8″=20,32 cm
10″=25,4 cm
12″= 30,48 cm
14″= 35,56 cm

pvc pipes have standard diameter… so if if you find a piece of pvc pipe which is smaller than drum shell diameter, it’s good!!

for example: pvc pipe 250mm good to build a 10″ chell      pvc pipe 200mm good to build a 8″ shell…
                  pvc pipe 315mm  impossible to build a 12″ shell…   you have to reduce the diameter…  difficult to do…
i have not found upper size for pvc pipe…
and you should have to find them yourself because these pipes are very expensive (250,315) just for one or two shells

if i remember (but not sure), Jesaipa (guy who has done this design) had tried to reduce pvc pipe diameter… by cutting the pvc pipe in diagonal…then he slided it, and glued it…     when it’s dry, you have to cut one more time in the shell …so you have to begin with an deeper shell than you finally want