ORIGINAL: ingynwendy
You should be able to get away with ball berings (not needle) for any HPI clutch bell heavy duty or not I've used them in Clutch bells all the way down to 11 tooth on my MT2. Note on non heavy duty and especially smaller tooth CB you will need to stack multiple berings to make up the gap or use washers. It took 3 and a small brass washer in a 12 tooth non-heavy duty for example.
I will not ever run a standard CB again as it wears faster then the plastic spur and then do to the wear on the CB it wears out the spur much faster. My spur on my Savage has held up much better then the one on my MT so that is why I am going to upgrade to the Savage set up. Yes I do wish they made something smaller then 13 teeth on the heavy duty CB but so far I have not needed any thing lower on my MT and only go down as low as 16 teeth on my Savage.
If you want to run ball bearings in you standard CB you need two 5x8mm bearings and one 5x8mm flanged bearing.