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Mustang Boss 429
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  Article Image gallery (10) 9F02Z172937 Specifications  
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Country of origin:United States
Produced from:1969 - 1970
Numbers built:1359
Author:Wouter Melissen
Last updated:October 06, 2020
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Click here to download printer friendly versionFor the 1969 model year, Ford added two high performance 'Boss' models to the Mustang line-up. Aimed at taking on the Chevrolet Camaro in the 'pony car' war, both cars were also conceived to homologate their respective engines for competition use. The Boss 302 would form the basis, while the 'semi-hemi' big-block V8 in the Boss 429 was developed for use in the NASCAR championship. Like many homologation specials, the two Boss Mustang production models became legendary in their own right.

Rarest of the pair is the Boss 429, which was built in 1969 and 1970. Its main purpose was to homologate the newly developed big-block engine for NASCAR. The unique aluminium heads feature a semi-hemispherical combustion chamber design that Ford called the 'crescent'. The heads were mounted on the block using the 'dry deck' system, which eliminated the need for a conventional head gasket. Instead, individual O-rings were used for the oil and water passages. This meant that a small leak would not immediately lead to catastrophic failure.

In its first iteration, the semi-hemi V8 was fitted with hydraulic cam lifters. From the 1970 model onwards, the big block engine was fitted with more conventional mechanical lifters. A single, four-barrel Holley carburettor was used during both model years. Officially, the engine was rated at 375 bhp. This power was sent to the rear wheels through a four-speed manual gearbox and a 'Traction-Lock' limited slip differential.

The bare shells of the Boss 429 or 'Boss-9' were built on the same production line as the other Mustangs of the same model year. They were then shipped in special carriers to the same Kar Kraft facility where the Le Mans winning GT40s had been prepared. Here the front-end was substantially modified to allow for the big engine to be fitted. The suspension mounts had to be relocated and completely new inner-fenders had to be made. There was also no room for the battery any more, which was moved to the trunk.

Compared to regular production Mustangs, the Boss models also featured a stripped down interior. Other model specific features included a 8,000 rpm tachometer and an additional oil cooler system. The engine cover also featured a manually adjustable air-flow scoop to aid cooling. The exterior of the Boss 429 was relatively sober to keep the weight down. Among the few distinguishing features are the Magnum 500 wheels, a small badge on the front fenders and a small front spoiler.

During the two-year production run, just 1,359 Boss 429s were built by Kar Kraft. The big-block engined machines were relatively expensive and demand dropped off in 1970. The car had certainly served its purpose as the semi-hemi equipped Talladegas won 29 Grand National races in 1969 and 1970.

Today, the Boss 429 is one of the most sought-after, non-Shelby Mustangs.

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  Article Image gallery (10) 9F02Z172937 Specifications