The seats in most aluminium cylinder heads are steel or alloy inserts that are press fit into the head and are then machined to the correct angles. In the case of this 2004 CRF250 the outer edges of the insert have sections that are broken away on both exhaust valve seats. Damage like this would require complete replacement of the valve seat inserts, which is a fairly involved machining process.
This is pretty common on old bikes with lots of hours on them where the valves get really hammered from wear or incorrect clearance, but I haven't really seen it happen on an engine this new. This is the first CRF250 that we've seen this happen on.
We've talked to a number of folks this week and it seems this problem might be a little more common than we originally thought. We aren't the only guys with a head like this in our shop. Time will tell how widespread this really is.
Here's the pics of the exhaust seats. You can see how the inserts on both exhausts have broken away up to the edge of the 45 degree angle on the seat.
I don't have an answer as to why other than maybe too tight a press fit during initial manufacture. I really don't know for sure.
If you don't live on the rev limiter the 05 head will produce a wider more rider friendly power curve due to it's smaller port area and the major change in the short side radius. The port approach to the valve is radically different on the 05 head. With some porting mods on the 05 head you can get some peak power back without killing off the new found torque. Most riders will like the 05 head better, but it's not for everyone. The changes to the 05 head are similar to what a lot of us were doing with the 04 heads when guys were looking for more torque and rideability. Using the 05 head is just a better way to accomplish the same goals without a bunch of epoxy filler in the port. So in a case like this where the valve seat insert will have to be replaced it's probably cheaper and easier to go with a 2005 CRF250 head.
Here's a few pictures of what I'm talking about:
http://www.ericgorr.com/images/crf250_seat_crack_1.jpg
http://www.ericgorr.com/images/crf250_seat_crack_2.jpg
.
This is pretty common on old bikes with lots of hours on them where the valves get really hammered from wear or incorrect clearance, but I haven't really seen it happen on an engine this new. This is the first CRF250 that we've seen this happen on.
We've talked to a number of folks this week and it seems this problem might be a little more common than we originally thought. We aren't the only guys with a head like this in our shop. Time will tell how widespread this really is.
Here's the pics of the exhaust seats. You can see how the inserts on both exhausts have broken away up to the edge of the 45 degree angle on the seat.
I don't have an answer as to why other than maybe too tight a press fit during initial manufacture. I really don't know for sure.
If you don't live on the rev limiter the 05 head will produce a wider more rider friendly power curve due to it's smaller port area and the major change in the short side radius. The port approach to the valve is radically different on the 05 head. With some porting mods on the 05 head you can get some peak power back without killing off the new found torque. Most riders will like the 05 head better, but it's not for everyone. The changes to the 05 head are similar to what a lot of us were doing with the 04 heads when guys were looking for more torque and rideability. Using the 05 head is just a better way to accomplish the same goals without a bunch of epoxy filler in the port. So in a case like this where the valve seat insert will have to be replaced it's probably cheaper and easier to go with a 2005 CRF250 head.
Here's a few pictures of what I'm talking about:
http://www.ericgorr.com/images/crf250_seat_crack_1.jpg
http://www.ericgorr.com/images/crf250_seat_crack_2.jpg
.
