By Ro McGonegal.
Pictures by Moore Good Ink:

Kaase Boss Nine Engine

Long-time Mountain Motor engine master Jon Kaase (Racing Engines, Winder, Georgia) says, “The stock Boss 429 parts were a masterpiece for their time, but slightly weak and difficult to work on. We made every effort [with the Boss Nine] to fix anything that was troublesome or failure prone.”

At the end of 2007, Kaase decided to take “the plunge and build all new Boss 429 Ford retrofit heads and related parts. I was betting on the fact that there were other Boss fans and Ford enthusiasts out there that wanted these new parts as badly as I did. Although they’ve only been out for a short while, I’m happy to say that they have been well received and successful in whatever projects they have been used in.”

Since build orders for the Boss 429 Mustang NASCAR homologation-specials ceased at 1,358 (859 were built in the spring of ’69; 499 more came to life as 1970 models later that summer), these units are rather scarce. Solving the cylinder block problem was easy. Kaase simply undertook the 429/460 big-block (in iron and aluminum) providing them with cylinder head oil drains in the correct location. The JKRE plan included using OE 460 head gaskets, so the deck surface of the Boss Nine heads is much thicker than the original dimension to provide the necessary clamping force. The Boss 429 was humorously under-rated at 375 horsepower while it probably made closer to 475 at the flywheel and without the parasitic drag of the accessories. The ports were huge, had poor low-speed velocity and didn’t begin to show their stuff until the tach was beyond 3,500rpm. Camshaft technology was light years behind the current form. Kaase knew that these two issues had to be addressed for a modern street-driven power plant that was originally created for the race track. 

To that end, he spent a lot of time rearranging the insides of the aluminum. Though the intake ports appear to be stock at the entry, they benefit from a revised short-turn radius and a revised approach to the valve seat. The floor of the exhaust port exit and the valve bowl under the seat were reduced in size. At all times, the quest was to make the Boss Nine heads appear like the OE castings.

Kaase modified the semi-hemi combustion chambers and relocated the spark plugs for resistance to detonation. Size of the combustion chambers has been reduced to 90cc so that flat-top pistons when fitted in a 460-inch engine will yield a compression ratio of 10.5:1. Further, the stock Boss 429 intake and exhaust manifolds fit the Boss Nine without modification. Moreover, the miracle of modern technology can produce camshafts with enough magic ground into them to puff up torque for low speed driving and provide a 750-800rpm idle, with duration that builds maximum cylinder pressure and different timing and lift for both valves.

The last area of contention, the wildly disparate rocker arm system, Kaase approached with his usual aplomb. He incorporated the rocker mounting saddle in the cylinder head casting, thus eliminating the mounting block, representing a savings of about $400 when purchasing the rocker arm set. These items are the produce of W.W. Engineering and are the very same that Kaase uses on his 820 cubic inch IHRA motors. The exhaust rocker arms, which have a ratio of 1.75:1, were designed to move the pushrods away from the block deck. In keeping with his “budget” approach to this project, Kaase uses Manley stainless steel intake and exhaust valves with BB Chevy-size 11/16-inch stems.

So what do you get when you apply Boss Nine heads to a 460-521ci short-block? How about an immediate increase of 50hp as compared to Kaase’s powerful P-51/SCJ heads? Swap these beauties on a 600ci version, and the output is 150 additional horsepower. And dig it ladies and gentlemen; all mechanical delicacies aside, you’d have to go a far piece to produce a wow-factor like the Boss Nine.

Check out the following outline for a behind-the-scene Boss Nine build.


Jon Kaase Racing Engines, Inc.
735 West Winder Ind. Parkway,
Winder, GA 30680,
Telephone (770) 307-0241

inserting pistons, Diamond custom pistons

Boss Nine readily accepts Diamond custom pistons secured to Scat connecting rods with Trend H-13 piston pins. Bearing caps utilize 12-point Grade-10 bolts and torque to 75 lb-ft. Depth of intake and exhaust valve notches is 0.250 and 0.350-inch, respectively. Intake valves measure 2.300 inches; exhausts are 1.900 inches.

This particular engine uses a hydraulic roller tappet version

The 385-series Ford block maintains large 2.505-inch bearing journals that lend stability that keeps the cam from flexing at high rpm. This particular engine uses a hydraulic roller tappet version.

Kaase Boss Nine Build

For most Boss Nine builds, Kaase prefers a Ford harmonic balancer. ATI offers a similar item but it is twice the price of the Ford part.

oil pump, high-performance 385-series big-block

Original oil pumps fitted to a high-performance 385-series big-block always succumb to vibration and inevitably break. JKRE adds its own device, one that will not fracture and break off under pressure

installing lifters, Trend 3/8-inch pushrods

Before the hydraulic roller lifters are installed they are soaked in oil. Sturdy Trend 3/8-inch pushrods accompany them.

W.W. Engineering, billet rocker

As with his 820 cubic inch Mountain Motor Pro Stock engines, Kaase employs the same billet rockers arms produced by W.W. Engineering in Dawsonville, Georgia, for the Boss Nine series. They operate with a rocker ratio of 1.75:1.

carburetor and timing adjustments, Kaase dyno-testing

Kaase’s cockpit temporarily vacant while carburetor and timing adjustments are being dialed in. Induction choices for this 700hp engine include single-carb, tunnel ram and two carbs, Hilborn stack fuel injection, or a Wilson 1388cfm throttle body and electronic fuel injection. Wilson 1-inch spacer provides an additional 130cfm.

Correct bore sizes, The boring process

A 520ci Boss Nine is the most cost-effective version to build simply because the aftermarket has the correct rotating parts (crank, rods, and pistons) in stock and ready to ship. The block arrives with a 4.340-inch bore. The boring process involves eight or nine cutting operations, each cut removing approximately 0.030-inch of metal. Correct bore sizes are finalized by the honing process—three strokes applied with 280-grit stones and three strokes with 400-grit.

Main bearing shells

Main bearing shells are dripping in oil and pressed into the saddles.

both ends of the seal are secured within metal

One end of rear main bearing seal (the first cap to be installed) protrudes beyond the flat of the block to coincide with the appropriately shorter piece in the bearing cap. That way, both ends of the seal are secured within metal, not butted up to it and inviting a leak.

Iron SVO and factory blocks

Iron SVO and factory blocks use 2-bolt main bearings for the front and rear. They have a hefty ½-inch diameter. Caps 2, 3, and 4 use four bolts each. All aluminum blocks utilize 4-bolt main caps.

Assembly lube is applied to the threads and beneath the head of each bolt

Assembly lube is applied to the threads and beneath the head of each bolt. Torque is applied in sequence and at 100 lb-ft.

Correct end play, measure end-play

Correct end play should measure 0.004-0.007-inch. This is determined by the thickness of the vertical walls of the thrust bearings in the center main cap. A dial-gauge is locked in place and then a large screwdriver (or the like) is used to move the assembly back and forth to ascertain the amount of play. This one had 0.006-inch.


bolt on heads

Fel-Pro 1018 head gaskets get a smoosh of sealant top and bottom. Aluminum blocks use studs rather than bolts. Kaase prefers 12-point Grade-10 bolts for iron block projects and tensions them to 110 lb-ft.

double-roller cam gear and chain

Most Boss Nine engines are fitted with a heavy-duty, double-roller cam gear and chain. Crankshaft snout is machined top and bottom for Woodruff keys. Cam timing can be altered by several degrees, plus or minus.

wet-sump oiling system

The wet-sump oiling system relies on a custom sump that features a one-way mesh screen bolted to the sump; a scraper mounted along the top of the mesh; and two one-way trap doors in the sump that operate like check valves, allowing oil to enter but not readily escape.

Kaase Boss Nine production cover

Production front cover is used, sometimes in conjunction with an adapter that receives a cam-driven water pump!

Kaase's Picture

Jon Kaase