start with the sub report with no links to the main report.
create your part number group
get your data and get your summaries.
are you using the summary wizard or manual runnning totals?
then create your shared variable in the sub.
you may need to create a reset formula for the shared varaible
whileprintingrecords;
shared numbervar x := 0;
x- is your variable name
then create you group by part number in the main report
place your sub report in the details of the group
then join your sub to the main by part number
you should see the values in the sub for the part number when you run the main report
then create your shared variable in the main report
make sure it is below the sub in the group footer of the main
DO NOT SURPRESS THE SHARED VARIABLE IN THE or you wont get anything.