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1966-1977 Ford Bronco Engine - Stroking A Bronco

400 stampeding horses from a 302 block
By Steve Dulcich
Photography by Steve Dulcich
1966 1977 Ford Bronco Engine

1966 1977 Ford Bronco Block Prep
Block prep is the foundation for any good engine build, with machining quality directly relating to the performance and reliability. Our block was machined on Speed-O-Motive's Rottler F-65 automated CNC, which bored the cylinders, square-decked the block, and corrected the main bores, as well as performed the crankcase clearancing for the stroker crank to come. After machining, the block was cleaned and painted and the cam bearings installed.
1966 1977 Ford Bronco Crank Shaft
The backbone of our power plans is a Scat 3.40-inch-stroke crank to replace the 3.00-inch stocker. The longer stroke will pack 45 more cubes into our little 302 Ford block, adding a serious increase in torque potential to the little Ford.
1966 1977 Ford Bronco Fullradius Fillets
While stock cranks are undercut in the journal radii, like any good high-performance or race crank the Scat stroker piece features full-radius fillets in the journals, adding fatigue resistance and strength. To ensure adequate clearance at the fillets, performance bearings are manufactured with a larger edge chamfer.
1966 1977 Ford Bronco Piston
In order to allow the stroker crank to work in the original block deck height, special pistons are required with a shorter 1.09-inch compression height, measured from the centerline of the piston pin up. These KB Silvolite No. 730 forged stroker pistons are dished 16.5cc for a moderate 9:1 compression ratio, just what we were after to allow our 347 to ingest whatever low-octane fuel we might encounter in the wild.
1966 1977 Ford Bronco Long Rods
To link the forged crank and pistons in a bulletproof assembly, the stock 5.090-inch-long rods were upgraded with a set of 5.40-inch Scat forged 4340 H-beams. Besides the obvious strength benefit, the longer rods' better rod/stroke ratio decreases cylinder wall side loading.
1966 1977 Ford Bronco Counter Weight
Stroker 347 combinations are typically externally balanced, which requires a counterweighted crank damper. An economical Professional Products damper was used for this application, providing the necessary balance factor via a bolt-on eccentric weight.
1966 1977 Ford Bronco Pin Installation
The rods are bushed for floating pin installation of the pistons, retained with Spirolox. The piston rings are SpeedPro's file-fit plasma moly, while the bearings are from Federal Mogul.
1966 1977 Ford Bronco Rings
File-fitting rings is a science unto itself. The rings were clearanced to specs at Speed-O-Motive on a power ring-filer, though low-cost manual ring filers are available from mail order suppliers for the do-it-yourselfer. Ring gaps were set to 0.018-inch, top, and 0.022-inch, second.

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1966 1977 Ford Bronco Engine 1966 1977 Ford Bronco Crank Shaft 1966 1977 Ford Bronco Fullradius Fillets
1966 1977 Ford Bronco Block Prep

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