From the February, 2009 issue of Circle Track
By Joel Goodman
Photography by Will Handzel
There are so many details in putting together a race engine that it's hard to cover the entire process and expect you to totally understand. Instead, we're going to focus on one very specific step in engine building, installing piston rings, so I can be completely thorough, and you will understand.
The installation of piston rings is a critical step that really takes both artwork and science to end up with a combination that seals the combustion pressure above the piston, scrapes the oil off the cylinders so it doesn't contaminate the combustion process, and transfers heat from the piston into the cylinder wall.
Besides fit, we use different rings for different applications, use some coatings, and try anything that will get more power to the rear wheels. But the basics to obtaining the optimum ring endgap, oil rails/expander fit, and reading rings when they come out of the engine, will get you in the horsepower game.
Piston ring fitment is very important and therefore warrants this discussion.
1. Installing piston rings sounds like it should be a box-to-piston process, but there is more to it than that. Learn what it takes to install rings for minimum cylinder leakage and maximum durability.
2. Piston rings come as a set wrapped in plastic and shipped in boxes. No matter what kind of rings you buy--moly filled, chrome, steel, and so on--inspect them for chips, cracks, rust, or any other inclusions that might cause a failure or combustion pressure leakage once installed. If a ring is damaged in any way, I won't use it.
3. I like to wipe the rings down with lacquer thinner while inspecting them. Then, I sand all the edges of the rings with 400-grit sandpaper to knock off any rough edges. Use very light pressure and make only one or two passes over the edge. Red Scotch-Brite is then used to remove excessive coating or manufacturers film from the outer edge of the ring.
4. The next step is to install the ring in the appropriate bore, using a piston-ring squaring tool. Often, the piston rings won't fit in the bore out of the box because they are oversize. If this is the case, don't force or twist the ring, simply file a little material off the endgap and start the slow process of getting the ring sized right.
5. Many companies, like Powerhouse and ABS, offer piston-ring squaring tools. I had squaring tools made specifically for the pistons in our engines. They locate the ring where it will be around TDC of the stroke. Since combustion happens in this area. I like to set my end gap at this point. Also, due to cylinder distortion when the head is torqued down and the excessive heat generated at combustion, this is the area to establish the proper end-gap dimension.
6. Properly fitting piston rings into the block is a complex process that requires consistent measuring between all cylinders. It is a measured gap, but a certain feel is needed to get that gap right. I use a MAC FG013 feeler gauge with three sizes out (in this case, 0.020-, 0.021- and 0.022-inch sizes) and with the ring square in the bore. I try to slide the appropriate feeler gauge into the gap starting at the inside of the ring. If it doesn't fit, I go down in size until one does. I consider a gap size true when the appropriate feeler gauge drags slightly going in and out of the gap and when the gap is tight enough that once the gauge is in the gap, the tension will just barely hold the entire feeler gauge in place.
7. In case you don't know, cutting the ring gap should be done in very small amounts. This is a process that requires time because you need to sneak up on the perfect size. Take your time to avoid oversizing the end gap. If you need to remove 0.020 inch, take 0.010 inch from each side of the ring gap to prevent a poor mating of the two ends of the ring.
8. Before I attempt to cut the ring for the proper endgap, I always cut a couple of thousandths off each end of the ring and check how the ends mate. I pinch the ends together as the ring would be in the cylinder while looking into a light. If the rings does not mate perfectly flat across the end, I adjust the anchor point for the ring file and go through the process again until the two ends mate perfectly. This way, all the other rings for this engine will have good endgaps. This is to minimize combustion-pressure leakage and oil passing through this area.
9. This photo barely shows a sliver of pie-shaped light through the inside edge of the ring endgap. This is why it is important to adjust the ring file for end gap squareness in the beginning.
10. After all the iterations of checking the top and second ring in the bore, grinding on them, and rechecking them for size (measure and keep the rings organized for each specific bore), the next step is to size the oil rails and tensioners. These are truly a feel install. I start with the tensioner, installing it square in the bore and dragging it slowly down and up the bore. I like just enough tension so that the roughness of the bore is felt, but not so much that the tensioner is catching on the surface.
11.To adjust the expander size, I slightly, and I mean slightly, bend the endtabs back on each side of the expander. The tabs need to mate together after you bend the tabs, so don't bend one a bunch and not touch the other.
12. Once the rings are sized for the bores, I check them all in the pistons. Before that happens, I use 400-grit sandpaper and an undersize feeler gauge to lightly sand the top ring land to knock down any burrs caused during the drilling of the gas ports. I'm not trying to remove material, just prevent the ring from hanging up on a burr.
13. Installing the top and second rings on a piston should be done with care. I like to spread the ring evenly using both hands (as shown). My experience has shown that if you twist a ring on and off the piston, it puts a set twist in the ring, which can prevent it from seating properly in the ring land during engine operation.
14. I then check the vertical clearance with respect to the ring and ring land. This will be different for various engines. For our situation, I want 0.0005- to 0.0008-inch vertical clearance on a restrictor-plate engine and 0.0008- to 0.0012-inch vertical clearance on our open engines. I use 0.001- (ENCO PN 615-5001) and 0.0005-(ENCO PN 615-5000) inch feeler gauges to determine these. Don't hold these gauges in your hand for long; they will change thickness due to temperature very easily.
Read more: http://www.circletrack.com/howto/1818/index.html#ixzz1k3JlzJZB
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