The Stovebolt Site Forums The Shop Area Engine Shop new dilema with 235 in frame rebuild.

I have been trying to do a budget rebuild on the 235 in my 55.2. Just wanted to replace rings, rod and main bearings, valve seals, oil pump and gaskets involved. Upon removing the oil pan and taking the pistons out, I luckily noticed the pistons had cracked along the edge near the wrist pin hole, but on the round edge. Each piston was cracked in same spot. Going to have to order new pistons on Monday morn. What would have made the pistons crack here, or at all? I did get the motor to start before I disassemble it, though it turned over hard. I will try to post a pic. Though it looks like the piston is chipped, it IS actually a crack that extends into the rough casting. cracked piston here is another shot... second pic

That type of crack is usually caused by early timing and/or a lean mixture, which sets up preignition and detonation. It's like hitting the top of the piston repeatedly with a sledgehammer just before the crankshaft reaches the top of the stroke. It can also hammer the bearing material flat on the upper bearing shell.
Jerry

Is there a stamped number on top of the pistons, like STD., 20, 30, 40, etc? If so, that's the oversize the cylinder has been bored to- - - -Standard, .020", etc. That's important to get the right size replacement parts. Also, look at the ring ridge, if any at the top of the cylinder. Since the rings don't come all the way to the top of the cylinder, there will be a short section of unworn cylinder wall above the top ring's travel area. If you've got more than .005" of taper wear it's not a good idea to do a re-ring job without boring the cylinder bigger to get it straight again. The wear usually happens in the first 1 1/2" or so at the top of the cylinder, which means the rings will constantly be changing size to try to maintain contact with the cylinder walls. That usually results in a VERY short-life rebuild- - - -10,000 miles or less before it starts burning oil again!

Check for taper wear this way- - - -take a top ring off a piston, and square it up in the cylinder right at the top of the ring travel. Measure the gap at the ends of the ring by stacking up feeler gauge blades until they're a snug fit in the gap, and add up the thickness you get. Write it down. Now push the ring to the bottom of its travel with a piston to keep it square in the bore, and measure the gap again. Subtract the bottom gap from the top gap, and divide the result by 3. You've got a good approximation of cylinder taper, which should be .005" or less.
Jerry