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Preparing Piston Rings

Discussion in 'T-Bucket Engine and Driveline Articles' started by Mike, Dec 12, 2011.

  1. Mike

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    Well-Known Member

    Oct 23, 2006
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    I spent 35+ years working in the high performance automotive aftermarket and was always amazed how people would spend hundreds and even thousands of dollars to find 10 horsepower, while they would ignore the free tricks that would gain them the same benefits. One of those areas is in proper piston ring preparation.

    I'm not going to get into different ring designs, materials and the use of ring coatings. The purpose of this tutorial is to demonstrate how easy it is to ensure rings are properly fitted to the piston and cylinder in which they will be installed, so you will realize maximum ring sealing, maximum oil scraping and maximum heat transfer to the cylinder walls.

    Once the machine work is completed on your block and you have pistons and rings in hand, set aside an area to keep individual pistons and their rings separate and identified. If you have a metal top bench, wipe the surface down with lacquer thinner and then use a marker to mark the bench with the numbers 1 - 8 (or however many cylinders you're working with). Get a sheet of cardboard and write your numbers on there. Whatever method you use, just be sure you can identify which piston and ring combination go in each cylinder.

    Wash the pistons with a quality solvent and get them laid out. Then take the rings out of the package. Soak a rag in lacquer thinner, pick up a ring and wipe it down as you carefully inspect it for rust, chips, cracks or any other defects. If you find a problem, you will not want to use that ring. Examine the ring facing, to ensure the face coating covers the face of the ring and is not compromised. If you have a piece of glass or granite you know is flat, cover the top of it with 400 grit sandpaper and place it in your parts washer. Get solvent running over the block and give the sides of the ring a few swirls on the sandpaper, so you can identify low or high spots. Once you are sure the ring is in good condition, dry it off. Then place the ring with your number 1 piston and grab the next ring, to repeat the process.

    OK, is everything verified and clean? Then it's time to move on to the next step. Take the number one top ring and fit it into the number one cylinder. Use a bit of care when compressing the ring and if it will not fit into the cylinder, the ends are probably butting. Get a small and fine flat file (point files work a treat for this) and carefully remove a bit of material from each side of the gap.

    Caveat - Work slowly and continually check to ensure you are keeping the ring ends square as you go. You can always remove a bit more material, but it is impossible to restore material, if you got too ham-fisted with the file. Remove some material, then close the end gap up and hold the ring up to a light, to make sure the ends are square and you're not creating a pie-shaped cut.

    Once you get the ring in the cylinder, you need to ensure the ring is square in the bore. If you're using flat-top pistons, it can be as easy as using a piston, upside-down, to square up the ring. Ring squaring tools are relatively inexpensive, so you might want to look into using one. If you can afford to buy some tools, I also recommend a taper ring compressor and a quality pair of ring installing pliers. Be sure you have a high quality set of feeler gauges on hand and you will want something that will get very small, down in the 0.0005" range.

    Once you get the ring squared up in the bore, get your feeler gauges out and see what you have for end gap. Determine what the ring manufacturer is calling for in gap, for your particular application. Different applications are going to call for different gaps, so do your homework.

    An old rule-of-thumb for a street application with a cast ring would be 0.004" end gap for each inch of bore. A 4.000" bore engine would use a 0.016" end gap on the top ring. The second ring should have 0.005" end gap for each inch of bore, giving that same engine a 0.020" second ring end gap. Please note, that is a rule-of-thumb and your thumb might be completely different. A high performance engine that is going to generate more heat might want the top ring gap to be more in the 0.0045" to 0.005" range, with the second ring gap in the 0.055" range. For something running a blower, you might want to get the top ring gap set as wide as 0.007" and the second ring gap somewhere between 0.0063" - 0.0073". See how quickly things can change?

    In any instance, oil ring side rails want to be set at 0.015" end gap, no matter the bore or the application.

    Read what the ring manufacturer recommends for your application and stick to it.

    If you're looking to get a 0.016" gap, try fitting a 0.016" feeler gauge into the gap. You want to develop a feel for a slight amount of drag as you insert the gauge from the inside of the bore and a slight drag as you pull the gauge out vertically. If the gauge will not fit, pull the ring out and adjust the gap.

    GO SLOWLY! If you need to cut 0.006", remove 0.003" from each end, so you are keeping the gap squared up. If you're fortunate enough to have a end gap cutter with a dial indicator, you can rush up on the first 0.004", before you start making touch cuts. Manley Performance offers a ring end gap cutting tool (P/N 41833) you can clamp in a vice and turn by hand. I suggest buying an additional grinder blade (P/N 41817), so you have a spare. These tools are nice, because you can get the ring laying flat on the bed of the cutter, before you make the cuts. If you use a point file, remember you want to keep the ends square on both the vertical and horizontal planes. You do not want any pie cuts in either plane, because this will decrease ring sealing.


    See that pie cut at the end faces? If the ring has this issue, you need to get it squared up, before you can start setting the gap properly. If you get gap set correctly on the outside of that gap, the inside gap is going to be too wide. Set the gap correctly on the inside and you risk the outside ends butting with heat.

    Remove a touch of material, fit the ring back into the cylinder, squaring it up as you go and then check the gap again. You're likely going to go through this process 3-4 times for each ring, so don't get frustrated. Take your time, double-check everything and you'll get the job finished the right way.

    Once you get the top ring gap set, dress all the end edges with a very fine honing stone or some 400 grit sandpaper, wash the ring, dry it and place it with your number one piston. Be sure to keep that ring identified as the number one top ring, because it might not fit the other bores properly.

    Grab the second ring and fit its end gap, the very same way.

    Get the oil ring expander and slip it into the bore. With your fingers, pull the ring up through the bore. You want to feel a bit of drag as the ring moves, just enough so you can feel the finish of the bore. If you feel the ring catching on the finish, you will need to adjust the end gap. And this procedure requires you to work even more slowly and carefully. Using a pair of needle nose pliers, you want to bend the end tab back, ever so slightly. Remember, those tabs need to mate with one another, so be sure to bend each tab an equal amount, keeping the bends square.

    Once you have the expanders all fit to each bore, get the side rails out and cut them for 0.015" gap. Be sure to dress the edges and wash them, keeping them in order with the corresponding piston.

    Now it's time to check the ring grooves in the pistons. Take the number one piston and the number one top ring. With the ring slipped over your fingers, insert the outer edge of the ring into the top ring groove and roll the the ring around the piston a time or two. If you feel a tight spot, it's time to start looking for a burr on the ring or in the groove. Since you were so careful checking the rings before, examine the piston closely. If you spy a burr in the groove, wrap some 400 grit sandpaper around an undersized feeler gauge and LIGHTLY sand the burr out of the groove. Work slowly and keep checking things as you go.

    Now you can check the second ring and its groove, the same way.

    Wash the piston and the rings again. Cleanliness is next to Godliness.

    Now, with your hands or a pair of ring expanding pliers, fit the second ring into the groove. DO NOT TWIST THE RING INTO THE GROOVE! Twist the ring in and you will put a twist in the ring itself, which will guarantee the ring will never seat properly on the ring land.

    Check the ring manufacturer's recommendation for vertical clearance. With your feeler gauges, check this clearance, all the way around the ring and the piston. You are going to scoff, but when you start working with feeler gauge blades in the 0.0005" - 0.001" range, you will not want to hold these gauges in your hand for very long, as the heat of your body will transfer to the blade and it will change the thickness of the gauge. So check a ring and lay the blades down to cool a bit before checking the next ring and groove.

    When you are finished, take the rings back off the pistons and you have only one step left. Wash everything again. If you have a clean area set aside for engine assembly, move everything into that area, keeping everything in order. If not, number up some freezer bags and place each piston, along with its corresponding rings into the proper bag and seal it up, so it will remain clean.

    Something I found effective with the race engines was to actually identify what piston clearances and ring end gaps were set at for each engine. Use an engraving tool to engrave the data onto the bottoms of the pin bosses on the number one piston and then the next guy down the road will be able to see what you've done. Do not, under any circumstances use letter and number punches, as you will distort the pin bores by pounding on the bosses.

    Yes, this took you a bit of time. But now, you know, beyond a shadow of a doubt what your ring end gaps and vertical clearances are like. You have set each one of them, so you know they are correct and you will not have to worry about butting end gaps or having a piston grab a ring. You have ensured ring seal will be the best it can be. You have ensured the rings will effectively remove oil from the cylinder walls, to prevent any risk of detonation. Which means the ring set will let you make as much power as possible. And all it cost you was a bit of your time.
    Spanky and smokeyco75 like this.

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