Help -7.5 Ford Gas engine exhaust manifolds keep coming loose?

UVHS

New Member
I bought a 1994 Ford E350 van a year ago on eBay. By the time my father in law drove it 1500 miles to me the exhaust manifolds were loose making a poping sound. Since that time I have had it in the shop no less than 4 times getting the manifold tighted, heads milled, new bolts, etc. I just had the right side tightened again today. Does anyone have an idea why they won't stay tight? I am going broke maintaining them. I think I have spent nearly $1200 on them in a year.
 
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