The output shaft spacers help keep the clearance between the shift gears, otherwise known as the float end, to a minimum to help the gears engage clean. The spacers in pre EFL PX engines only have 1 stability board and the later shims (after DFL) have 4. The later EFL version can be fitted to the previous PX engines rear. The PX EFL shaft output requires the use of at least 2 spacers and also 2 snap rings, one of each being at the ends of the gear assembly. The more stable PX EFL gearbox parts are usually used in tuned PX engines. Due to the higher power levels produced by the more performance oriented motors the correct backlash between the gear cogs becomes essential. To allow for a more precise end distance of the float shaft, to the hundredths of a millimeter, to be achievable SIP have produced the thinnest grades of gearcase spacer to complement the existing range of dimensions produced by PIAGGIO. Your exchange box will thank you in the end for the quieter (and longer) life. Tip: The end float distance of the output shaft mount should be 0.15 to 0.4mm when it is just put together. If you are experiencing during a major service it should grow under 0.5mm otherwise you will require new (thicker) spacers ... SIP-TIP: The feeler gauge for measuring the axial clearance of your output shaft makes this job much easier. It is available in our shop, Art. No. 60824000. Don't forget to order a new cross, barrel selector and fresh gearcase oil to get the job done properly.