6R09K.com

The 5^th digit of all 1964-1973 Mustangs designates the engine that the car left the factory with.  The 289 Hi-Po engine was characterized by the letter “K.” Thus, the reference to Mustangs with the Hi-Po engine as a K Code.

See this page for a complete break down of VIN information.

If you’re serious about purchasing a K code Mustang or already have one and don’t own this book, I strongly suggest it - The 289 High Performance Mustang by Tony Gregory, Third Edition. The 4^th Edition is currently being put together. You can order it here.

There are other unique perculiarities to the K codes, but for now since it's the lifeblood of the car, let's focus on the engine. The following is a very brief overview of the items specific to these engines:

Block – the 289 block is the exact same as the 65/66 A and C code engines.

Engine internals -

̃    Main caps – 15/16” instead of tapering to 5/8”

̃    Connecting rods – used heavier bolts and nuts

̃    Crankshaft – included unique hatchet and counter weight

̃    Camshaft – mechanical

̃    Solid lifters

̃    Pistons – aluminum flat top

̃    Narrow timing chain

Engine externals –

̃    Heads – screw in rocker studs and cast spring seats. Also will have 19,20, 21 cast in them

̃    Intake – same as A code 4v

̃    Carburetor – 1.12 venturi 600 cfm, manual choke

̃    Exhaust manifolds – different design for better air flow

̃    Water pump – 6 blades on impeller fan

̃    Fan – four blade fan with fomoco, SC, and 911960

̃    Alternator – larger pulley

̃    Motor mounts – for ’65 rubber insulators

̃    Valve covers – chrome

̃    Fuel pump – made by Carter. ‘65s used 3939s ‘66s used 4201