How big is a 168 tooth flywheel?
The size isn’t fixed by the tooth count alone. A 168-tooth ring gear or flywheel refers to the number of teeth that mesh with the starter motor, but the actual diameter depends on the tooth pitch (the size of each tooth). Depending on whether the gear uses a metric module or an imperial diametral pitch, the pitch diameter can vary significantly. In practical terms, you’ll see pitch diameters roughly spanning from about 6.5 inches (170 mm) to about 20 inches (510 mm), with the outer diameter always a bit larger due to tooth geometry. For a precise measurement, consult the vehicle’s specifications or measure the part directly.
What 168 teeth tells you about the part
In most cars and light trucks, a 168-tooth designation refers to the starter-ring gear that attaches to the flywheel. The key point is that the tooth count indicates the circumference on the pitch line, not the exact outer size. The ring gear must match the starter pinion and the engine’s flywheel geometry, so variations exist across makes and models.
- The tooth count corresponds to the pitch diameter when you know the tooth size (pitch).
- The ring gear must match the starter motor’s pinion gear: the count helps ensure proper engagement and timing.
- Outer diameter depends on tooth height and overall gear geometry, not just the number of teeth.
Understanding these points helps explain why a 168-tooth ring gear can have different outer diameters depending on the engine family and manufacturer.
Estimating size from tooth pitch
To estimate the physical size of a 168-tooth gear, you need the gear’s tooth pitch. There are two common systems: metric (module) and imperial (diametral pitch). Here are the two standard approaches.
Representative metric-module values illustrate how the pitch diameter changes with tooth size:
- m = 1.0 mm: pitch diameter Dp = 168 × 1.0 = 168 mm (6.6 in); outside diameter Do ≈ Dp + 2m = 170 mm (6.7 in).
- m = 1.5 mm: Dp = 252 mm (9.92 in); Do ≈ 255 mm (10.0 in).
- m = 2.0 mm: Dp = 336 mm (13.2 in); Do ≈ 340 mm (13.4 in).
- m = 2.5 mm: Dp = 420 mm (16.5 in); Do ≈ 425 mm (16.7 in).
- m = 3.0 mm: Dp = 504 mm (19.8 in); Do ≈ 510 mm (20.1 in).
These figures show the broad range you can expect depending on tooth size in metric designs. The pitch diameter (Dp) is the primary size reference; the outer diameter (Do) is slightly larger due to tooth height. Use OEM specifications for exact figures on a specific engine family.
Alternate approach using diametral pitch
For gears designed to imperial standards, the diameter is calculated with the diametral pitch (DP): pitch diameter Dp = Z ÷ DP in inches. Some typical DP values and their implications are as follows:
- DP = 8: Dp = 168 ÷ 8 = 21 inches (533 mm).
- DP = 12: Dp = 168 ÷ 12 = 14 inches (356 mm).
- DP = 16: Dp = 168 ÷ 16 = 10.5 inches (267 mm).
The outer diameter will be slightly larger than the pitch diameter due to tooth height. Because addendum/dedendum and overall tooth geometry vary by manufacturer, these are approximate figures. Always verify with the vehicle’s service manual or the part’s specification sheet.
Practical guidance for identification
If you’re trying to identify or replace a 168-tooth flywheel/ring gear, follow these steps:
- Consult the vehicle’s service manual or parts catalog for the exact part number and dimensions.
- Measure the ring gear’s pitch diameter by counting teeth and determining the tooth pitch, or measure across the centers of selected teeth to infer pitch size.
- Match the measurement to OEM specifications to ensure compatibility with the starter motor and flywheel assembly.
In practice, the “168 teeth” designation is a constraint that must be paired with the correct tooth pitch for a proper fit. If you’re unsure, a professional cross-check with the dealer or a trusted repair facility is recommended.
Summary
A 168-tooth flywheel or ring gear does not dictate a single universal size. The pitch diameter depends on the tooth pitch (module in metric terms or diametral pitch in imperial terms). This yields a broad size range, roughly from about 6.5 inches to 20 inches in pitch diameter, with outer diameter a bit larger due to tooth geometry. For precise sizing, rely on OEM specifications or direct measurements of the part in question. The critical factor is matching the tooth pitch to the starter gear and the engine’s flywheel design, not just the tooth count.
