Fitting Magnatraction magnets to the older Scalextric cars can be difficult as the chassis was never designed to take a magnet. Cars like the Formula 1 cars with the large Superslix tyres on the rear and full chassis plate are relatively easy as a flat plate magnet 1.5mm thick will fit between the motor and rear axle on the underside of the chassis.
Those cars that use the small Superslix tyres on the rear can be difficult as the chassis is too close to the track to fit a magnet on the underside of the chassis. Also, there’s not really enough room to place a magnet on the inside of the chassis between the motor and rear axle.
On some cars the motor is angled up and away from the track. If this angle is steep enough then a magnet can be secured to the underside of the motor.
Then there are the open chassis Scalextric cars. With these you’ll be into fabrication to make a location to fit the magnet.
Knowing how close to the track rails to fit a magnet can be difficult too. To help with this there is a magnet downforce calculator available. This will help you work out the best location for the magnet on an old Scalextric car.
Another great insight from Scalextric Car Restorations