Paksik , it looks like its Dio , not gilera 172?
here some dimension for you thats works for everything from 50cc to 172cc :
mid section : 110mm diameter 110mm long
back cone , 160mm long ( 110mm diameter going to 20mm muffler for 172cc , 13mm for 50cc , 16mm for 70cc ..
front cone 160mm ( 110 mm going to 40mm )
total pipe lenght is LONG : 1000 mm ( from cylinder to end of last cone)
so rest is 570 mm long pipe going from cylinder to that 40mm front cone --
depending on how many CC is engine , and if you have good transmision ( CVT ) and clutch parts and tuned perfectly and cyliner etc etc etc -->you shorten that 570mm pipe 20mm at a time to find peak rpm and power .. you must change transmision parts ( rollers , spring etc )like 20 times etc etc ..
if not have those , leave it long .. 1000mm
dont waste your time measuring those racing pipes , they will not work unless everything in engine changed ( only cartel is not changed )
to get cones paterns go to
www.pulserate.com and down load program , then print out and cut paper ..