100121 - 21T Gear Adjusting  Inserts (Losi 5ive)  (5) 
Comes complete with new bolts.
When choosing the correct spacer for big bore gear sets, simply count the teeth ratio difference your going for from stock.

Ie. For 27/50 gearing (ideal for 46cc) , this is 8 teeth larger on the pinion but also 8 teeth smaller on spur, therefore it needs the same spacer as stock (19T)
 
For 29/50 gearing (Ideal for 65cc), this is 10 teeth larger on the pinion but only 8 teeth smaller on spur, therefore it needs a 21T spacer (2 sizes up from stock spacer)