I chipped the valve seals off my '75 CB750 yesterday. They seem to be made of Bakelite. These seals are half the reason I opened the engine, but it didn't burn much oil at all. Now I wonder about the other seals -- alternator, ignition, kickstart, and countershaft. They wouldn't be subject to the high heat of the head, but I still wonder. The engine doesn't leak. I may just split the case and replace them all. I hadn't intended to do that since the engine has shown no problems; this started as a preventive teardown. The cam-chain guide and tensioner show tiny chips missing; another indicator that I need to split the case and replace all the soft parts.