Are 350 and 454 motor mounts the same size?
In short, no—the 350 small-block and 454 big-block motor mounts are not the same size and are not directly interchangeable in most configurations. This article explains the differences, why they matter for swaps, and how to verify compatibility for a given vehicle.
Why the sizes differ
Motor mounts are paired to the engine block and frame, and the 350 small-block and 454 big-block use different mounting pad spacings and brackets. The 350 typically uses a narrower mounting arrangement, while the 454's mounts are built to fit the larger, heavier block, which changes the distance between the mounting holes and the cradle geometry. That means you can't simply swap the mounts or mount brackets without addressing the entire fitment geometry.
- Mount pad spacing on the engine block is different between small-block (SB) and big-block (BB) engines
- Frame-side mounting brackets have different bolt patterns and locations
- Overall engine height and clearance for exhaust, intake, and accessories can differ
- Exhaust manifold clearance and steering/axle geometry may be affected
Because of these differences, swapping a 350 into a chassis that originally used a 454—or vice versa—typically requires BB- or SB-specific motor mounts, and may necessitate crossmember relocation or adapter kits. Always verify part numbers and fitment for your exact vehicle and model year.
Interchangeability myths
Some aftermarket catalogs propose universal or cross-block mounts, but these are generally for particular swap kits and are not a universal, one-to-one replacement across all models. Always cross-reference vehicle year, model, and drive configuration.
What to check before attempting a swap
To navigate compatibility, consider the following steps.
- Identify the exact engine family and generation (SB Chevy 350 vs BB 454) and confirm the vehicle’s year and chassis type
- Compare the engine-block mounting pad spacing and the frame bracket pattern to determine if SB mounts or BB mounts are required
- Look up OEM or aftermarket motor mount part numbers for your specific vehicle and engine combination
- Inspect the frame, crossmember, exhaust routing, and steering components for potential clearance issues
- Consider crossmember relocation or using a purpose-built swap kit if necessary; consult a professional if unsure
If you are unsure after checking these items, consult a professional to avoid misalignment, engine movement, or clearance issues.
Bottom line
350 and 454 motor mounts are not the same size and cannot be assumed interchangeable. For any engine swap or mount replacement, match the mounts to the engine family and vehicle year, and verify part numbers with manufacturers or a trusted auto shop.
Summary
Key takeaway: When swapping engines or replacing mounts, don’t assume compatibility. Measure the bolt pattern and mount spacing, verify clearance for exhaust and accessories, and rely on OEM or kit-specific guidance to ensure a safe, proper fit.
Are all motor mounts the same size?
There are all kinds of engine mounts that all come in different sizes and types, selecting the right one can be a daunting process. However, through customization processes and analysis, the engine mount can be designed to meet certain specifications and optimize performance of the system.
Are BBC and SBC motor mounts the same?
Both sbc and bbc use the same motor mounts and bellhousing layout and the distance from CL of motor mount to bellhousing is the same dimension. Only difference is a big block is about 2" longer in front of the motor mounts. So your radiator/fan spacing will be more, especially if ya want a blower.
Will a 454 bolt in place of a 350?
basically EVERYTHING is different. however since some of them came with 454s, its still a "bolt in" swap since all the things needed do exist. you may even need to change front springs as a big block is 250lbs-300+lbs heavier, though an aluminum intake and water pump help....
Will a TH350 bolt up to a 454?
Yes, a TH350 will bolt to a 454 engine, but you must use a flexplate that is specifically designed for a 454 because it is an externally balanced engine. Additionally, be aware that while the transmission will bolt up, the TH350 is not built to handle the high torque of a 454, so it may be short-lived if you plan on heavy use like towing or racing.
Key compatibility requirements
- Flexplate: The most critical part is the flexplate. You need one that is balanced for the 454 and has the correct bolt pattern to connect to the TH350's torque converter.
- Engine mounts: The engine mounts from a small-block are compatible with the big-block 454, as per First Generation Monte Carlo Club.
Performance and longevity
- Durability: A TH350 is not as strong as a TH400 and can be prone to failure under the stress of a 454, especially with frequent hard acceleration or towing.
- For light use: If the vehicle is a daily driver or used for light duty, a TH350 can work, but it's wise to be prepared for potential issues and drive conservatively, according to First Generation Monte Carlo Club and Reddit users.
- Upgrade option: For heavy-duty applications, a TH400 or other heavy-duty transmission is a more suitable choice for long-term reliability, notes Facebook users.
