How to tell if a motor is a 350 or 305?

[LeEmma]

The most accurate way to identify a Chevy 305 (5.0L) versus a 350 (5.7L) is to locate and decode the engine's casting number or stamped suffix code. These numbers, found on the block itself, provide definitive proof of the engine's original displacement, year, and application, eliminating guesswork.

For a guaranteed identification, find the casting number on the rear driver’s side of the engine block, near the bellhousing mating surface. This number is cast into the metal during manufacturing. Common examples for 305 blocks include 14010201, 14093638, or 10066036, while 350 blocks might be 3970010, 14016379, or 10243880. Cross-reference this number with a reliable Small Block Chevy identification guide or database.

Additionally, check the stamped suffix code on a machined pad located on the front passenger side of the block, just ahead of the cylinder head. This code, often a combination of letters and numbers, specifies the exact engine variant as installed by the factory. For instance, a code ending in "H" or "HE" often denoted a 305 in certain years, whereas "CLB" or "CJM" could indicate a 350. This method is highly authoritative as it comes from the factory's own records.

If the engine is disassembled, physically measuring the cylinder bore is conclusive. A true 350 has a cylinder bore diameter of 4.00 inches, while a 305’s bore measures 3.736 inches. This 0.264-inch difference is a fundamental design distinction.

Identification Method	Chevy 305 (5.0L) Key Indicator	Chevy 350 (5.7L) Key Indicator
Primary: Casting Number	Look for numbers like 14010201, 14093638.	Look for numbers like 3970010, 14016379.
Secondary: Stamped Code	Suffix codes vary; consult a decoder guide.	Suffix codes vary; consult a decoder guide.
Definitive: Bore Diameter	3.736 inches (94.9 mm).	4.00 inches (101.6 mm).
Visual Clue (Later Blocks)	May have "5.0" cast at the rear.	May have "5.7" cast at the rear.
Component Hint	Often uses a thicker harmonic balancer.	Typically uses a standard balancer.

Some physical differences can offer clues but are not absolute identifiers. Many 305 engines use a thicker harmonic balancer. While some forum discussions suggest oil dipstick location (passenger vs. driver side) as a clue, this is unreliable due to variations across vehicle models and years. Cylinder head characteristics, such as smaller 58cc combustion chambers or smaller intake valves (1.72") on some 305 heads, can hint at the engine size, but heads are often swapped, making this a secondary clue at best. Always rely on the block's own numbers or physical measurements for a trustworthy verification.

[Angelo]

As a mechanic who’s seen hundreds of these engines, I always go straight for the casting number. Grab a flashlight and look at the back of the block, on the driver’s side, right where it meets the transmission. It’s grimy back there, so wipe it clean. Those raised numbers tell you exactly what you have. Forget the dipstick rumor—that’s a great way to get it wrong. If the heads are off, just measure the bore with a caliper. The 350 is a full 4 inches. No guesswork needed.

[LeChase]

When I’m restoring a classic Camaro or truck, verifying the engine is correct for the year is everything to me. The stamped suffix code on the front passenger side is my bible. It confirms not just the 305 or 350 size, but also the original horsepower and which car it left the factory in. I’ve learned that while “5.0” or “5.7” cast on newer blocks is helpful, it’s not on all of them. The harmonic balancer thickness is a decent visual tip in a junkyard—the 305’s is usually chunkier. But for my build sheet, only the factory codes count.

[OWeston]

Here’s the simple breakdown from a parts counter perspective:

1. Best Bet: Find the casting number. Decode it online.
2. Quick Check: Look for a stamped code near the front passenger-side head.
3. Sure Thing: Bore measurement.
4. Heads-Up: Don’t use the oil dipstick location to decide. You’ll order the wrong gasket set. A 305 and a 350 look almost identical. The main differences are inside the cylinders or in those numbers. If you’re buying one, insist on seeing the codes. It’s the only way to be sure you’re getting the cubic inches you’re paying for.

[SanJeremiah]

I was swapping an engine into my old Chevelle and needed to confirm it was a 350, not a 305. The guy selling it swore it was a 350. I checked the back of the block and found the casting number: 3970010. A quick search on my phone at the salvage yard showed that was a classic 350 block from the late ‘60s and ‘70s. I also noticed the harmonic balancer looked like the slimmer type. The stamped code was weathered, so the casting number was my hero. It saved me from a costly mistake. The lesson? Trust the permanent metal stamps, not the verbal guarantees. Always have a decoder guide bookmarked on your phone—it’s the most important tool you’ll need for this job.