How do you tell if a small block Chevy is a 4 bolt main?
The quick answer: count the main-cap bolts. Four bolts per cap means a 4-bolt main; two bolts per cap is the standard 2-bolt setup. This article walks you through practical ways to verify the configuration, using both visual checks and reference data.
Visual verification: main-cap bolt pattern
In the field, the simplest way to identify a 4-bolt main is to inspect the main bearing caps themselves. Here’s how to proceed.
- Count the bolts securing each main cap. If you see four bolts per cap, you’re looking at a 4-bolt main. If there are only two bolts per cap, it’s a 2-bolt main.
- Check all four main caps for consistency. A genuine 4-bolt main block should show four bolts on each cap; an inconsistent pattern across the caps may indicate replacements or modifications.
- Access may require removing the oil pan or lifting a splash shield to view the underside of the block. Be cautious to avoid damaging seals or gaskets.
- Understand that aftermarket or rebuilt blocks can alter cap configurations. While bolt count is the strongest indicator, confirm with casting data when possible.
Conclusion of the visual check: a true 4-bolt main block will display four bolts per cap across all mains, whereas a standard stock 2-bolt block will show two bolts per cap.
Verifying with casting numbers and codes
When the bolt pattern isn’t visible or you want documentation, use the block’s casting number and any suffix codes. This helps confirm whether a block is a 4-bolt main design and provides year/model context.
- Locate the block casting number, typically cast into the block on the passenger side rear, near the bellhousing, or in a similar accessible area. Some blocks display the number near the lifter gallery or timing area.
- Record the casting number (and any suffix code) and consult a reputable Chevrolet engine casting database or factory catalogs to identify whether that casting corresponds to a 4-bolt main variant.
- Cross-check year and model context. GM produced 4-bolt main small blocks for certain high-performance applications in specific eras; databases will help confirm which configurations were offered and when.
- If the data is unclear or you’re evaluating a used engine for a build, have a qualified machinist inspect the block to verify main-cap integrity and journal sizes.
Relying on casting data provides a robust second line of verification, especially on engines that have been rebuilt, swapped, or re-capped.
Practical notes and caveats
What to watch for in the real world
- Some engines may have had main caps swapped or reworked during rebuilds, which can complicate a quick identification. Verify both the bolt pattern and the casting data for accuracy.
- Aftermarket blocks can mimic the 4-bolt pattern while using different main-journal dimensions or cap designs. For accuracy in a build, measure bearing bores and confirm cap dimensions with the machinist.
Summary
Identifying a 4-bolt main on a Chevrolet small-block primarily comes down to two checks: the bolt count on each main cap and the block’s casting information. Four bolts per cap is the definitive physical sign of a 4-bolt main; two bolts per cap indicates a 2-bolt main. When possible, corroborate with the block casting number and suffix code in a reliable reference to confirm the configuration and avoid misidentification, especially on engines that have been rebuilt or swapped. A trained engine builder can provide an expert assessment for any high-performance planning.
