In concept, the 409 was supposed to be simply a bored-out 348 with a longer stroke. The production unit, however, ended up having few interchangeable parts. Cylinder blocks and heads for both engines were machined on the same lines, though, which was a vital bit of help from the cost angle. In boring out the 348 block, Chevy carefully avoided any changes in the casting. For one thing, it was considered imperative to retain the full-circle water jackets around the bores. This limited the maximum bore increase to 3/16-inch (0.1875-inch), giving a final bore dimension of 4.3125 inches. Stroke was increased from 3.25 to 3.50 inches, which called for a new crankshaft. Both the 348 and 409 had forged-steel crankshafts, though the latter had longer crank throws. Bearing sizes were also shared - 2.50-inch diameter for the mains and 2.20-inch diameter for the crankpins. The 409's crank demanded heavier counterweights, however, and consequently weighed 8.2 pounds more for a total of 67 pounds.
To keep the same deck height, it became necessary to shorten the connecting rods. This had the drawback of increasing maximum rod angularity and therefore side thrust on the piston. While the 348 had permanent-mold cast aluminum pistons, the 409 was fitted with forged-aluminum pistons for greater heat resistance. The 348 employed offset pistons pins, which had the benefit of reducing piston-skirt slap after a cold start. That makes a truck engine more civilized, but has no real importance for a high-performance car. The 409, however, had no piston-pin offset, so there was no need for separate lefts and rights. In the 348, valve relief cavities were on opposite sides of the topland. The 409 pistons had milled valve reliefs, all on the same side.
Like the entire basic layout, the valve gear design for the 348 was taken from the small-block engine. This meant ballstud-mounted, stamped-steel rocker arms and valves arranged in a line at a 12-degree inclination above wedge-shaped combustion chambers. Cylinder heads for both the 348 and 409 were made from the same castings, but the 409 had a wider pushrod holes and different valve-spring abutment faces. It also had stronger valve springs than the 348 for sure closing at high rpm, as well as stronger, thicker pushrods. The 409 used a single coil spring with a flat steel damper per valve, while the 348 had dual valve springs. Both intake and exhaust valves on the 409 were inherited from the 348, the intakes measuring 2.066 inches across the head and the exhausts 1.72 inches.
As a reworked 348, the 409 naturally featured higher compression (11.25:1) and a wilder camshaft. Intake-valve lift was raised from 0.406 to 0.440-inch, and exhaust-valve lift from 0.412 to the same 0.440. Intake-valve opening duration was extended from 287 to 317 degrees, and overlap (the period during which both valves are open) from 66 to 70 degrees.
In 1960 the 348 had been offered with a triple two-barrel carburetor setup that boosted rated power from 340 to 350 bhp. No multi-carburetor manifolds were devised for the initial version of the 409, because its big four-barrel Carter had almost the same airflow volume as three deuces.
The 409 debuted as a mid-1961 option. In its most powerful form it delivered 360 bhp at 5800 rpm, and generated peak torque of 409 pounds-feet at 3600 rpm. It weighed 664 pounds, only 34 pounds more than the 230 cubic-inch Chevrolet six. A 409-bhp option with dual four-barrels by Carter became available for 1962. The following year, a full 425 bhp was claimed for this combination, thanks to an 11.0:1 compression ratio and solid lifters. The same engine with a single four-barrel Carter was rated at 400 bhp. The twin four-barrel option was discontinued at the end of the 1964 model year. For 1965, the hottest 409 was rated at 400 bhp. It sported a big-port aluminum intake manifold, 11.0:1 compression ratio, a high-lift/high-overlap camshaft with solid lifters, single four-barrel carburetor, and special low-resistance exhausts manifolds. A 340-bhp version was carried over unchanged from '64, with 10.0:1 compression, single four-barrel carb, and hydraulic lifters.