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Honda Odyssey CR250 Head |
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Adding a '73 - '74 Honda CR250 Head to a Honda Odyssey FL250If there is one area where the Honda Odyssey FL250 needs some help, then it's in the area of engine cooling. Of course, the lack of a rear suspension is a big issue too, but I'd say that the engine overheating issue is a bigger concern. With the engine tucked in behind the seat, it's no wonder that it tends to get hot after extended runs. I mounted a Digatron Tach and Cylinder Heat Temp (CHT) gauge to my Honda Odyssey, and I was seeing over 400F head temps after extended runs with a lot of wide open throttle. At the same time, I could hear detonation (NOT good!) at the higher head temps, even though I was running with a 96 octane mix of higher octane fuel. Carb jetting was also set rich just to try to reduce temps, but the engine still would run hot after an extended time. To make matters worse, the stock Honda Odyssey cylinder head comes from an older Honda dirt bike which had some of the cooling fins shortened to clear the tuned pipe as it came up and over the head. These shorter cooling fins do not help matters when it comes to cooling. There are other options...
73-74 Honda CR250 Cylinder Head & 76 CR250M Copper Head Gasket Fortunately, a 73 - 74 Honda CR250 head will bolt on to a Honda Odyssey FL250 engine. The 73-74 Honda CR250 cylinder head has taller cooling fins and they are taller across the entire head. No shortened fins to clear a tuned pipe since the earlier CR250's had the tuned pipe go down and along the bottom of the bike. What this means is that 73-74 CR250 head can help provide a little better cooling capacity than the stock head. They can be picked up on eBay for a reasonable price if you keep your eye out for them. In addition to the 73 - 74 CR250 head, it's also possible to run with a 76 Honda CR250M copper head gasket. The advantages of this copper head gasket are that it helps conduct heat better between the cylinder and head. In addition, this copper head gasket is around .020" thick (around half the thickness of stock FL250 head gasket). This thinner head gasket boosts compression and also tightens up the squish band a little. On the downside, the copper head gasket is less forgiving to imperfections on the sealing surfaces and is more prone to leaking combustion gases. The stock Honda Odyssey FL250 gasket is more forgiving and will seal better in many cases. You'll know the copper gasket is leaking because you'll see oil wetting around the gasket area between the cylinder and head. That's what happened in my situation when I first tried the copper head gasket. So, I bought a new copper gasket and carefully cleaned the top of the cylinder deck. Then I tried to clean up the cylinder head and noticed some nicks and dings on the gasket sealing surface that were definitely not helping matters at all. Since I don't have convenient access to a machine shop, I just decided to try a "redneck" method of cleaning up the head sealing surface. I took a scrap sheet of flat stainless steel sheet metal that I had laying around. I then taped a sheet of fine grit sandpaper to the sheet metal and placed it on a known flat section of concrete floor in my shop. I used a 1/2" drill with some adapters and a spark plug socket to slowly spin the cylinder head on top of the sandpaper. I went slowly because it's hard to control to spinning of the head if the drill is running too fast. I used a soapy dishsoap solution to lubricate the sandpaper (it was wet/dry sandpaper), and I frequently stopped to check the head surface to keep an eye on progress. I just ran it long enough to clean up the entire gasket sealing surface of the head.
Poor Man's Method of Lapping Cylinder Head Surface Turned out pretty well consider the redneck method I used. Obviously, if you are not careful, you could screw things up worse if you don't have a flat surface to do this lapping of the cylinder head. Doing all this definitely helped with sealing. I still need to run it some more to make sure that I have no more leaks long term. If I do, then I will just switch back to a stock Honda Odyssey FL250 head gasket and call it good enough. In my opinion, more important than the advantages of the copper head gasket is to ensure you get a tight seal on the combustion gases. And, better than using this redneck method would be to have the head professionally resurfaced by someone that has access to a machine shop and knows what they are doing. For most people, it'd probably be easier to just run with the stock head gasket and not worry about it since it is much more forgiving of sealing surface imperfections. The CR250 head on a Honda Odyssey helps a little with cooling, but the engine will still run hot because of how it is positioned behind the seat and shrouded from a direct blast of cooling air. Probably the best way to knock down engine temps is to get one of the water cooled conversion heads available for the Honda Odyssey. These water cooled heads are not cheap though, so the 73 - 74 CR250 head is still an option for those with a Honda Odyssey FL250. Here are some links to pages showing some of what I've done on my Honda Odyssey so far: Honda Odyssey Airbox Mod Honda Odyssey Tuned Pipe Honda Odyssey CR250 Head Honda Odyssey Digatron Tach Honda Odyssey Powerbloc Clutch Honda Odyssey FL250 Driven Clutch Honda Odyssey Overheating Solution Honda Odyssey at Beverly Sand Dunes Honda FL250 Water Cooled Head Return to Main Honda Odyssey Page
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