When an engine runs for many years, the cylinder walls wear along with the piston rings. Since the top piston ring stops about 1/4 inch from the top of the cylinder bore, wear on the cylinder wall will be limited to the same range. As such, the bore will remain at original diameter in the top 1/4" while it wears out a little larger from there down. This leaves a ridge in the top of the bore that you might catch with your finger nail. Forcing the piston up past this ridge for removal can cause the piston rings to catch on the ridge, which could damage the ring grooves in the piston. This may not be too important if you know you will be reboring the cylinders and replacing the pistons with oversized parts. But new pistons are expensive, and so is the machining cost for reboring. If the cylinders are not too badly worn it may be prudent to save the pistons and just replace the piston rings. In this case it is best to remove this ridge (cut out the material) before you try to remove the piston from the cylinder.

Actual function of the ridge reamer may not be apparent at first glance, but it's easy to figure out when you have one in hand. First rotate the engine crankshaft to lower the piston near the bottom of stroke. Insert the ridge reamer tool into the top of the cylinder bore, and adjust the cutter(s) to come into contact with the ridge. Oil the cutters and rotate the tool with a hand wrench to cut away the ridge as you progressively advance the cutter(s). When the entire ridge has been cut away the cylinder bore will be flush all the way up. Then you remove the tool, and you can easily push the piston with rings out the top of the bore with out damaging the ring grooves in the piston.

A ridge reamer may cost $25 to $50, with the cheaper ones available from J.C Whitney or one of the discount tool stores. For a single use you might even borrow one from one of the larger discount parts stores like Autozone. Be sure to get one that will work with the bore size in your engine. If you want to use it with different engines you might shop for one with an adjustment range that can be used with all.

Ridge reamers come in variety of designs, but the ones pictured here are typical of low to mid priced models. I had some recent experience with the one shown at right, and I was not happy with it. The base expands to be secure in the cylinder bore, and the top part turns to carry a single spring loaded cutter. This looks and sounds good in theory, but in practice it did not work well. Apparently the spring force was insufficient, or perhaps the cutter was dull, but in any case it didn't cut much and never would remove the complete ridge from the cylinder. About all it did was remove some carbon deposits.

This next ridge reamer I have been using for many years, decades even (click for larger image). This one has a steel cone in the base which is pulled upward by a center screw to expand three cutters outward against the cylinder wall. The steel tabs on top fold in for storage, and fold out to be secured with the screws for operation. These tabs rest on top of the cylinder block to control the height of the tool, so it will work only where the top of the block is perpendicular to the cylinder bore (as with most piston engines). Drop it into the cylinder with tabs resting on top of block, tighten the center screw to expand the cutters, oil the cutters liberally, and turn clockwise one revolution with a large socket wrench while holding the tool down in contact with the top of the block. Since the cutters are not spring loaded, you have to stop after each turn to tighten the center screw to expand the cutters again, and you have to repeat this several times until the ridge is completely removed from the cylinder. The cutters will advance only 0.001" to 0.002" each time, so it may take several minutes of wrenching to finish each cylinder. But with three cutters working at once it goes fast enough (for an occasional use tool), and it certainly does get the job done.

At 04:32 PM 4/1/2008 -0600, William Higinbotham wrote:
"I saw your article about the reamer and discovered how bad is the design of the top unit. I could tell that it was going to cut out of round while trying to set it up. We ended up getting the LISLE® Roller-Action Ridge Reamer from JC Whitney. It was fairly easy to setup and worked flawlessly." - Note: This one is more expensive, but also works on engines with canted top of block.