I have a 1977 F150 ranger with a 5.8l 351m and the speedo says 12,699.im trying to wonder why is puffing blue smoke out the exhaust,any help?

 

Rings shot, cylinder walls worn, valve stem seals hardened, valve guide wear .... speedo saying 12,699 could be 112,699 or 212,699 for all we know.

Just guessing here ....

 

If it's only on start up, that points to the valve seals/stems. If it's a light haze all the time, then more often than not it will be the rings or cylinder walls.

 

So that means simply rebuilding the heads?

 

It could be just rebuilding the heads, yes.

Does it smoke On start up only or constantly?

If it's on start-up you could yank the valve covers and do a visual inspection. It's an old truck with an old engine, this kind of thing is normal.