I bought an R3 race bike recently that had some minor work done to the engine. The seller claimed that the head had been ground down, thinner head gasket was installed, and slotted cam gears were installed. I decided to pull the spark plugs and both plugs sheared off from the threads leaving the entire threaded portion stuck in the head. I tried using an easy-out to get the plugs out and was able to get 3/4 of one out after it sheared off from the remaining threads. I pulled the head off and found that there were some small cracks radiating from the spark plug hole. I have considered sending the head to Millennium Technologies for repair but I'm not sure it is worth the cost on this bike. Several online parts warehouses have a new OEM head assembly for under $300. If I were to go this route and swap over the valves/springs/etc, would I need to have the valve seats cut on the new head or would I be able to get by by lapping the valves in their respective seats? Another concern I have about swapping heads is that I will be reinstalling the cams with the gears degreed as they were removed from the original head. Should I consider going back to stock cam gears? I don't really care about losing the extra performance from the mods that were done to the engine previously, at this point I just want to have a reliable setup. I will probably have the bike re-tuned as the compression will be different if I replace the head. Thanks for any advice.
That thing must have been tuned/running like garbage to kill head like that. If you go back to stock head and set up yes either put the stock cam gears back on or have the cam timing set by good builder so you can get back to the best available power from what you have.
I believe the cracks were of my own doing. I tried to easy out the remaining threads of the spark plug after the head was removed. The easy out gripped, but the threads would not budge. I noticed the cracks when I was cleaning everything up so that i could grind and chisel the remainder of the plug threads. I will have the tuning looked at regardless of what route I go. My biggest hurdle at the moment is what I will need to do if I purchase a new cylinder head. I will probably just get some stock cam gears to eliminate any problems there. I'm still not sure if I can get by with lapping my current valves into the new head or if I will need to send it out and have the seats cut by a machine shop.
^that If you go back with a stock thickness head use the stock cam gears and timing marks. You can use the adjustables but you’ll need to have the timing set by someone with experience degreeing cams. I’m surprised Yamaha doesn’t sell a complete head with valves and spring already installed.
