503 pinion size
 

Can anyone give me a suggestion on a good starting point re pinion size running stock spur. Running a 8.5 motor (may go down to 7.5 later) on indoor carpet tracks, midland raceway (tight and windy) and also Gloucester (again quite tight except for long back straight).
I was thinking 19....?

Yes sounds a good shout a 19t, I used a 20/21 on my 7.5 motor but was a medium size track with a good flow to it

Cheers for that, I'll start there then!