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.
In Kaase’s quest for power increases, he revised the shapes of the Boss Nine combustion chambers, the intake and exhaust ports, and changed the spark plug locations. For reliability, a new rocker system was devised in conjunction with W.W. Engineering to eliminate the adverse offsets of the stock Boss parts. Boss Nine heads adopt stock 460 head gaskets (Fel-Pro 1018), intake gaskets, and exhaust gaskets and uses 429-460 stock internal oil drain-back holes.
Bare cylinder heads $3600.00 pair.
Complete cylinder heads starting at $4350.00 pair.