6.9 head resurfacing

[OldIron82]

So I finally got my heads dropped off at a local machinist that came highly recommended from everyone I talked to. 42 years in the business. He did a double take when I said 6.9 idi. He was thrilled to see nostalgic old iron but quickly changed his tune and said the pre chambers suck. Lol. My only concern was the valves. I just wanted a valve job. He checked the straightness of the heads with a straight edge and feeler gauge and was happy with it but unfortunately the one head has a very nasty fire ring groove in it. I suppose it's from rust over the years on the head gasket fire ring that just eats it's way into the head? Anyway, he said it's my final call, but the man is a perfectionist and hates to see me leave with anything but perfectly flat heads. Doing the research I have found everything from one end of the spectrum to the other. "You can't cut those heads"!!! to "ahhh, cut the same heads multiple times. No big deal" What do you awesome idi'ers have to say on the matter? I read if only one head is cut, I will have to shave the opposite side intake bank so it will engage properly.

I don't believe the man will get to my heads for at least a week or two so I'll have time to call him and say cease and desist on the shaving if the general consensus is no way.

In other news, I found a running 87 6.9 C6 NP208 (i think) for $600.00. The truck rotted out around the engine. I think I'm gonna grab it and use that short block assembly with my heads regardless of the mileage as opposed to my 85,000 mile 1983 ******* block. I'll still grab that savage yard 6.9 out of that ambulance for the fire breather build one day but I'm sure it dont run at the moment.

Thanks everyone.
OI82

 

[OldIron82]

Ok so I just found the article in the tech section I obviously missed before. Never mind I have all the info I need. Feel free to delete this thread. Oops. My bad.

 

[cpdenton]

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Not a problem to resurface as long as minimum thickness isn't compromised and valve recession is accounted for.

Make sure your machinist has this spec sheet.

 

[icanfixall]

That is the same scification manual I live by. I found it correct in every way so far. All idi engines have a min max head thickness. Most have no idea how to measure it too. Notice the valve cover gasket face is a machined face. We measure from there to the bottom of the head using a 4 to 5 inch micrometer. Valve recession is one of the most important items when doing a valve job. The other is the recession protrusion of the precups. Not many shops have the cutting tools to machine the head and the precups at the same time. Some will use a belt sander on the precups.. Not the best idea either but it gets the job done.

 

[OldIron82]

Thanks for the spec sheet. Could anyone please explain WHY the specs on these heads are so critical? Will valves burn or is there a clearance issue? I'm just curious. Thanks

 

[79jasper]

Clearance.

Sent from my SAMSUNG-SM-G890A using Tapatalk

 

[OLDBULL8]

In short. All 6.9L and 7.3L Ford diesel engines from 83 to 94 have flat head surface to block surface.

Unlike a gas engine that have a doomed combustion chamber where the valves are inset so the pistons cannot come in contact with the valves. If you'll notice the diesel pistons have a relief in them, because the stroke (TDC) is so close. Any machined miss-measurement could be catastrophic.

Does that explain it?

 

[icanfixall]

Bill the "relief" in the top of any idi piston is not for valves. Its design is for the flame front out of the precups and this is called a Recardo Cup piston. We have many pics of valves hitting the pistons and they show what a valve recession in the pistons might look like. In the posting by hydro idi you can see what happened when he installed a new cam but did not replace the valve springs. His shop rebuilt the heads but missed the weak springs. They were 40 to 50 lbs and the oil pressure was enough to hold wide open the valves byt a pumped up lifter. Our heads are flat and the ONLY reason we do not have pistons hitting the heads is the thickness of the head gasket prevents this.
I too have asked why our head min thickness is so small because we can recess the valves in the seats easily.