Cummins/Onan — Cylinder Block Repair

Repair & overhaul

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

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Procedure

1. A hone can be used to rebore a cylinder. Remove stock to 0.002 inch undersize of finish bore with coarse hone (100 grit), then complete honing with finish hones (300 grit).
2. Anchor block solidly for either vertical or horizontal honing. Use a drill press or heavy-duty drill which operates at approximately 250 to 450 rpm.
3. Lower hone into cylinder until it protrudes 1/2 to 3/4 inch past end of cylinder. Rotate adjusting nut until stones contact cylinder wall at narrowest point.
4. Turn hone by hand. Loosen adjusting nut until hone can be turned.
5. Connect drill to hone and start drill. Move hone up and down in cylinder approximately 40 cycles per minute. Usually bottom of cylinder must be worked out first.
6. Check diameter of cylinder regularly during honing. Check size at six places in bore; measure twice at top, middle and bottom at 90 degree angles.
7. When cylinder is approximately 0.002 inch within desired bore, change to fine stones and finish bore. Crosshatch formed by scratching of stones should form an angle of 23 degrees.
8. Clean cylinder block thoroughly with soap, water and clean rags. Do not use solvent or gasoline.
9. Dry crankcase and coat it with oil.

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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