In case it helps anyone down the road, here is how I successfully removed a rubber seal that fell into my engine while the cylinder head cover was off.
- First, try to locate the part you've dropped. I used a flashlight and a small mirror to try to see down the cam chain passage. If you're lucky and you find it, hopefully you can reach down and grab it with a long pair of tweezers from a hobby shop. One of those antenna-type magnets is also useful if your part is metal.
- If #1 doesn't work, you'll want to drain the oil and hope it comes out through the drain hole. There are also the two large bolts on the under side of the crankcase, and you might get lucky if you remove those. If this step is unsuccessful, or if the part is too big to pass through any of these holes, you'll need to remove your exhaust (if necessary) to access and remove the pan. Hopefully it will be in there.
- If not, then now it gets tricky. Stick your hand into the engine from underneath (i.e. through where the oil pan would be) and feel around as best you can. You can't reach in too deep because of the transmission, but try to get your fingers into every area you can reach. Maybe you'll get lucky and find it. If not, proceed to #4.
- If you have an air compressor, blow air into the engine from above (i.e. down the cam chain passage). You can also remove the oil filter cover and blow air in from the front as well. Hopefully the lost part will fly out. A lot of oil certainly will. If not, go back to #3 and feel around inside. This is when I found my seal. Coat hangers and other bendable wires will be helpful for poking around inside to try and dislodge the missing part.
- Put everything back together, re-fill with oil, and never tell anyone about your stupidity
If none of these steps work, you're probably going to need to remove the engine from the frame and start digging. Or of course, you could just be really careful and not drop anything inside to begin with.
