AmSnow.com is now SnoWest.com
The search is on
The search for parts began in my garage. Through years of repairing and restoring snowmobiles, our family had accumulated a fair amount of spare parts. While large, our collection did not provide all the components I needed.
So I searched swap meets and eBay while also parting down several donor sleds, including a 1987 Ski-Doo Formula MX and a 1988 Ski-Doo Escapade. Selecting what parts to use was based on my experience working on and riding Bombardier products. But I also had to study both current and vintage parts catalogs to identify items that would work on my project. Some Ski-Doo aftermarket parts were acquired from Allied Leisure Corp of Plymouth, Mich., and Central Snowmobile of Green Bay, Wis. My parts search lasted about 18 months.
As the parts were accumulated the design evolved. Unlike a professional engineer, I never put the design on paper. Instead, it was an accumulation of ideas acquired as I visualized the final result.
The build begins
First I stripped the ’73 TNT Silver Bullet down to the last nut and bolt. The hood, chassis, chaincase, secondary, front axle/sprockets, handlebars, and dash were saved for later use.
All of the other parts were sold on eBay or at swap meets to help other collectors restore their ’73 Silver Bullets, and the proceeds helped fund my project.
Major modifications were done to the chassis to accept new front and rear suspensions. The tunnel’s rear was tapered to give the machine a modern appearance. The hood required a couple of modifications to accept the ’88 Escapade’s instrumentation and to provide clearance for the front suspension.
The dash required modifications to accept the 503 engine, which is much larger than the original 340cc motor. Spacing on the front drive sprockets was modified to fit the new track. With the exception of a change in the gearing, the secondary clutch, brake, and chaincase are original.
Fabrication continued with the modification of an ’87 Formula MX PRS rear suspension. The original MX suspension was designed for a 16 ½-inch tunnel and was modified to fit into a 15 ½-inch tunnel.
That mod was more involved than just lobbing off the ends of pivot points. Rather, it required the modification of every swing arm in the system and also involved modifications to the slide rails to fit a 124-inch track. Five inches was added to the slide rails’ length. The longer track from a 1985-’94 Tundra was needed because the front axle is located further forward in the early machines with the engine located behind the front axle.
With the original 114-inch track, the rear axle was too far forward in relation to the chassis’ back-end. Mounting points for the rear suspension began with those from the Formula MX donor sled, but were recalculated based on the difference in front sprocket size between the two sleds.
Fabrication on the front suspension, began with the shocks, trailing arms, and swing arms from a ’97 Formula 500. First I removed the original ski post mounts located inside the front belly pan. Then a main mounting plate was designed for each side to accept mountings for the swing arms and upper shock mounting.
Finally, the trailing arms had to be shortened to match the back edge of the hood and mountings were fabricated to accept the trailing arm rear pivot points. All of this had to be designed to properly work with the height and action of the previously modified rear suspension. Proper ski pressure was a major factor in the final setup.
Challenges to overcome
Early it was obvious that the steering system was going to be a challenge. It was not possible to run tie rods directly from the center post to the outer steering arms without interfering with the front tunnel. So a sliding box tub system was designed to provide horizontal motion to points outside the tunnel’s width. Then much shorter tie rods were designed from the original Silver Bullet parts to make the final connection to the outer steering arms.
The next major challenge was to fit the 503cc Rotax engine into the space designed for the much smaller 340. To simplify this process the complete wiring harness from the ’88 Escapade was used.