Originally Posted by rick
Have done minor rebuild a couple times on Chevy 350s, so have no fear of doing this myself. Spec I was looking in manual for, but didnt find was the valve to piston clearance. Have checked this on V8s by puttng a small amount of putty on the top of piston and mounting the head as you would to run it, then slowly rotating the engine in its normal rotation. Then disassemble and measure the thinnest portion of the putty. This is useful for determing how much the heads can be milled to correct minor worpage, and increase the compression a small amount. Understand how bad dropping a valve can be, just helped a friend replace his motor due to loosing both exhaust valves on the front cylinder. He found a motor for 1,200 with just over 8,000 miles. This bring up the question of, would it be better to just find a low milage motor or do a rebuild on mine.
Excellent points Rick. Unfortunately bike engineers don't leave us much to work with when it comes to re-machining anything and so the service limits are what they are to prevent these pesky little piston/valve collisions.
If you stick with the head/jug warpage/flatness, valve seat/valve cutter diameters/angles, valve guide, piston/cylinder clearance service limits and then adjust your valve clearance this is how you achieve the proper piston to valve clearance and also avoid valve guide wobble/piston wobble.
Clean the area you're hand-cutting really well, apply machinist's die to coat the intended valve seating areas, use quality cutting oil, rotate your valve seat cutters and lap your valves very slowly gently by hand in whatever direction (i.e. clockwise or counterclockwise) that is easiest for you but remember these things:
1. Do not remove any more surface material than necessary to achieve a smooth finish.
2. Do not cut in opposing directions back and forth as though scrubbing the inside of a drinking glass. Instead turn in one direction only until you see die being removed but do this ever so softly and gently. When cutting ensure to keep the cutter perpendicular to the surface being cut. Most importantly double check your cutters to ensure they are the right specs for the area you are cutting and that they are not damaged or too worn past the point of being useful.
3. Do not use too much die. A machinist's and mechanics best work is done when there is the lightest coat of die possible on the area being worked on.
4. Do not grind the seats too much and pay close attention to your service limits. I'm sure you already have access to a quality caliper and micrometer to measure things so that's a given with your experience.
5. Lap your valves to their seats in the same slow gentle manner first with coarse grinding compound until you have a smooth matched surface on both seat and valve. Repeat the process with fine grinding compound and ensure the smooth finishes are in the middle of the valve face area. Mark the valves with a Sharpie to correspond with their seating areas. I mark parts this way sometimes when they need to be replaced to the area being machined, lapped or fitted but it's my own personal preference to do it this way.
You can only go in so far into the head until its service limits are exceeded and anything beyond service limits will slam-jam a bike engine something fierce. The putty trick is a good one to use to double check your work from the inside after you get it all put back together. That way you can see what that piston to valve clearance might be like even though it isn't called out by the manufacturer since overgrinding will drop the valve too deep inside of the head, make it impossible to adjust your valve clearance and force you to buy a new head.
Wish we lived in the same area because I know it would be fun riding and wrenching with you friend. Keep us posted.