Lapping valves

So, you have heard of "lapping valves", "fresh top end", or "top end rebuild"? Not sure what lapping valves means? Read on.

If your engine has low compression, you may need to lap the valves. The other possibility may be piston rings, but that's another post. Start by removing the engine, cam cover, cam, and head. Once you have the head free, you will need a valve spring compressor, lapping suction tool, needle nose pliers, and grinding compound. Beware-- all engines are different; this may not apply the same to yours.

Set the spring compressor up with the short end on the valve face. I actually made this spring compressor from a cheap C-clamp and 2 bits of steel pipe. I cut a slot in the longer piece, then welded it to the screw end. The short piece was welded to the flat end.



Through the slot in the pipe, the valve keepers are accessible with a few cranks of the screw. Pop the keepers out (visible in the bottom of the pipe under the valve tip). Back off the spring, and the valve is free.
Remove the spring or springs together, and mark which end goes towards the head.


Valve is now free. The keepers are visible below on the table by the pliers. You can peek in the top exhaust port and see the light and the valve stem.


The valve can be pulled out of the head, being careful of the valve guide. Clean the carbon off the valve a bit with some light sanding. Beware the sliding surface; it should not be sanded! The carbon should also be removed from the port. Be careful not to score the head, and beware the valve guide. 400 grit sandpaper works well.




Now you are ready to lap. Unless your valves are really pitted, you should only need "fine" grinding compound. Smear some around the edge of the valve and put the valve back in the head.

Now take the suction tool and stick it to the valve face. Using a "fire starting" motion, rub your hands together with the tool between them. You will hear a scratching sound as the valve rubs back and forth, but it will get softer as you go. When the hand-rubbing/valve-spinning becomes quiet, pull the valve out a bit with the suction tool, turn it 90 degrees in the head, and put it back in and repeat. Do this 3-4 more times, then check by wiping off the grinding compound.

You should have a shiny seat and valve edge that don't photograph well. In the picture below, the grinding compound is still on, but the valve and seat are shiny all the way around. If you have a large gap of non-shinyness, you may have a bent valve. Bent valves would also cause compression leaks, but are generally preceded by a catastrophic engine failure of some sort, so you should know before you get to this point.

So, now that I have a shiny new valve seat and everything cleaned up, I'm ready to reinstall the valve. But first, I will replace the valve guide oil seal, which was under the springs on the cam side of the head. This is fairly straight forward, but the old ones can be a bear to remove. Be patient and get the needle nose pliers to pinch the oil seal off from the side. The new one should just press on, but it may take some persuasion with a mallet.

With the new bits in and valve reinstalled, the process is the same for the rest of the valves. Lucky me, this engine only has 4. The old Honda 750 I ride has 16. I hope this helps solve the mystery of valve lapping.