Wednesday, November 6, 2013

cappra 125: stuck piston

I worked on the Montesa MX'r until about 2am last night. The motor was seized, so I started disassembly by taking the side covers off. The spark plug was missing so it had been exposed to the elements for quite some time. The inside was filled with water... and oil. Water has a way of turning oil into a sticky dark chocolate sludge over time. I got the clutch off and couldn't decide how far down she was frozen. The piston was for sure stuck, but I couldn't tell of the crank was or not. I went to work with some penetrating oil, some fire, a block of wood and a sledge. I can imagine my neighbors didn't appreciate the amount of hammering that I was doing late last night, so I called it quits a little after 2am. By that time I had managed to free the lower end and was able to get the cylinder to lift off the crankcase, but the piston was still severely stuck. I went inside a did a little research on the topic and found some interesting stuff on using grease and a grease gun to create pressure inside the bore. Also read about igniting gasoline, different concoctions involving acetone, ATF, PB blaster, etc. All of which seemed interesting, but I figured I would stick to what I knew. Today I went and got a propane torch and heated the steel liner up and then cooled the piston with ice. I lubed her up and gave the piston a few healthy strikes and finally got it to budge. The piston on the Cappra is super tall, so I had to drive it all the way through the stroke of the bore. I finally got it though. There is considerable scarring on the bore of the cylinder and I know I did some damage to the crown of the piston, but I knew it was gong to need bored and the next size piston installed anyway. The 125 has a super thick liner since the engine was based on a 250, so I have a ton of material to work with to get it down to a smooth bore. I'm checking into the cost of a piston right now. I can now proceed with de-sludging the lower end and reconditioning it. I think I'm definitely going to need new bearings throughout the motor. Good news, the trans is good and it shifts through the gears fine. Shifting on the right should be fun to get used to again. My old Aermacchi 350 is right hand shift and I have had a Triumph and a couple Montesa's over the years that are this way. Shifting isn't usually all that bad; it's the braking on the left that throws me. My left foot works like a switch for some reason. It's either "on" or "off"; nothing in between. Makes for some harsh braking technique. I always worry about instincts in a panic situation as well.  Though by the looks of things I'm a ways off from having to worry about this.

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