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Message: [QUOTE="glovemeister, post: 1542477, member: 3867"] Some reading on it: "Piston Ring End Gap On Ford/Navistar 7.3L T444E Diesel Engines The AERA Technical Committee offers the following information on piston ring end gap for Ford/Navistar 7.3L T444E diesel engines. This engine's second compression ring uses an increased piston ring gap. However uncommon, this engine does require a minimum second ring end gap of .062". This design is similar to earlier V-8 diesel engines built by this manufacturer. The minimum gap is larger than what is commonly seen for a compression ring end gap in other engines. Navistar uses the wider end gap to aid in oil control, and to lower engine emissions. Using a wider end gap in the second ring land evens out the pressures in the ring land area of the piston. AERA members are advised to adhere to the following piston ring end gap specifications when installing rings in a 7.3L T444E diesel engine. Top - .014"-.024 Second - .062"-.072" Oil Ring - .012"-.024"" A large second ring gap is supposed to help prevent top ring flutter and allows some compression to leak by the second ring to help prevent oil from migrating into the combustion chamber. Cylinder Bore Diameter*, Standard size: 4.1096-4.1103 in. * Maximum out-of-round — 0.0002 in.; maximum taper — 0.003 in. Piston Skirt Diameter**, Standard size: 4.10475 in. ± 0.00025 in. ** Measured at 90 degrees to the pin, at 1.684 in. below the oil ring groove at room temperature. From 2002 Shop Manual: " Ring Side Clearance (second ring only): 0.002" - 0.004" Ring Gap (Compression): Top= 0.014" - 0.024", Second= 0.062" - 0.072" Ring Gap (Oil): 0.012" - 0.024" Piston skirt diameter: 4.10475" +/- 0.00025, measured 90 degrees to the pin Cylinder bores - 4.1096 - 4.1103 Max out of round and/or taper: 0.0005 From a look it does appear that the out of round/tapers, and piston skirt diameter are slightly different. 97 is .0002 out of round, .0003 max taper. Piston Skirt 4.10475 in. ± 0.00025 in 99-03 Same skirt, but max .0005 max combined oor/taper. I then got to thinking the 99-03 has larger injectors/more boost, and that could be a reason. But seems weird given that the blow by is reduced more. My thought is that surely has to be related to emissions somehow, but I am failing to see where that would be? [/QUOTE]