3/8s inch internal thread .... what ever that equates to for its external thread ....

try an engineering supplier ... possibly RS  its an odd thread tho .. its got a wide pitch

if you want something helicoiled don;t mess about with bike shops just take it to a small egineering company and they'll do it for about a tenner. if you wanna buy new cranks, middleburns are fine on squares do em up tight and they will last forever. To get mine off my trials bike when i came to sell it it took me haning off the spanner on the extractor bouncing on it while my mate held the work stand steady to get the things off, they were such a good fit.