4.0 head resurfacing - maximum allowed?

[Maui-sprint]

1991 XLT extended cab
4.0 L
200k miles
Auto-to-manual trans conversion


Quick history on my issue: Got the truck from my next door neighbor back in 2016 for free. He had overheated the motor and blew a head gasket. I picked up some new heads and reused the valve train. Ran perfect for a couple years and then ended up blowing head gasket again due to minor overheating. Had heads resurfaced 0.010" to clean up and installed new head gaskets. Ran for about a year no problem before developing a "minor' head gasket leak once again. For this one, I used Blue Devil sealer and haven't had any issues for the last two years until now. Over the weekend truck began misfiring and blowing white smoke and water out the exhaust. It did this WITHOUT overheating (coolant system has been working perfect). inspection of plugs shows fouling in Cylinder #1. I have always used Felpro gaskets.

My plan is to fix it right (again). I plan to remove the heads and bring to my machinist for inspection. If they need to be resurfaced, how much more can I safely go? What can I expect in performance after resurfacing (ie: detonation, etc...)?

 

[gaz]

@Maui-sprint ,
Svenn Pruett of the NHRA claims that 91-94 OHV 4.0l blocks can be milled up to 0.040". It is worth noting that even 0.030" will increase the compression ratio at least 1 whole point of compression; from stock 9:1 to at least 10:1.

NOTE
a head mill of even 0.010" may require the lower intake manifold to be matched.

Svenn Pruett wrote the book "How To Modify the Ford 60°v6 for Power"

 

[Maui-sprint]

I did not machine the intake at all when I got the heads resurfaced (0.010") last time. Is there a chart available with recommended corresponding intake machining depths?
Does this mean I will need to use higher octane ($$$) fuel from now on?

 

[Shran]

Watching this thread just for my own curiosity. I'm wondering why the intake would need to be machined if not a lot was taken off the heads - does it create gasket alignment issues? I can see that being the issue if quite a bit was removed from the head.

If you have a set of heads that have been worked on with no documentation, is there a way to measure how much material has been removed vs a brand new stock head?

The engine I'm working on has heads that have been machined at least once and the intake sits flush with the heads around the valve cover surface. I seem to remember other 4.0's where the intake valve cover surface sat below the head just a hair.

 

[gaz]

@Maui-sprint ,
The valley's outer walls are the intake side of the heads. If the heads sit lower, they change how the lower intake manifold's sealing surfaces align.

You have a used engine with heads that you had resurfaced. The engine may have previously been worked on (machining) causing an alignment issue with the new heads

Since you are taking these heads to a machinist...bring the lower intake manifold too. If the machinist asks you why you brought the lower manifold, I would find another machinist. Matching a lower manifold to machined heads is both a math equation and art, it does require a skilled, experienced machinist.

If it is determined that this set of heads has or will be milled a total of 0.030" or more, than yes...using premium fuel MAY become a necessity with the resulting 10:1 +/- compression ratio. If the head mill does end up between 0.030"/0.035", then you will have gained 4.5hp to 6hp; depending on your driving habits, with increased fuel economy due to the HP to weight ratio improvement.

You mentioned 200k when you took possession, add your miles...
• I recommend a performance valve job
• resurface valve seats if needed
• new valve guides
• inspect the rockers and push rods for wear

@Shran
• It can be determined how much a head was milled by measuring the volume of the cylinder bowl remaining in the head compared to known new volume
• or comparing the deck height to the known height of a new head