It is important that the piston ring set and the piston be engineered to complement each other from a standpoint of clearance between the back of the piston ring and the bottom of the piston groove. As an example, a 4.030 KB hypereutectic piston requires a 0.0065-inch per inch of bore – or 4.03 x 0.0065 = 0.026-inch. Very interesting. The space behind the ring is calculated by subtracting the ring radial wall thickness from the root depth of the ring groove in the piston. I would also go along with the comments that a round true bore is much more important in keeping a cylinder sealed than pinching a couple thousands of ring end gap. Minimum recommended clearance for intake & exhaust valve would be 0.100" deep and.050" radially. I follow the .004 per inch of bore, for N/A engines, and the ring manufactures recommendations across the board for nitrous, blown, etc. The concern that increased back clearance will adversely affect engine performance has been dismissed by engine testing. I have had engines leak as much a 40% with no real impact on performance, at least on the drag strip, but I know it is getting worn. SPEED-PRO PISTON RINGS - DESIGN, FEATURES, and. Safe top ring end gaps can be found by multiplying the bore diameter by the appropriate ring end gap factor from the clearance chart below. How much ring gap? Depending on the application, the vertical clearance is typically 0.001-0.003-inch. Re: Top piston ring gap - whats really required? by Nut124 » Sun Oct 04, 2020 1:58 pm, Post The second compression ring reinforces the top ring. Optimum back clearance for a racing application is.000". I would hone cylinders to different surface finishes, ↳   Vintage/Classic/Historical Engine Tech, ↳   3V Performance Engine & Valve train Development, ↳   Race Cars & Parts - For Sale or Wanted. Side Face. Top piston ring gap - whats really required? Insufficient clearance can cause the piston to seize in the cylinder. A hypereutectic piston _____. by german4inline » Sun Oct 04, 2020 1:46 pm, Post I have a vacuum tester used for checking valve seal, it has a disc big enough to vacuum check a cylinder on say like a 350 chevy, with the head off. The amount that the face of the ring is below the ring lands is the ring back clearance. The broken piece may cause further piston or engine damage. A back clearance of 0.008-0.012 inch is typical for racing and high-performance pistons. by rebelrouser » Mon Oct 05, 2020 9:50 am, Post If there is no back clearance, the rings take the side thrust. Many may say I am not correct, but I use a cylinder leak tester on my engines on the stand before they are fired, and anything over 20% leak, I take it back apart and find out why. by bigmike » Mon Oct 05, 2020 12:27 am, Post If the axial height of the ring is reduced so that a large clearance is found. It should not protrude beyond the lands when the ring is bottomed in the groove. .0045" to .0050" per inch of cylinder diameter is safe and will work very well. With today's thin rings , some racers run side axial clearance as tight as 0.0004 - 0.0005-inch, using gas ports to supplement ring pressure. There are two compression rings. http://www.youtube.com/user/MaggiFinn1/videos, https://static.summitracing.com/global/ ... 902-30.pdf, https://www.youtube.com/watch?v=cxn-LxwsrnU. uses about 16% silicon and is a cast iron piston. All rings use cylinder pressure behind the top ring to increase sealing load on the ring. B: Ring width (h1) 8. When I give the customer their engine, the cylinder leak is noted on the build sheet, I strongly recommend to my customers the leak tester as a way to gauge the health of the engine. 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