How to read an ACDelco spark plug reference

ACDelco identifies plugs by a part number plus a specification sheet (dimensions). Here is how to read the code and the ranges.

An ACDelco code such as R44XLS or 41-101 combines a resistor mark, a heat/reach class and design-feature letters. ACDelco also publishes the exact dimensions (reach, gap, insulator diameter) per part number.

1. R — resistor

An R at the front marks a resistor-style plug (interference suppression). Most modern ACDelco plugs are resistor type.

2. The number — heat / reach class

The digits identify the plug type (heat range and reach class) within the range. Compare with the equivalents on the page to confirm the exact fitment.

3. Suffix letters — design features

Trailing letters (e.g. X, L, S, C) mark design features such as projected tip, extended/long reach, special gap or copper core.

4. Ranges

5. Dimensions

ACDelco lists, per part number: insulator barrel diameter, installed height, reach (mm and inch), spark location, gap and terminal type. Unlike NGK or Champion, ACDelco does not use a strict letter-by-letter key — always confirm the fitment with the listed equivalents.

6. Tightening torque

Torque depends on the seat, thread diameter and cylinder-head material. Flat seat (with gasket): 12 mm 15–25 N·m (iron) / 15–20 (alu), 14 mm 25–35 / 25–30, 18 mm 35–45 / 35–40. Taper seat: 12–14 mm 10–20 N·m. Always follow the value printed on the box.