Preventive Maintenance , 426

1 reply to this topic
  • Scotty426

Posted May 27, 2006 - 03:58 AM


I hear you check the valves on a 4 stroke for wear and stretch. I bought the bike used so I don't have the first idea how much they have stretched from the start, just that, with a feeler, they were in the legal range last time I checked (cam change, -legal before and after). I plan to pull the head off and inspect the valves. I also plan to replace the timing chain and probably the valve springs on the theory that preventive maintenance will save me big heartache later. She starts easily and runs clean up and down the range. The questions are:
1. How do I look at a valve with unknown history (except my year of riding) and tell if it needs replacement? Am I looking for wear on the valve face (they're titanium with a coating). Is there a stretch limit I need to worry about?
2. Assuming I find a mess, has anyone switched to stainless valves and found they induce valve float at a lower RPM? I trail ride the girl and don't rev it high much, but occasionally I do.
3. What fails on the bottom end? Time for bearings? (3 years on the bottom end).
Thanks in advance.

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  • Birdy426

Posted May 27, 2006 - 08:17 AM


A good rule of thumb for evaluating valve wear or stretch is when you have to go down more than two steps in shim thickness, it's time to replace them. If you want to tear the head apart and inspect, it's more a visual thing than a measurement thing. You are looking for wear in the sealing face of the valves (where they contact the seats). I've seen photos of worn vs new valves, but I can't seem to find them right now. Try the search function here, or google. For what it's worth, the center intake on my '01 WR needed to be replaced after about 6000 miles. I put 3k of those on the bike, and the previous owner told me he had racked up about 3k as well (of course, that might have been 6K or 1K, so take it with a grain of salt). The progression on my bike was very quick one it started...checked the valves, and all was good. about 1000 miles later, it got hard to start. (this is at a total of about 5500 miles). Replaced the shim on the center intake with one step smaller, and all was well. By the time another 300 miles rolled over on the clock, it was hard to start again. Checked the valves, and the center intake again was tight. Went down two steps this time to get within spec. Put another 200 or so miles on it and the primary drive nut backed off, tearing up the crank. When I tore it apart for the rebuild, the center intake was again out of spec. If your valves aren't "moving", they are probably in ok shape. (That doesn't mean it isn't worth a look, though)

I looked at stainless when I rebuilt my bike, but opted to stick with Ti. In reviewing the info here, opinions are split about 50/50 for and against Kibblewhites vs factory. My thought was that the KW valves used significantly stiffer springs, increasing contact pressure and closing force, which would increase loads on the valve seats. As I was using stock (freshly ground) seats, I thought stock was best. I have about 600 miles on the rebuild with no issues.

As far as the bottom end, they are very near bullet proof. I have seen some reports of crank pins breaking in the bottom end at high mileages, and of the cam chain gear (which is machined into the left crank half) wearing beyond use, but that's about it. On my bike, the primary drive nut came loose, and the primary drive gear beat the heck out of the crank-side splines. Not catastrophic, but still required a new crank. If you're going to have the bike far enough apart to do the head, you might as well pull the cylinder and freshen up the rings (or new piston w/rings...13.5:1?) and also measure radial float and side clearance on the big end of the crank. Not much more work, and will give you some level of confidence in the bottom end. The main bearings are damn near indestructible. I've never heard of one failing. Again, I replaced mine as a precaution when I have the bottom end apart to replace the crank, but the old parts felt fine.

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