I climbed in my 84 van this morning and turned it on, and boy did it sound
rough.  I figured it was just cold (it was probably 15 degrees F here in
Utah this morning) and I figured the van needed to warm up.  Well, it
sounded very rough the whole ride and when I got home I looked underneath
and took a look.

Where the collector pipes (attached to the heads) meet the next pipe (the
pipe with the O2 sensor in it), there are four bolts (two for each pipe),
and I only had one.  Somehow three of the bolts came undone!  I put a new
exhaust gasket (it was missing where the bolts were missing) and put in
some new bolts, but why did this happen?

I recently put on a new exhaust system (less then a month ago), and I am
fairly sure I tighten everything down good.  Just to be sure, I checked
all the exhaust bolts and there were a few that were looser then I liked
(so I tightened them up).

What do you think?  Was I careless, or was the heating and cooling cycle
on a new exhaust system enough to loosen the bolts?  Or was it something
else?

Blake