Hi All,
bought an 86 f-150 with an overheating issue and need to replace the engine 6 cly 300. My mechanic friend says the 85 & 86 years are different than the rest of the years they made the I -6’s. Does anyone know if there would be engine block compatibility issues with non 85/86 year engines, or would it just be the differences in the heads? Thanks!

 

First welcome to FTE

2nd find a different "mechanic" maybe not a friend
The 300 block are all the same from the 70's up to the last somewhere in the 90's
The ONLY difference is in 87 the motors went to EFI so there was no need for a fuel pump mounted to the block so no opening but they say the bolt holes are drilled and taped for one?
To over come this you can run a electric fuel pump but to be safe needs to be wired up right so when the motor is not running the pump will not run, not hard to do we can help there.

If you get a EFI motor hope you get the EXH manifolds as they flow better than the log manifold on the truck now.
There is a little work to bolt them up but not that hard at all and then to the choke and intake heat, again not hard to do for a little more power.
Or just swap the manifolds as a direct swap, the EFI head will have a few more bolt holes but not a big deal.

The EFI motors use 1 fan belt just unbolt all that and bolt on your stuff.
All 300 motors use the small block, 302 / 351W, bolt pattern so again any 300 will work just the fuel pump deal.

NOTE: early trucks, < 83?, use clutch linkage and the motor side pivot is screwed into the block.
The later 83 or 84 use a juice clutch. So when they went to EFI the juice clutch had been used for years so I dont know if they had the boss drilled and taped for the pivot or not? Being you have a 86 and if manual would have a juice clutch it is not a issue.
Dave ----

 

There are two main version of the 300. One with AIR or Thermactor in the exhaust manifold, the other had a cylinder head with AIR in the head itself.

Don't fault his mechanic too much. He gets paid by the hour. He wants something that will fit and go. The mechanic paid by the hour is not going to want to spend time modifying and fitting things to make them work.

 

There was no consideration to having your engine rebuilt? I know local machine shops are getting harder and harder to find. If you live in a larger town you may still have one.

 

The engine needs to be replaced because it overheats? That does not make sense. Did it get too hot and damage the engine? Why not figure out why it is getting hot and fix the problem?