What is the biggest size Engine that will fit in a 1955 Chevy Belair convertible?

There's no single definitive answer to the biggest engine that will fit in a 1955 Chevy Bel Air convertible. It depends heavily on what you consider "fit." Simply fitting the engine block is different from fitting the engine with all accessories, exhaust, and considering the necessary modifications.

While some have successfully installed big-block Chevy engines (like a 454 or even a 502), it requires significant modifications:

* Firewall Modifications: The firewall will almost certainly need significant modification or replacement.

* Suspension Modifications: The extra weight of a large-block engine may require suspension upgrades to handle it properly.

* Steering and Exhaust: Steering clearance and exhaust routing become significant challenges with larger engines.

* Transmission and Drivetrain: You'll need a transmission and driveshaft capable of handling the power.

In short, while *technically* a very large engine *might* be crammed in, practically speaking, a significant amount of custom fabrication would be necessary. A big-block Chevy is likely the largest that would be considered reasonably feasible, even then with substantial work. Anything significantly larger would likely be impractical and require extreme modifications, possibly compromising the car's structural integrity.