SKU: JEPJG3108-4000-7
UPC: 193564240842
MPN: JG3108-4000-7
The JE PISTONS JG3108-4000-7 is a file-fit piston ring set built for 4.000-inch bore eight-cylinder engines, sized for racing and modified street builds where ring sealing decides how much power makes it past the rings. Each order ships as eight matched sets covering a full V8, with ring thicknesses of 1.2mm top, 1.5mm second, and 3.0mm oil. File-fit construction lets a builder dial end gaps to the actual cylinder bore and intended use rather than settling for a fixed factory gap. This is a component for engine builders working at the precision end of the engine and component catalog.
Material selection follows the work each ring does. The top ring is carbon steel with a gas-nitrided treatment and plasma-moly inlay coating, a combination that holds combustion pressure and resists scuffing under high cylinder loads. The second ring runs an iron-phosphate finish in a Napier design, which scrapes oil down the cylinder wall more effectively than a plain second ring and reduces oil migration into the chamber. The carbon-steel chrome oil ring rounds out the three-ring pack. Low ring radials keep cylinder wall drag down, which matters in a high-rpm engine where every bit of friction is parasitic horsepower. Actual bore spec measures 4.005 inches per web research, with the 4.000 figure naming the nominal size. Builders pair these with matching forged pistons and a fresh set of bearings.
Reach for this set when you are assembling or freshening a 4.000-bore V8 for track or aggressive street duty and want control over end gap. File fit means the gaps get set to the bore using a ring filer, so plan that step into the build. With the rings sorted, the rotating assembly comes together with new rod bearings and a main bearing set to finish the bottom end. The set carries a Prop 65 warning and is made in the United States. Browse the rest of our piston ring sets if you need a different bore size for your build.
The JG3108-4000-7 is sized for a 4.000-inch nominal bore, with actual ring specs measuring 4.005 inches per web research. Because it is a file-fit design, the end gaps are set to the actual finished bore of your block during assembly.
The order includes eight matched sets, enough to cover a full eight-cylinder engine. Each cylinder gets a 1.2mm top ring, a 1.5mm second ring, and a 3.0mm oil ring, sold together as a matched set for consistent sealing across the bank.
File-fit rings ship with end gaps slightly tighter than final spec so the builder files each ring to the exact gap the cylinder bore and application call for. This requires a ring filer and a gap measurement step, but it lets you tune sealing to your engine rather than accepting a fixed factory gap.
The Napier second ring has a hooked lower edge that scrapes oil down the cylinder wall more effectively than a plain ring. That improves oil control and reduces oil migration into the combustion chamber, which matters in high-rpm and high-load applications where oil consumption and ring seal directly affect power.
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