The X5 transmission has evolved since the original in all kinds of ways over the last couple of years. At one point there was a dual idler option which had a 2nd idler that meant more wrap around the drive pulley and lifted it higher, this needed the XXX4 belt which is longer. Since then the machined pulleys have come out which particularly the drive pulley is so much better skip with good tension is virtually eliminated. You can't get the dual pulley anymore as Losi stopped making the right pulley.
I must admit that I still use a home made 'hold down' roller that sits a fraction off the belt on the drive pulley so if the belt tried to lift it stops it and keeps the teeth engaged, hence no skip.
With both of these (new pulleys and hold down roller) things I have no skip what so ever and a very free transmission.
Bearing in mind the various benefits of the squared chassis and the clearances of the new shell etc I personally would get the whole upgrade kit, have an evening transferring it all over. Run the car, the belt sometimes stretches a fraction so it's not unual to need to nip the tension up a bit for the first few runs and then you shouldn't have to touch the transmission for months. Mines not been apart I don't think since prep for Belgium in the summer.
HTHs
Dan
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