Cummins/Onan — Piston and Rings

Repair & overhaul

From a Cummins/Onan service manual covering: 0.6MAJ, 1.0MAJ, 1.5MAJ, 1.5MAJ-224R, 1.5MAJ-232R, 2.5MAJ. Check it applies to your exact model before use.

Applies to: Cummins/Onan 2.5MAJ

Procedure

1. Remove piston and connecting rod assembly through the top of the cylinder.
2. Clean carbon deposits from piston ring grooves.
3. Check oil return holes in lower ring groove are open.
4. Before installing new rings, check ring gap by placing each ring squarely in cylinder at bottom of its travel.
5. File rings slightly oversize as necessary to obtain correct gap; do not use rings requiring too much filing.
6. Standard size rings may be used on a .005 inch oversize piston.
7. Use .010, .020, .030, .040 inch oversize rings on corresponding oversize pistons.
8. Tapered rings marked TOP must be installed with mark toward closed end of piston.
9. Space each ring gap one third around piston, with no gap in line with piston pin.
10. Fit bottom groove with oil control ring and two upper grooves with compression rings.
11. Piston has full floating piston pin held by two lock rings; ensure lock rings are in place.

Source: Cummins/Onan service manual. Steps and values are as documented; always cross-check against the official manual and any later revisions for your exact serial number before servicing.

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