GM starter bolts do not have a single universal size. Common diameters you’ll encounter are 5/16 inch and 3/8 inch (UNC), and some modern GM engines use metric bolts such as M8x1.25 or M10x1.5. Always verify against your engine’s service manual or by measuring the existing bolts.
GM’s varied lineup across decades means the starter mounting hardware has changed with engine families, platforms, and production years. This article outlines the typical sizes you’re likely to encounter and how to identify the right bolts for your GM vehicle so you can replace them safely and correctly.
Common starter bolt sizes you may encounter
Below is a snapshot of the bolt sizes most GM technicians and DIYers run into. The exact size for your car depends on the engine family and year, so verify before replacement.
Key factors that influence bolt size
Engine family, production year, and market (US vs. international) determine whether the mounting hardware uses imperial or metric bolts, as well as the required length and thread pitch. When in doubt, consult the service manual or compare to the original hardware.
- 5/16-18 UNC mounting bolts — the most common size on many older GM V8s and small-blocks; lengths vary by starter and engine block.
- 3/8-16 UNC mounting bolts — used on some larger GM engines and in certain heavy‑duty applications; lengths vary by installation.
- Metric bolts: M8x1.25 or M10x1.5 — appear on some modern GM engines and non‑US market models; lengths vary by starter mounting thickness.
- Specialty or OEM kits may use alternate thread pitches or spacers; confirm by comparing to the original hardware or service literature.
In summary, you’re most likely dealing with either 5/16-18 UNC, 3/8-16 UNC, or a metric M8/M10 bolt depending on the engine generation. Always verify the exact size and pitch for your application.
How to identify the correct size for your GM engine
To avoid cross-threading or improper mounting, identify the correct bolts by measuring the existing hardware or consulting the vehicle’s service documentation. Here are practical steps you can follow:
- Remove the starter and inspect the mounting bolts for diameter and thread pitch.
- Measure the bolt diameter with calipers or a precise ruler; determine if the threads are imperial (UNC) or metric (M).
- Count threads per inch (for imperial) or measure the pitch with a thread gauge (for metric) to confirm the exact specification.
- If in doubt, bring the bolts or the starter to an auto parts store or consult the GM service manual to obtain the exact replacement bolt size and grade.
Always torque bolts to GM specifications and use bolts of the correct grade. If you’re unsure about the exact size, opt for OEM hardware to ensure a proper fit and safe operation.
Summary
GM starter bolts vary by engine family and year, with the most common sizes being 5/16-18 UNC, 3/8-16 UNC, and metric M8/M10 on newer models. To replace them safely, identify the exact size for your engine by checking the service manual or measuring the existing bolts, and use the correct grade bolts to ensure a secure mounting.