This spare parts bag includes dog bones and axles for the TT-01 chassis. It includes 2 long and 2 short axles. The short axles are used for the front of the TT-01, which will give the TT-01 a track width of 185mm. The long axles are used for the rear of the TT-01, which will give the TT-01 a track width of 190mm. Note: You will need the long axles only if you are using a body set which has a wider rear tract/width. Example: The Ferrari Enzo and the Porsche 911 have a wider rear track, therefore long rear axles are used to widen the rear of the TT-01 to ensure a true to scale appearance. When using Tamiya body sets that are 185mm wide for both the front and rear only use the front axles as replacements or purchase item 9005713, which includes 4 short axles.