Hi,
SG shaft is much longer. A standard shaft has threads all over while the SG/Pilot shaft is partially threaded at center. Nowadays, most r/c cars would use engines with SG shafts because they are simply more convenient for mounting flywheel and clutch.
SG/Pilot shaft:
http://www.os-engines.co.jp/english/...alog/13960.htm
Standard shaft:
http://www.os-engines.co.jp/english/...alog/11830.htm
Regards
Joaquim