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Piston Ring Removal and Replacement
Replacing rings is a fairly easy process to stretch the engines usability between major rebuilds and cut down on that blue smoke and high oil consumption. Frequently, replacing only the rings is not advisable because by the time you do the teardown and measurement of the components, the pistons are often beyond the manufacturers specification or something else you find will signify the need for a major rebuild. Still, there are those times when inspection and measurement will show that the pistons and cylinders are still serviceable as is. On these lucky occasions, you can get by with simply replacing gaskets and the rings. The process is pretty straightforward but if done incorrectly may result in your engine needing a major rebuild or just unsatisfactory results.
When should you replace the rings?
If you have a trace of blue smoke and are using quite a bit of oil, you may be in the market for rings. Before you can come to this conclusion, you need to check your piston to sleeve tolerances and surfaces plus make sure your valve guides are not sloppy. The valve guides can also exhibit the same symptoms as do worn out pistons and sleeves (or cylinders).
Rings can be replaced when you have verified all the following:
There are several conditions that warrant more extensive work than ring replacement. Engines frequently get stuck because of ring problems and depending on how it sticks, a stuck engine can lead to other extremely expensive problems such as bent rods, damaged crankshafts, and block casting damage. It is wise to make sure that your engine is a good candidate for just replacing the rings.
Where do you start?
The disassembly should be covered in your service manual with all the specifics you will need but here are some general tips that should apply to most machines. The disassembly process is usually begun by draining the fluids (water/antifreeze and oil) and pulling the oil pan. On some machines you can perform the process without removing the head by pulling the pistons out from below by simply removing the rod bearing caps. Caution is advised here because this leaves out an important step that may come back to haunt you. If you don't remove the head, you will not be able to remove the ridge at the top of the cylinder. This process is done from the top with a tool called a ridge reamer. These are commonly available at auto parts stores, Sears, or nearly anywhere that quality tools are sold. The ridge reamer is a cutting tool that will equally cut around the very top edge of the cylinder to remove the ridge. The ridge is formed naturally as the area of piston and ring travel in the cylinder wears and the portion above it does not. If you don't remove this ridge, there is a possibility that your new larger and very sharp-edged rings will be broken by contacting the ridge. This means another teardown and sometimes sleeve replacement due to scoring. Removal from the top requires cutting off the ridge to avoid damaging the piston ring lands (the surfaces that support the rings) as you pull the piston out. In any case, removal of the head lets you test the valves for good sealing and allows you to measure the tolerances of the head and block surface. Remove the head, remove the ridge, and remove the rod bearing caps. You can then pull the pistons out the top and carefully remove the rings from piston.
Cleaning up the Pistons
After you have removed the pistons, examine them and the cylinders for scoring that would suggest more than just ring replacement. Then remove the rings by very carefully spreading them from the break (called a ring gap). A tool can be purchased called a ring spreader that can make this a bit simpler. An inexpensive ring spreader looks like a pair of pliers that open when squeezed. More expensive rings spreaders have the same design but also have a band to wrap completely around the circumference of the ring to ensure that you don't elongate or spread the ring gap too far. This is not that important on your old rings since you are throwing them away but on the new ones it is critical.
Once the rings are removed from the pistons, examine the grooves that the rings fit into and make sure they are not damaged. You will need to clean the grooves carefully to remove carbon and dirt that would hamper the correct seating of your new rings.
What size rings will do the job?
Sizing your new rings can be done by measuring the bore and determining what oversize will completely fill the gap when the piston is at the top of its stroke and the manufacturers required ring gap is taken into account. The ring gap is the clearance left at the split in the ring when the ring is as compressed as it will be in your cylinder (this is usually the top of the pistons stroke).
Chances are that using one oversize up from the existing rings will suffice since only replacing rings usually indicates very little wear on the piston and cylinder.
Honing the Cylinders
Cylinder hones can be purchased at any real auto parts store. The hone is a device with 3 grinding stones and will chuck up into your 1/4 inch drill to allow you to eliminate the smoothness of the cylinder bore. This smoothness, called a glaze, would keep your new rings from seating properly. The intent of honing is to get a nice cross-hatch surface on the cylinder. To get this you must move the hone up and down as the drill runs. Never allow the drill to run in one spot. Be sure there are no large particles on the bore or hone surfaces that will cause scoring. Cover the crankshaft rod journals while honing to keep them protected from falling debris.
Filing the Ring Gap
When you receive your new rings, you will place them in the cylinders and measure how much clearance is left at the ring gap at various points in the cylinder. Slipping feeler gauges into the gap lets you determine if the clearance is sufficient and matches your tractors suggested specifications. To increase the ring gap, spread the gap enough to insert a file and move the ring back and forth occasionally checking the gap in the cylinders till you reach the desired clearance.
Reinstall the rings on the piston using your ring spreader or very carefully spread them by hand and slip them into the ring grooves starting with the lowest ring (oil ring) and ending with the top ring (compression ring). Insert them into the bore by using a ring compressor. This is a sleeve that fits around the piston to compress the rings such that the piston can be slipped into the bore. Be sure your ring compressor is perfectly clean on the inside and put some oil on the surface where the rings will be sliding as you push the piston down into the cylinder. You will also want to put a bit of clean motor oil on the cylinder walls prior to slipping the piston into the cylinder. Then follow your service manual's instructions for reassembly of the engine and other components.
Other items to check when doing a ring job
Though these are not related to actually replacing the rings, doing this job presents an ideal opportunity to check them. First, while the rods and pistons are out, you may be able to slip a feeler gauge in between the Camshaft and its bushings to see if they have exceeded their useful life. The other opportunity that shouldn't be missed is the adjustment of your rod bearings. Since the rod caps must be pulled off, you should perform measurements to see if the journals need adjustment or replacement. On many machines adjustment will simply involve removal of one or more shims but if you don't do it while you are doing your rings, you may have to go through the whole process again in the near future.