Cummins/Onan — Piston and Rings Service

Repair & overhaul · Pistons and rings

From a Cummins/Onan service manual — check it applies to your exact model before use.

Applies to: Cummins/Onan MCCK

Procedure

1. Remove wear ridge with ridge reamer before removing pistons.
2. Turn crankshaft until piston is at bottom of stroke. Remove nuts from connecting rod bolts.
3. Lift rod bearing cap and push rod and piston assembly out top of cylinder with handle end of hammer.
4. Keep connecting rod bearing caps and bearings with their respective rods.
5. Remove piston rings using a piston ring spreader.
6. Clean piston ring grooves with groove cleaner or end of broken ring filed to sharp point.
7. Clean rod bores and back of connecting rod bearings thoroughly.
8. Mark each piston to ensure rod assembled on same piston. Remove piston pin retainer and push pin out.
9. Inspect pistons for fractures at ring lands, skirts, pin bosses.
10. Check wear at ring land using new rings and feeler gauge.
11. Replace pistons showing bad scoring, burring, excessive skirt clearance, wavy or worn ring lands, fractures or detonation damage.
12. Replace piston pins showing fractures, scored bores or bores out of round more than 0.002 inch (0.051 mm).
13. Use new piston pin to check pin bushing in connecting rod for wear.
14. Before installing new rings, check ring gap by placing ring squarely in cylinder at bottom of travel.
15. Space each ring gap one third around piston; no gap in line with piston pin. Bottom groove oil control ring with expander; two upper compression rings.

Full service-parts list: Cummins/Onan MCCK →

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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