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A valve job is a fantastic way to get your bike running better, especially if it has low compression, a stuck valve, is blowing smoke, or has any one of the hundreds of other problems that come along with an out-of-whack engine top-end. Today, I am going to talk about measuring motorcycle valve guides.
Measuring motorcycle valve guides is just one step in what can be a rather long, complicated (but also incredibly satisfying) process. Don’t fear the engine top-end. Sure, there are some sections of a valve job that a home mechanic simply cannot do himself or herself due to the need for specialized equipment (such as valve seat cutting and valve grinding).
However, I still STRONGLY encourage you to do everything you can yourself and only bring the heads to a shop for the precision machine work – I mean if I encouraged anything else, that wouldn’t be very “HappyWrench-like” of me would it? And you will save a ton of money doing some of the work yourself.
I will also try to talk about all sections of a motorcycle engine valve job over a series of posts – linking them all together – but today it is just going to be about measuring motorcycle valve guides.
Valve guides are just like they sound. They are the little “tubes” inside the head that the valves travel inside as they open and close during the intake and exhaust cycles of the motor. Getting your valves and valve guides set up properly comes down to clearance.
Clearance is the space between between the outside diameter of the valve stem and the inside diameter of the valve guide. Too much space and the valve will wiggle inside the guide, potentially slap into stuff, and allow oil to trickle down the stem into the combustion chamber. Too little space and the valve can actually get jammed inside the valve guide as everything heats up. This is actually the story with Cal. The valve guide for the front exhaust valve was too tight and the valve was completely stuck.
You will need a micrometer for this process, which is a precision measuring tool (not expensive, just precise) for measuring the outside diameter of the valve stem and a set of small ball gauges for measuring the inside diameter of the valve guide. I picked up a set of the latter for $20 off Amazon. You can potentially use a very high quality caliper for the measurements, but I still think micrometers are more precise.
Clean your valve stems thoroughly and begin measuring the stem along the “shiny” section. This is the section of the valve stem that is actually traveling through the guide. The shiny-ness is created by this movement. Measuring in 3 or 4 spots multiple times until you are confident that you have a close to accurate measurement. Measure out to the third decimal place.
Now take your small ball gauges. These generally expand or contract by turning a little screw or knob on the gauge shaft. The key here is to find the diameter that barely fits inside your valve guide and is still able to slide up and down inside the guide. Measure in a few spots. Hopefully they are close to the same as a lot of taper to the valve guide could mean it is time to replace the valve guides themselves. Once you have got the small ball gauge set to the inside diameter of the valve guide, measure it with your micrometer.
You should now have two measurements – the outside diameter of the stem and inside diameter of the corresponding valve guide. Subtract the second from the first and you have your valve clearance. Check your manual or the internet, to find the “acceptable” valve clearance. These clearances will be different for the intake versus exhaust valves. Mine are 0.004 to 0.006 for exhaust and 0.002-0.004 for intake.
After completing the process. There are really only two options – too loose or too tight. For too loose you can buy oversized valves if they make them, and if they don’t make them you will likely have to replace those valve guides (a topic for another blog post). Too tight and you have the option of reaming out the valve guide slightly with a valve guide reamer or some sort of home fashioned combination of cylindrical rod, sanding apparatus. I do not suggest trying to make the valve smaller. Valve guides are meant to be reamed, while valves have precision coatings to make them smooth, etc. Therefore, don’t mess with the valves themselves when trying to get the clearance right.
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