Mark Houlahan
Tech Editor, Mustang Monthly
March 13, 2012

Today, some 46 years later, the K-code "Hi-Po" Mustang is still one of the most sought after classic Mustangs. With just three years of production and something less than 13,000 produced, the K-code certainly has higher numbers than, say, a Shelby Mustang. But when you consider the overall production numbers of the '65-'67 Mustang, the K-code represents barely one percent of production. That's what makes the K-code rare.

The K-code option has had many stories written about it over the years, most based on accurate information, but there have been some discrepancies over the years as information is gathered from various places. It doesn't help that, like most Mustang production information, Ford often made assembly line changes during production shifts just to keep the assembly line moving. Thankfully, the K-code engine itself, built at Ford's Cleveland Engine Plant No. 1 in Brooke Park, Ohio (now home to Ford's EcoBoost V-6 engine), had very few production changes.

With the help of Bob Mannel, author of Mustang & Ford Small Block V8-1962-1969, we've delved deep into Bob's book, original Ford literature, archive photos, and even tore apart a K-code engine ourselves locally that was in need of a rebuild. Along the way, we learned plenty of K-code details and have a new-found appreciation for the performance small-block that Ford initially offered in the '63 Fairlane and eventually installed in Mustangs, as well as the Shelby 289 Cobra, G.T. 350, and several other vehicles. We hope this inside look at the Mustang's K-code 289 Hi-Po engine answers your questions, helps you find that K-code Mustang you're looking for, or assists you in building an accurate Hi-Po 289 for your restoration project.

Engine Block

We'll start with the foundation for every 289 Hi-Po: the engine block. Let's get one thing out of the way right at the start--the Hi-Po block was NOT a special high-nickel block or cast just for Hi-Po builds. The Hi-Po block started life at the same foundry with the same casting numbers as regular 289 D-, C-, and A-code blocks. While there are earlier Hi-Po casting numbers, the one to concern yourself with for '65-'67 Mustangs is C5AE-6015-E (and C4OE-6015-F for '64-1/2). What made the Hi-Po block different was how Ford inspected blocks for Hi-Po use. Ford used a special dye to inspect the block for minute cracks and imperfections. Essentially, Ford used "perfect" 289 blocks as the starting point for the Hi-Po and then added the coveted larger heavy-duty main caps for strength.

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Other misnomers through the years include screw-in oil gallery plugs (a few early '63 blocks had them but it was Shelby that added them during his blueprinting for Cobra and Daytona Coupe use that started this rumor). Standard 289 blocks, including the Hi-Po, had standard 1/2-inch press fit plugs. Hi-Pos did not have screw-in core plugs either. As a matter of fact, the block had no discerning outward differences except that most had the vehicle's VIN stamped in the side of the block (regular 289s did not) and the often used "HP" designation in paint inside the bellhousing area, though not every Hi-Po block got this marking.