Cummins/Onan — Valve Clearance Adjustment, Four-Cylinder JC
Adjustments · Valve train
From a Cummins/Onan service manual covering: 10.0JC, 12.5JC, 15.0JC, 4.0JB, 5.0JB, 6.0JB, 6.1JB, 7.5JB, 7.6JB. Check it applies to your exact model before use.
Applies to: Cummins/Onan 10.0JC, Cummins/Onan 12.5JC, Cummins/Onan 15.0JC, Cummins/Onan 5.0JB, Cummins/Onan 6.0JB, Cummins/Onan JB engine, Cummins/Onan JC engine
Before you start
- Early four-cylinder engines do not have a BC mark on the flywheel.
Procedure
- Adjust valves in firing order 1-2-4-3 after cooling period; set timing and clearance for cylinder 1.
- For cylinder 2, turn flywheel clockwise 180 degrees (1/2 revolution) to between 10 and 45 degrees past BC mark.
- Adjust number 2 cylinder valve clearance.
- For cylinder 4, turn flywheel clockwise 180 degrees to between 10 and 45 degrees past TC mark; adjust clearance.
- For cylinder 3, turn flywheel clockwise 180 degrees to between 10 and 45 degrees past BC mark; adjust clearance.
Tools required
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- Intake .012 in. / Exhaust .015 in. (Begin Spec D)
Full service-parts list: Cummins/Onan 10.0JC →, Cummins/Onan 12.5JC →, Cummins/Onan 15.0JC →, Cummins/Onan 5.0JB →, Cummins/Onan 6.0JB →
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.