumping a plateau jump. The stock rev limiter only works on acceleration, there is no protection when downshifting. One of the intake valves floated beyond the limits of the valve spring and clipped the piston crown and shattered the intake guide. The fragments bounced around in the engine and damaged the piston and head. I was surprised that the cylinder didn’t get scared. I replaced the head, piston, and one intake valve, spring, collets keepers, and gaskets. The parts were very cheap. Service Honda sold me the head for about $215; compare that to an YZF, which is a staggering $750 for a new, bare head. One minor problem that I had when assembling the new head, the shims that I needed were on back-order. In fact I ordered a complete set of shims for the CRF and I finally received all the shims at the end of June. Instead of waiting for the Honda shims I went to a local multi-franchise motorcycle dealer and matched up a shim with the same diameter from another common street bike. I’ve shimmed the valves three times over the course of about 200 riding hours. The valves were tight each time. That’s a bit often compared to an YZF, but considering the difference in cost of the parts, the Honda head is designed to be disposable like a piston. For the low price of a head it doesn’t even make sense to port it or refinish the seats and replace the guides like the old XRs. In my opinion, that’s a good idea. Most of the YZF heads that I see for service in my shop have never had the valve clearance shimmed and they’re real clapped-out. The Japanese manufacturers have been struggling with the American dirt biking public over maintenance of high performance 4-stroke singles. Hey guys, that pitch that the magazines give you about 4-strokes being no-maintenance is a myth. These engines aren’t the 1975 Country Squire station wagon, they’re 1/10 of a Formula 1 racecar engine! They need the oil changed frequently and the valves adjusted periodically. Consider this, the average cost of an engine rebuild in my shop for an YZF is $1,000 on a 2-3 year old bike. For the west coast hop-up shop currency exchange rate, multiply that times 1.9 You guys can buy a lot of oil and filters for a grand. Big Bore Alternatives Its been about six months since the cover of MXA featured a test on a Wiseco kitted CRF with a 500cc displacement. Here is the status of that project. Wiseco in Ohio offers an exchange program whereby you send them a clean core cylinder and they send you a modified cylinder. This is how the cylinder is modified. The cylinder is bored and fitted with a forged aluminum liner then TIG welded at the top. The stock cylinder wall isn’t thick enough to support the 101mm piston and that’s the main reason why a liner is required. The cylinder bore is plated with nickel composite by Millennium Technologies in Plymouth Wisconsin. A Wiseco piston kit and Cometic gasket kit is supplied in the kit. The part number for the kit is CK137 and retails for $995 without a core exchange. Wiseco is in the process of casting a new cylinder with larger water jackets in an effort to reduce the price down to the $750 range. I have one of the lined cylinders on order and will report to this forum my experiences with installing and riding the bike with this kit in the near future. [/QUOTE]

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