I'm dismantling the vehicle in order to replace the clutch. Sub frame is out... Tried to pull the starter mounting bolts. Bottom one came out just fine, but the upper will not budge. I'm using air tools and can get a direct shot to the bolt. However it's not moving. I'm spraying it with pb blaster, a periodically trying.

Any experience with this?

Not sure if I should try heat or some other approach.

 

That's what I would do. Can get fancier penetrating oil like Kroil. Are you using a decent air impact or a cheapy? My 1/2" Ingersoll Rand is way stronger than the 3/4" Harbor Freight model I tried...

 

It shouldn't be that much of a fight, have you tried a breaker bar?

It's not often I need to use a breaker bar to get that bolt loose, but I have had to do it a couple of times.