Kaase launches new Boss Nine engine kits

By Titus Bloom, July 24, 2014

BossKitRedValveCov-Created for use on common Ford 429-460 big-blocks
-Simple assembly with conventional parts

Winder, Georgia: For engine builders, and enthusiasts with ambitions in hot rod engine assembly, Jon Kaase has introduced the Boss Nine in a new kit form.

Among the kit’s more prominent components, Kaase includes his noted semi-hemi cylinder heads with accompanying pistons, pins and rings as well as pushrods, shaft-mounted rockers and induction system. Everything to complete the full assembly is supplied.

KasseBossValveCoversThough power production may vary from 500 to 1,000hp in naturally aspirated form and up to 1,500hp under forced induction, it is the engine’s evocative appearance and heritage that heightens its universal appeal. Predictably, options abound and powder-coated cast valve covers are available in silver, red and black. Indeed, in any color that can be identified by a paint code. In addition fabricated sheet metal covers are offered in natural aluminum finish.

In performance the Boss Nine’s magic is ignited by increasing its stroke length from the original late-nineteen-sixties specification of 3.590in. “Those big-port heads,” contends Kaase, “don’t like stroke lengths shorter than 4in., and respond enthusiastically to 4.150in, 4.300in or 4.500in, all of which we use.”

Because the longer 4.500in stroke causes the piston to protrude from the cylinder at bottom dead center, Kaase recommends a Race block or a “79” block, which has a 0.250in longer cylinder wall. Produced from 1979 to the mid-‘90s these can be identified by the nomenclature D9 on the block’s external surface. “They’re robust,” declares Kaase, “and we have one at the shop. It is 0.030in over-bored with 2-bolt main bearing caps and has taken the abuses of fourteen years of dyno testing. It usually generates between 900 to 1,000hp and we’ve used it on all Boss Nine and P-51 tests—it’s still going strong.”

First flush of life in 2008, forty years after the original Boss 429 Mustang

To appreciate the full measure of the Boss Nine it is helpful to return to its origins. I never planned on building a Boss 429 head,” says Kaase, “until driving back from the Engine Masters Challenge in 2007. Though virtually everything we’d built at my shop had been a derivative of that engine, I knew if we used stock parts in an EMC contender it would fall apart. So we decided to build it with new, revised components. This approach allows anyone to build the engine using the popular passenger car 385-series block. It seemed a commonsense approach, but we wouldn’t have started the venture if not for the EMC. And six months later we would have dismissed the entire project because the banks were failing and everyone worried if they’d still have a job!”


The deck thickness of the cylinder heads (about 0.625in) is greater than the original and though the valves reside in the same place, the rocker arms attach to the head in a more simplified manner, making the assemblies less expensive.

Despite the racing successes of the original Boss 429 Mustang the semi-hemi cylinder heads were weak. The combustion chambers cracked and their thin decks leaked around the O-ring head gaskets. Also the original intake valves suffered premature wear as the unusually short rockers with poor operating geometry caused them to hammer the seats.

“Most of the revisions applied to the Boss Nine,” explains Kaase, “were incorporated to make it stronger and easier to work on.” The deck thickness of the cylinder heads (about 0.625in) is greater than the original and though the valves reside in the same place, the rocker arms attach to the head in a more simplified manner, making the assemblies less expensive.

Also, the exhaust rockers are a little longer, which moves the pushrod away from the deck. This revision improves pushrod clearance and eliminates grinding the block close to the water ports. Moreover, the Boss Nine combustion chambers are a little more efficient.


The exhaust rockers are a little longer, which moves the pushrod away from the deck. This revision improves pushrod clearance and eliminates grinding the block close to the water ports.

For enthusiasts looking forward to a romp through the springtime landscape, 429/460 BBF engines are readily available and inexpensive ($100) and the Boss Nine now flourishes in kit form.

For further information contact:

Jon Kaase Racing Engines, Inc.
735 West Winder Ind. Parkway,
Winder, GA  30680,
Telephone (770) 307-0241 or email: JonKaaseRacing@gmail.com



  1. What is the price of a complete kit (Boss Nine). I’m looking for 850 to 900hp and need a real screamer with red line 8000 to 8500rpm

  2. Building 850 to 900 HP is very attainable, however if you’re wanting to build an engine that will REV in the RPM range you’re looking at you’re going to have to build it will FULL RACE components (Rotating assembly, Block, and Valve Train) Contact our offices directly at 770-307-0241 to go over your needs to determine what will work best for you.
    Jon Kaase Racing Engines.

    • How much are the Boss 9 top end kits?

      • Earl,

        There is no set price as each kit is different; we match our components with those you already have and to the goals you’re trying to achieve. If you could call (770) 307-0241, we’ll review your objectives and together determine a package for you.

  3. T/R power for Boss Nine: is it able to be done 3/2 or 2/4?

  4. I have a 1971 429 block complete which was tired and was given to me. I’ve been told these have the thicker cylinder walls and can be bored bigger, but what about the longer cylinders at the bottom, as I would like to stroke to 552/557 with aluminum heads and forged crank, and possibly a Pro Charger installed later. Also, I’d appreciate a suggested compression ratio regarding either my current 1971 Torino 500 with 351C, 4bbl., 471 gear, 4-speed, 12-second car, or my 1964 Falcon station wagon daily driver, which will become a Gasser. Lastly, advice on an oiling system upgrade would be appreciated.

    Thank you,

    • Jim,

      If you contact our shop (770) 307-0241 we can go over your goals with this project and point you in the right direction.

  5. I had visited a race shop in my area of IL and they were working on a 1970 Boss 429 that a customer bought to flip! They were having trouble getting it to fire, so not to be rude I left! I was wondering how your kits seal the heads to the 429 or 460 blocks and if you use O-rings like the 1969-70 models.

    • Rich,
      The Kaase Boss Nine heads have been designed to eliminate the O’rings and instead utilize conventional head gaskets. This allows you to convert any 385 series Big Block Ford (429/460) engine into a Boss.

  6. What EFI system would you recommend for the 460 Boss 9 block stroked to 572? Thank You!

    • Jim,

      We offer a couple of different routes to go on the EFI side of things. If you are looking to run an air cleaner to give the engine more of the stock BOSS 429 look then we would typically modify a carbureted intake to install an injector in each runner then run a throttle body in place of the carburetor under the air cleaner. Another option could include our Stack Injection system that would look like eight old-style Weber carburetors. Either system could be run using either Holley or FAST fuel injection. You can find more information on our website jonkaaseracingengines.com or you can call our office at (770) 307-0241.

  7. I recently acquired a 429 4V from a 1973 Mercury and intend cleaning it up and returning it to 1970’s horsepower plus. I’m wondering what you would suggest and provide relevant costs.

  8. Dean,

    If you’ll contact our office at (770) 307-0241, we can go over your goals and come up with a combination that will work for you.

  9. I have a Ford 429 engine from a 1971 Thunderbird that I’d like to stroke and rebuild to reach 700hp using one of your Boss 429 kits. The engine will be used in a Gasser-style 1956 Ford Fairlane for Bracket racing operating with a C6 transmission, 4.56 rear gears, and 29.5-10.5-15 drag slicks. I want to build a reliable engine capable of many trouble- / maintenance-free passes. Can you price me the parts list needed to build such a engine? If I find it cost-prohibitive, what would a comparable kit with the same 700hp be if I used the P-51 or SR-71 heads. I really love the appearance of the Boss 429, but if cost-prohibitive, I want to use one of your other sets of heads for this build.


  10. Mike,

    If you contact the guys in our office, they can review the numbers more thoroughly, but to provide you with an approximation, figure on $3,500 for a rotating assembly (forged crankshaft internally balanced, H-beam rods with ARP bolts, and forged pistons, rings and bearings). Then, there’s the Boss top end package that amounts to about $9,500, which includes cylinder heads, intake manifold, valve covers, camshaft, lifters, pushrods, rockers, gaskets and hardware. A Boss-headed engine like this with a hydraulic roller will easily exceed your 700hp mark; it will also be user-friendly. Alternatively, a set of SR-71 cylinder heads would allow you to build similar horsepower and cost about $3,000 less for the top-end components.


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