So here is my big upfront spoiler alert about motorcycle valve lapping – a true valve job including cutting valve seats, etc. is and should always be the work of a professional motorcycle shop. Why? Because valve seat cutting is a very precise procedure requiring specialized equipment. A good valve job is an art/science that can last a very long time.
That said, a valve job also shouldn’t be terribly expensive if you bring in the heads yourself (most of the labor is actually engine disassembly). I have seen shops charge as little as $15 per seat. Not bad considering the importance of properly seated valves for compression and to avoid the dreaded blue smoke.
Then you might be asking, what is the point of this blog post? To explain the purpose and place for motorcycle valve lapping. The issue that I have noticed is that many DIY mechanics think that motorcycle valve lapping can solve a valve seating problem when the reality is that nothing replaces a proper valve job. That said, valve lapping is important when the time and place is right, so it is worthy of its own discussion.
For example, if your valves and valve seats show pitting and wear, then it is time for a valve job. Lapping is not and never should be an aggressive or power-tool based procedure. There is a reason they make those stupid wood sticks with the suction cup. Proper lapping is done by spinning the tool between your hands (going in both directions and lifting frequently) like someone trying to light a fire using two sticks. Do not and I repeat DO NOT place your valve in a power drill and try lapping that way. That is way too much force and you will end up getting a valve job anyway. Something to keep in mind is that as you push the valve into its seat and start lapping, the lapping compound will get pushed out of the sides and only remain in the middle of the seat – overlapping under these conditions will create a concave shape to your valve seat if done too aggressively. A lighter touch is better.
To oversimplify things, valve cutting and grinding is the part of the process done to make sure that the valve and valve seat create an air and liquid tight fit. Valve lapping is the final step in the process (sort of the icing on the cake) to make sure things are perfectly mated. To be honest, sometimes valve lapping isn’t even necessary if the valve job was done right. I lean toward not lapping after a good valve job (the shop might do a little lapping as part of the process for you) and doing DIY motorcycle valve lapping when I am trying to rectify only the tiniest of issues.
For example, if I am replacing a single valve because of an issue, I might lap the new valve into the old seat provided the old valve seat is still in good condition. Essentially, valve lapping can be used to marry two pieces (that are in good condition!) together that weren’t mates in the past. It can also be used for the tiniest amount of valve and valve seat cleanup. Again, if you see heavy pitting or scarring, then valve lapping will not be enough. Remember to always test your valve seal afterward by pouring gasoline into the combustion side of the chamber with valves and valve springs reinstalled. Leave overnight and see if any leakage occurs. There should be none.
Final piece of advice on motorcycle valve lapping. Make sure when you are done lapping that you VERY carefully wipe up and clean out all lapping compound. This stuff is highly abrasive and will harm the engine if it gets inside there.
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