Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Special Engine Marks
Information identifying undersize crankshaft, oversized tappets, low compression, oversized cylinder bores, engine built that day and shift are stamped at various locations on block.
M - (M-2-3..) Indicate 0.001" (0.02 mm) undersize No. 2 & 3 main bearing journals.
R - (R-1-4..) Indicate 0.001" (0.25 mm) undersize No. 1 & 4 connecting rod journals.
M-10 - Indicate 0.010" (0.25 mm) undersize ALL main bearing journals.
R-10 - Indicate 0.010" (0.25 mm) undersize ALL connecting rod journals. These codes are stamped on center crankshaft counterweight.
A - Indicates 0.020" (0.50 mm) oversize cylinder bores. Located top of front pad right side of block.
- Indicates 0.008" (0.20 mm) oversize tappets. Located top of front pad right side of block.
O/S - Indicates 0.005" (0.12 mm) oversize valve stems. Located front of head on thermostat boss.
NO RELATED
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.