Honda Valve Train

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Honda CRF models
2004 CRF250
Although no specific service limit is advised in the Honda service manual, you should replace all the valves and springs at the same time as the piston. Honda recommends changing the piston every 15 hours, and in practice I estimate the valve train and piston to have a useable service life of about 30 hours. In fact the valve train usually wears out before the piston. The dangers of leaving the valves in place until the engine wonÂ’t start anymore, is that the valve seats will become damaged and require an expensive service of valve seat reconditioning. In a case like that, considering that new head is so inexpensive, it may be cheaper to buy a new head rather than fix the old one.
Another problem that is rare for the CRF250 is a failure of the left side cam bearing. If the engine becomes particularly noisy at idle, or if there is a thick black residue of rubber present under the valve cover, then the cam bearing is worn and placing a greater side load on the cam chain guide. The rear tensioner chain guide is coated with rubber, which accounts for the thick black residue.

2002-04 CRF450
The two most common problems with this model include intake valve wear on all model years and cam chain tensioners that go slack on the 2002 and 03 models. At first the intake valves wear was attributed to dirt bypassing the air filter, causing wear to the protective oxide coating on the titanium intake valves. Some of the other theories touted on the Internet included valve seats that are too hard, too soft of a valve spring, and too steep of a closing ramp on the camshaft. The most reliable solution for the intake valve problem is to install a Kibblewhite spring kit and Black Diamond stainless steel intake valves. The parts cost about $300. Ferrea also makes stainless steel valves and their product features lighter weight and are designed to work with RD brand single coil spring kits. Ron Hamp Cycle offers a choice of lightened Ferrea stainless valves or solid titanium valves with DLC (diamond-like coatings) in standard and oversized along with custom machine work.
There are two options for the cam chain tensioner. The least expensive choice is to buy a tensioner from a 2004 CRF450, which sells for about $52. The 2004 and later Honda part is greatly improved but it should be replaced every 100hrs. Factory Racing of Italy chose a different approach to the problem; they make a manual cam chain tensioner. However manual cam chain tensioners require frequent adjustment and a careful touch. Another area of concern on the Honda is a worn exhaust rocker arm roller. Look for a deep groove to appear in the center of the roller. That indicates that the bearing is worn. You can’t replace the roller itself because it isn’t available from Honda separately. You need to replace the entire rocker assembly, which sells for about $120. If you are a fervent Internet news group reader there are two things that are recommended that should never be attempted. They include grinding down the valve shim pads to give valve to tappet clearance, and installing Honda ATV steel intake valves with the stock springs. When a valve is worn so far that Honda doesn’t offer a small enough shim, its junk and needs to be replaced. With regards to changing valve materials, when you switch from a lightweight titanium valve to a heavier steel valve, you must also install a stiffer spring. One last thing that contradicts the Honda manual and Internet myths, never attempt to use valve grinding compound to pre-finish a titanium valve prior to installation. The gritty compound will damage the oxide coating designed to protect the valve.