The Modern Bajaj Chetak Rides On
 
The muffler bolt, known for coming loose, is metric: 10 millimeter hex head bolt with a 14 millimeter head and coarse metric  threads and 25 millimeters in length under the head. Available at local box hardware stores. Bajaj part no.  was 39008004. 

The Modern Bajaj Chetak Rides On The Modern Bajaj Chetak Rides On