The Stovebolt Site Forums The Shop Area Engine Shop 235 piston question

I have pulled apart my 56' 235 for an overhaul. It was worked over at some time before I got it because the rod bearings are 10 under and the cylinders have very little wear on them. The pistons have 040 on the dome. I'm thinking that means they are 40 over? Also does anyone have the measurement of what the standard piston is?

Thanks.

If they say 040 then they're 40 over.
STD on the 235 is 3.9/16, you could go 060 and still be safe.
If theres little wear showing on a bore gauge though Id just throw a set of rings in and hone up the walls real well and call'er good.

Good Luck!

Ditto on the 3 9/16" standard bore. That's 3.5625" in decimal measurement. A .040" oversize cylinder should measure 3.6025" if it's not worn oversize. The majority of the wear in a cylinder happens in the first 1 1/2" of piston travel at the top of the bore, so it doesn't wear uniformly from top to bottom. If there's more than .005" of taper wear, a ring job usually won't last long due to the piston rings constantly changing size trying to maintain contact with the cylinder walls.

Here's a quick way to check cylinder taper if you don't have an inside micrometer or a dial bore gauge. Take a compression ring off one of the pistons and place it in the cylinder at the top of the ring travel. Square it up in the bore by turning the piston upside down and using it to align the ring in the cylinder. Make sure you don't measure at the very top of the cylinder, because the rings don't travel that far up the cylinder. Stack up blades of a feeler gauge until you can measure the ring gap accurately. Record the measurement. Now use the piston to push the ring down to the bottom of the ring travel, or at least close. This depth isn't critical, because there will be very little wear if any at the bottom. Measure the gap again. Subtract the bottom gap from the top, and divide the remainder by 3.14. That's the cylinder taper.

Example:
Top ring gap is .035" Bottom gap is .020" Difference = .015" .015 divided by 3.14 = .004" taper. You can glaze-break and re-ring this cylinder. The taper isn't enough to fatigue the rings and cause a short-life rebuild.
Jerry

Edit: The piston diameter at a 90 degree angle to the wrist pin and the same height as the wrist pin from the top of the piston should be about .002" smaller than the bore size. The piston is egg-shaped, with the largest diameter at the center of the skirt, so be sure and measure it at its largest diameter when it's cold. It gets round as it heats up due to its internal construction. To cure a sloppy-fitting piston during a rebuild, we used to run a knurl on the face of the skirt. Surprisingly, it worked pretty well. Chevy cast iron pistons wouldn't take a knurl, so there was a spring steel expander available that got installed inside the skirt with a special pair of spanner pliers to cure piston slap on those engines.
Jerry

Thanks! This is exactly the info I needed. There is very little wear on the sleeves in fact practically no ridge on the top at all so my plans were to just replace the rings. Not due to a compression issue as they all read 110 plus but the head is cracked and I had no idea what shape the rest of the motor was in when I got so I am doing rings, rod bearings, and oil pump to give me piece of mind. It ran great already minus it having a cracked head. I will use the taper test to check that also. Thanks again!

Be sure to run a "glaze breaker" hone or a button hone in the cylinders to break up the slick surface so the new rings can seat, and then scrub the cylinders vigorously with HOT soapy water and a stiff-bristle brush to get the hone grit out of them. Follow up with a rinse with the hottest water you can get to remove the soap. I use a strong Tide solution and an old round toilet brush, followed by a rinse with a garden hose connected to the drain fitting on my water heater. Don't worry about getting the crankcase wet, but do tie some throwaway rags around the connecting rod journals on the crankshaft or wrap them in duct tape to keep grit away from them during the honing process. When the cylinder is clean, it will flash-rust as soon as the hot water evaporates, so oil it slightly as soon as the rust begins to appear. No rust- - - -not clean enough!
Jerry

Thanks Jerry for the input. I already have cut what very little ridge, almost none, there was on the top of the sleeves and did some honing. I want to take measurements before I go any further with the honing process.

bztguy,
Hotrod Lincoln gave you some good advice. I have a 56 235 and was faced with some similar issues. I did exactly that and it is still running daily after two years. Still good compression readings too.
Normbc9