Factory big-block K-members for late-'60s B-Bodies aren't exceptionally rare, unless it is a Hemi K-member because of their altered perches and bracing. Otherwise, the standard big-block K-member bolts to all 383s, 400s, and 440s and was found in all B-Bodies, be it an R/T 440 four-speed Coronet, a 383-powered Road Runner, or a 400 b-block Plymouth wagon. Performance and suspension upgrades were only manifested in stronger torsion bars, bigger brakes, and anti-roll sway bars, uneffecting the engine cradle itself.
Though many will want to sell you on the rarity or exclusivity of any K-member, the truth is unless you're building a 100-percent perfect, concours restoration, the differences between one K-frame and another are almost indistinguishable. With that in mind, we didn't feel too bad hacking up the original engine cradle for our '69 Dodge Charger project car.
The factory K-frame came about during the development of the unitbody design. While GM was struggling with eliminating body twist and structural flex in their chassis platform vehicles, Chrysler and Ford were exploring the design benefits of incorporating the chassis or frame into the body components. While forward and rear framerails protruded from the central cab, certain crossmembers were used to tie the exposed rails together. While GM's powerplants were married to the chassis, both Ma Mopar and the Blue Oval worked at creating separate engine cradles that not only wielded the motor mounts, but also housed the lower control arms (while the upper A-arms were mated to the inner fenderwells and forward framerails). Assembly images showed how complete K-frames with the engine and transmission were married to the vehicle when the body was lowered down from the elevated assembly line. This design of a complete engine cradle bolted to the framerails drastically improved the vehicle's rigidity, as engine torque was dissipated not only via the engine mounts through the K-member, but also through the framerails, unlike the GM chassis design that suffered extensive cracking, twisting, and distortion under the torque of their larger "rat" motors.
Factory K-frames were made from two pieces of stamped and molded heavy-gauge sheetmetal, spot-welded together into the characteristic "K" shape from which it got its name. Thick plates were welded on for the steering box perch and motor mounts. Three K-members were produced for the B-Body line, an engine cradle made specifically for small-block and Slant Six, B and RB engines, and a heftier Hemi K-frame. Due to the obvious rarity of Hemi-powered vehicles, those purpose-built K-members are significantly more desirable, as mentioned previously. the stamped design of factory K-frames are surprisingly strong and are credited with withstanding upwards to 1,000 hp in some Pro Stock race cars with the minutest of modifications. Many racers still retain their factory motor mounts while incorporating an engine plate for extra integrity.
Though unnecessary, we took...
Though unnecessary, we took a wire cup wheel on a grinder and a smaller buffer wheel to strip the K-member as clean as possible before we began the fabrication.
Since we had a short delay...
Since we had a short delay between our prep stages and the day we would be able to jump into our fabrication, we decided to shoot the naked engine cradle with a quick coat of primer and semigloss black paint. If modifying your K-frame isn't up your alley, spending a Saturday cleaning up the stock K-member will also prove worthwhile.
Finally ready to start cutting...
Finally ready to start cutting into our piece, we used a cutting wheel on a hand grinder to sever the larger portions of the engine-mount bracing and break the factory welds. If you plan to retain your factory engine mounts, leave them. Since our cutting wheel wasn't able to reach in between those tight places, we decided a little brute force would suffice. A simple see-saw action snapped the 37-year-old welds loose.