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Dirt Bike Discussions By Brand
Honda MX & Off-Road Dirt Bikes
2004 Crf450
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[QUOTE="Rich Rohrich, post: 1054329, member: 16241"] The seats have to be [b]machined[/b]. Even if the seat wear is minimal, lapping the seats won't give you a proper sealing surface for a new valve, and you won't be able to get the seat narrow enough for good airflow. It's a very narrow seat that you need to have machined to bring it back into spec. Valves and seats that don't seal properly bleed off pressure and really kill performance at high rpm , and seats that are too wide really hurt airflow which can cause all kinds of weird performance issues with intake reversion near TDC. It's pretty rare to see a CRF that doesn't need the valve seats recut to get a proper seal even on a BRAND NEW HEAD. Lots of folks try lapping the new valves in to get a good seal, but that doesn't do it. Even if you do the other work yourself I would strongly suggest taking the head to a performance machine shop and getting the valve seats recut to ensure you have a concentric seat. Have them measure the exhaust valve guide clearance as well. The intake guides don't wear much but the exhaust guides need to be checked each time you change valves. You also have to set the spring installed heights for all 4 valves to ensure you have the correct seating pressure. A height micrometer or a height standard guage is used to measure the distance between the base washer and upper retainer (the installed height) . This distance is changed with precision ground shims that fit under the base washers. The shims are available at any speed shop or performance machine shop. Other than that it's not really much different than changing OEM parts. [/QUOTE]
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Dirt Bike Discussions By Brand
Honda MX & Off-Road Dirt Bikes
2004 Crf450
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