The 3rd and 4th gears are a known weak point of the C64 transmission and are the gears which are loaded up the most, especially when doing track day driving.
Simply Sports Cars in conjunction with Hollinger Engineering have developed a stronger 3rd and 4th gear kit that have the same ratios as OEM parts. The kit comprises of 3rd and 4th driving and driven gears only. The dog rings are transferred from the existing 3rd and 4th gears.
The gears are still helically cut which reduces operating noise but due to the teeth being larger they are a little louder than stock.