Originally Posted by NJ-Brett
Glad your rod is not toast.
That piston sure got hot though, looks like some old Triumphs I had.
On those bikes, if you broke them in easy, the rings did not seal well the they ran very hot and looked like your piston.
In your case, its likely from lean running?
That piston was indeed toast, and it was most definitely from a lean condition. Thankfully that's all behind me at this point, and as of a few days ago.. the bike is running! Put another 50 miles on it this morning and she's loosening up wonderfully, and stronger than ever before. For the sake of finishing off this particular rebuild, here are the rest of the photos and so on...
The replacement cylinder head arrived in great condition, with one little exception... Whoever was disassembling that particular XR had sheered off one of the carb boot bolts. A quick message to the seller, and I was refunded twenty bucks which went towards the purchase of a Heli-coil kit. I'm not a huge fan of Heli-coils in high stress situations, but this is just the intake boot which has around 7-8 ft/lbs of torque on it. No problem. Removed the valves, and all of the seats & valves were in fantastic condition.
My first method of attack was to apply a little bit of heat and try to twist out the remaining edge of the stud with a pair of Vise-grips. No dice. Second attack was via Ez-out, which promptly started to sheer off. I stopped before it broke and abandoned removal, instead opting to simply drill it out and install the aforementioned Heli-coil. Worked like a charm.
Fast forward another week or so... At this point, I finally dug into the rest of the engine and removed the clutch side cover from the engine case. I'd need to remove the clutch basket in order to replace the cam chain. Via another ADV member's XR650L rebuild thread, I constructed a tool from flat aluminum in order to hold the clutch posts in place while I removed the center clutch nut with a breaker bar. I don't own an impact wrench, and it seemed like it would do the trick...
The other 'tip' to help in this case is to hold the rear brake down with the bike in the highest gear possible, and THEN twist on the center nut. I spent a day cleaning up the rear of the bike, but when I tried to bleed the rear caliper, discovered that the damned thing had seized. Ugh! Couldn't get the damned piston out of the caliper for the life of me. I began scouting ebay for another caliper, and meanwhile gave the clutch nut a go with my newly made tool. I figured it would still do the trick.
Sheered off one of the posts... and then two more in rapid succession. Not good! Honda's tool is a little different from the one I constructed, but the general idea is the same. Holding those clutch spring 'posts' further away from the actual clutch plate simply put too much torque on then, and that was that. I was pissed at that point with my $40 dollar mistake, and walked away from the bike for another week and a half, or so. In the meantime, I placed an order for a few more parts..
Clutch side cover gasket, brake seals, swingarm slider, pressure plate, clutch nut, exhaust crush gaskets, and clutch springs.
In regards to that rear caliper... Checking on Ebay and other various sources, it appears that there are a slew of bikes which share the same rear caliper as the XR250L. Older XRs, CR500s, XR600Rs and so on. Couple Suzukis and Yamaha's too. Apparently that particular caliper was used by many Japanese manufacturers throughout the 90s. The mounting bracket for the XR-L is L specific, but the caliper 'plugs' into that bracket, which then slides on to the bike and retains the rear axle.
I was about to pull the trigger on an ebay caliper when I caught wind of a trick to 'unstick' a frozen caliper. Using a grease gun, you can apply grease via the brake bleeder valve, and it should push the piston right out. The PSI output from a grease gun is rather astounding, and it worked more beautifully than I could have imagined! The piston and caliper was filled with grease, which took some cleaning to get rid of, but was definitely salvageable at that point with a set of new seals, as the old ones were absolutely trashed. After some wet sanding with 2000 grit paper, the caliper and piston were once more in great shape. I installed the seals, cleaned out the rear master, and had the rear brakes working good as new.