So, Ford has used emission thermactor plugs, for small block heads for many years.
These emission thermactor plugs are useful when installing a pair of emissions legal heads on a non-emissions engine.
All (OEM) and aftermarket ford smog legal heads have thermactor holes, one on each side of the head.
So, Ford designed these emission thermactor plugs to have multiple uses. Consequently, Ford drilled and tapped a hole in both ends of the heads for a 5/8-inch national coarse (N.C.) thread.
So, If the engine is using Emission/Thermactor hook ups with a smog-pump, the rear hole are left open.
These emission thermactor plugs can do two things:
- Plug the rear holes if you are not using any Emission/Thermactor hook ups.
- Note: That the plug is internally drilled and tapped for a 3/8-inch N.C. thread.
- Allow you to attach accessory-drive brackets to the front of the heads. ( Most accessories use a 3/8-inch N.C. bolt )
Sometimes in an emissions-exempt vehicle, the rear hole may be left unplugged by mistake.
Left open, the holes will generate a tremendous exhaust leak. Consequently, Sometimes blamed on leaky header or exhaust manifold gaskets.
So, When not using the smog pump, the O.E.M. Ford 5/8-inch blind hole insert (Ford PN 351418-S) is a simple and inexpensive fix to this problem.
They are also available from Pioneer Engine Products using part # PF580
You will be able to screw these plugs into any head that has the holes. Consequently, They change the thread size to allow you to use your factory accessory drive brackets. You may want to use these plugs to close off any air passages in your cylinder heads. If there were no plugs you would have an exhaust leak.
Now there are some cases when you don’t need the special plugs for brackets. You may just want to plug the holes.
You can go to your local bolt supplier and find 4- 5/8-11 x 3/4 stainless steel bolts.
Don’t forget to use a good thread sealer as the pitch of these bolts are quite coarse.
So, These plugs have multiple uses but, if forgotten you may have that unforeseen exhaust leak.
Thank You !