For those stymied by frozen pistons, don't bother trying to beat them out with heat and various 'liquid-wrench-type concoctions. The aggravation and associated mechanical mayhem isn't worth it.
Disassemble the engine, remove the cylinder studs and mount the top case with cylinder and crank in a hydraulic press so that the cylinder bores are vertical . I used a wooden frame made of 2x4s, but whatever you make it of make it sturdy. In the days before go ahead and squirt some of your favorite anti-rust potion and heat things up with a torch, but it's still going to take a surprising amount of pressure to get the pistons to budge, as apparently the rings 'weld' themselves to the cylinder through electrolysis ( expert opinions welcome).
This particular CB engine had been sitting under my house for the past 25 years and no way was a little liquid wrench, heating and pounding gonna free it up! However, once apart, everything inside was in very good shape and looked to be a low-mileage engine.
Freeing up 'Frozen' 305 Pistons
Freeing up 'Frozen' 305 Pistons
Do what you've always done and you'll get what you've always had.
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- honda305.com Member
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Re: Freeing up 'Frozen' 305 Pistons
About 20 yrs ago, as an experiment, I unstuck a CB77 that had been seized for about 20 yrs. As an indication of the poor condition of the bike, the tank was completely rusted though the bottom. Over a two or three month period, I applied Kroil about 2 times a week and with the engine in a high gear, rocked the bike back and forth against the seized piston. Over the course of the weeks little changed and then after a couple of months of this process the engine went bang and the piston unseized. I then repaired other problems on the bike and with new oil I fired it up. It ran for a few months until the piston ring that was seized broke up and that cylinder lost most of its compression. Before that though it ran quite well. I think patience can unseize most cylinders.
I have also had luck using hydraulic pressure to unstick a cylinder. This only works if the engine is not at bottom or top dead center. Make a hydraulic adapter with a grease zerk that screws into the plug threads and with the valves closed pump grease into the cylinder until the seizure gives way. This is less harsh than using a press but only works if there is travel before bdc.
Honda twins are particularly susceptible to seizure on the left cylinder because they run cast iron rings in cast iron bores. If the bike is left on the side stand for months with the petcock on, the fuel washes the oil off the cylinder and moisture will cause them to rust together. I have encountered this several times, always on the left cylinder on twins since that is the low side on the center stand.
Best to all. M
I have also had luck using hydraulic pressure to unstick a cylinder. This only works if the engine is not at bottom or top dead center. Make a hydraulic adapter with a grease zerk that screws into the plug threads and with the valves closed pump grease into the cylinder until the seizure gives way. This is less harsh than using a press but only works if there is travel before bdc.
Honda twins are particularly susceptible to seizure on the left cylinder because they run cast iron rings in cast iron bores. If the bike is left on the side stand for months with the petcock on, the fuel washes the oil off the cylinder and moisture will cause them to rust together. I have encountered this several times, always on the left cylinder on twins since that is the low side on the center stand.
Best to all. M