Rmz 250 Seat Height

08-12-2011, 10:56 AM

There are a few inherent design flaws in the late model 4-strokes that usually require a few well planned steps to substantially increase reliability. The biggest problem with most is that the factories have given us valves made of the most exotic, light weight material and then left the job of controlling their action up to a $6 valve spring…and this tends to cause problems.
The materials used in an aftermarket valve/spring ‘kit’ are probably secondary to how far the company went with the design and implementation of said ‘kit’. Like the old saying there is more than one way to skin a cat.. Most of the reputable aftermarket suppliers have a solution that works if properly installed. There are two that stand out IMHO.
What most people don’t know is that there’s a big difference between swapping parts and actually setting up a head the right way so that it will work properly and reliably. There is a lot less leniency here than doing a top-end on a 2-stroke; good machine work and attention to detail are paramount in the success of your rebuild. Finding someone that will do a good job is going to be just as important as picking out the parts.
A ‘proper’ valve job on a high-end 4-stroke should start with checking the valve/guide relationship. Any slop in the valve/guide relationship is going to translate into a lack of concentricity between the valve face and the seat causing accelerated wear of the valve and seat, oil consumption, poor performance, etc. Not only that but if you re-cut the seats with a worn guide you’ll trash the seats due to the same lack of concentricity. Basically if your valve/guide tolerance (intake - 0.0004' to 0.0015', exhaust - 0.0006' to 0.0021') is sloppy you need to replace the guides before you do anything else. FWIW, this is probably not something everyone would feel comfortable doing..
Once new guides are installed and the valve/guide relationship is acceptable you can re-cut the seats. This will definitely need to be performed after installing new guides since the new guides will most likely have a different point of concentricity than the old guides. A good 3 angle valve job will be in order using the fresh guides as the pilot. A 3 angle valve job will require three cuts, a 60ish* throat cut, a 45* valve sealing cut and a 30-32ish* final cut, all made using specialized seat cutting tools that fit precisely into the valve guide. By carefully using the throat and final cut you can set the width of the 45* valve sealing cut along with its location on the valve. Valve sealing width can vary, but generally you want to try and be 0.9 to 1.5mm wide on the intake side and 1.1 to 2mm wide on the exhaust side. A smaller width 45* cut will flow better and have more performance potential at the expense of valve/seat life. Also, in regards to the exhaust side, a valve/seat relationship place high up on the valve's sealing surface (closer to the stem) can have a detrimental effect on valve longevity. Once the seats are cut you can check them using some machinist's blue (search: Dykem) to check the valve/seat relationship. If you’re using uncoated steel valves it is possible for you to lap them if you wish.
Now that your seats are cut and your valve/seat relationship is perfect you now can worry about the installed height of the valve spring. Since your valve seats won’t be cut to exactly the same depth your valves won’t seat at exactly the same height. This translates into variances in valve spring installed height, and it's the installed height which dictates seat and over-the-nose spring pressure acting on the valve. A few thousands either way on your valve spring installed height can really reduce the life of your valve train components. It's the inconsistencies in valve spring pressure, either too much or too little, that can destroy all your hard work. By measuring installed height of the valve you can compensate for these variances by installing valve spring shims of various heights to consistently set your valve springs to the correct installed height.
FWIW, if you’re going through the whole motor you should definitely spring for a timing chain and tensioner while you’re in there. I would also look at replacing the crank seals and cleaning the pick-up screen on the oil pump while it's apart. When you’re done shim to the tight side of the manual spec and check your clearance often for the first 15 hrs.