Will this primary work with a 50cc?


Im buyimg the DR 85cc kit and primary 24-72.

if I in the future want to go back to a 50cc cylinder can I use this cog 21t 

21T cog

would be pain to open the engine to change the hole primary gear to 18/67 again.


A 50 ccm cylinder is quite weak. It definitely needs a corresponding primary transmission like the 14-69 or 15-69 or so.

If you want to perform speed tests on the Utah dry salt lake test range, with an aerodynamic lining on your vespa, you can keep the 24-72.
But it only works if you do not have opposite wind or any hill.

I was more thinking if I change to 50cc and to 21/72=3.4 (24/72=3.0)

Is 21/72 to long for a 50cc? I thought standard was 18/67=3.6 for vespa 50.

is it a big difference in 3.4 and 3.6 for a 50cc ?

Please specify your question.
Which cylinder kit is on your engine? Which exhaust? Which carburetor?
Please remind that the primary transmission is exactly adapted to the torque of your cylinder.

You cannot just simply change the primary gear and think this is a good idea.
If you really want to gain more speed, you should install more power and change the primary gear.

If you have for example a DR 50 and a SHB16/16 carb plus a Banana exhaust, it will have about 3.3 HP. With the 16 teeth Pinasco you will get around 60 km/h.

... so this is what you can do without openeing the whole engine