Head To Block Thickness Questions

[icanfixall]

For some time now I have been researching the deck to block thickness of our heads. The measurement is taken from the machined valve cover rail seat surface to the underside of the head machined surface. Factory oem specs are anywhere from 4.805 down to 4.795... Now if the valves are all set to the proper depth in the heads why can't the heads be less than the 4.795 thickness... We have a hydraulic roller lifter than will "find" the balance it should run in. The rocker geomotery wont change because of the thinner heads. I'm just not finding why we must stay in this 10 thousands tollerance. A head cut down 10 od 20 thousands with the valves all set properly in the heads will not be changing the compression ratio. Our heads are flat without a combustion chamber in the heads like a gasser has... Anybody have some ideas on this....

 

[alienturtle]

dont the pistons come up out of the block on these motors??

 

[The Warden]

dont the pistons come up out of the heads on these motors??

Yes they do, but only a very tiny bit...I think it's just enough that, with a compressed head gasket (i.e. one that's had a head torqued in place on top of it) in place, the top of the piston would be just about even with the top of the gasket.

Gary would be able to say better than I would, regarding the risk of the pistons actually hitting the bottom of the cylinder heads if the heads were milled down too far...

 

[icanfixall]

No amount of head milling will cause the pistons to contact the heads. Only milling the block will cause the pistons to come up out of the cylinders more. Remember the heads are flat. Also why don't we run 7.3 heads on a 6.9... I realize the head bolt diameter differance but thats only around 62 thousands differance in size... Thats 31 thousands per side of the head bolt in the bore.... Not a big deal. The wahsers will handle that easily.... I know why we don't run a 6.9 head on a 7.3 but... a member here did drill out a 6.9 head to fit a 7.3 block so it has been done... Ebay has a good looking set of 7.3 heads that were just used for dyno runs... then removed.... For $350.00 seems like a fair price... I have no idea who did the valve job or what the work or thickness looks like....

 

[The Warden]

Dumb but semi-related question...I still contend that the "ideal" setup would be to take a 7.3l block and sleeve it down enough to use 6.9l pistons, so that you minimize the risk of cavitation but still get the larger head bolt/stud size. If you were to do that, could you use 7.3l heads ans gaskets with 6.9l pistons, or would you want to use 6.9l heads and gaskets (with the bolt holes widened)?

 

[icanfixall]

Both of the head gaskets are the same size. The differance is in the lower edge cooling port on the 6.9 gaskets.... The 7.3 does not have the holes for the coolant... In the heads those passages are plugged with a freeze plug..... I bought a set of 6.9 head gaskets on ebay just to compare the firerings... they are the same size....

 

[The Warden]

Both of the head gaskets are the same size. The differance is in the lower edge cooling port on the 6.9 gaskets.... The 7.3 does not have the holes for the coolant... In the heads those passages are plugged with a freeze plug..... I bought a set of 6.9 head gaskets on ebay just to compare the firerings... they are the same size....

Oh wow, didn't realize the fire rings would be the same size...does that mean that, other than the cooling ports and the head bolts (and "internal" things such as the precombustion chambers and injector/GP placement), the heads are similar enough that they can interchange?

Just trying to learn something new and I haven't sen a 7.3l and 6.9l head side-by-side...thank you again

 

[racer30]

I think there is no problem using a "thin head" as long as the valves are set correctly into the seat. I was thinking of setting my valves deeper into the head just for the fact that I will be using a reground cam and will have a smaller base circle. I haven't made up my mind yet, still looking into valve spring install hights on some posible replacments.

 

[6 Nebraska IDIs]

Here's my thoughts.
IMO a 7.3 head on a 6.9 motor is much more feasible than a 6.9 head on a 7.3.
Using a 7.3 head on a 6.9 is practically the same as using a 351W head on a small block ford. My family has done the W head on a SBF with my brothers shelby mustang and its really not that difficult. A special set of shimming washers are required, these washers slide down into the bolt hole of the head. These washers take up the clearance that was the difference between the 7.3 heads bolt hole and the 6.9 head stud. You just need someone with some experience in the machining industry, and I have that with my brother. If you don't understand the washers I'm talking about I can take some pics of the leftover ones we had made for my brothers shelby engine.

 

[OLDBULL8]

I've got 6 heads in the shop, Will measure each for thickness.

 

[idiabuse]

I have 8 left over spacers made to put 6.9 head on a 7.3


Javier

 

[6 Nebraska IDIs]

Spacers shouldn't be required for putting 6.9 heads on a 7.3, drilling is required.

 

[david85]

$350 for a set of heads is a good price? I seem to remember my local engine machine shop had a set of heads from a 6.9 that were drilled out to work on a 7.3 but they decided it would be too expensive to get a set of custom bolts for the engine. The heads are rebuilt and were sitting for a few years already by 2008 when I was rebuilding my engine. He said he would have taken $250 for the pair (I THINK thats what he would have taken for them...) But other than the head bolt dilema, they were completely finished and ready to go. Guess it was a good deal but we figured it wasn't worth the risk.

After seeing the washer size for the 6.9 bolts however, he advised against using his heads and I was able to find a remaned set out of alberta for waaaaaay more than I should have paid

If I remember right, the heads I bought were shaved by 5 thou from deck to deck and the valves were trimmed by 10 thou compared to the original heads. I still have the original heads if anyone needs some measurements. Only reason I didn't reuse them is they would have required resurfacing due to some erosion where a fire ring was damaged and no local machine shop could have done the work (Prechambers required a special surfacing equipment according to him).

 

[idiabuse]

Spacers shouldn't be required for putting 6.9 heads on a 7.3, drilling is required.

spacers are needed after the drilling is done.


Javier

 

[bike-maker]

2 Issues with knocking the heads down past the recommended amount that I can see: Thinning the heads down brings the valves closer to the pistons when they are open. This is probably not an issue with these motors, and is probably a mute point if the valve seats are reground.
The other factor is the thickness of the castings themselves, as well as the core shift within the casting. This could vary between each individual casting by a fair amount; not enough that I would think an extra .010" or .020" would ever be an issue. I've had to remove more than .030" on heads that were warped from overheating.
All this being said; if you're not shaving the heads to adjust the compression ratio, what's the point in it?
I can just about guarantee that if you take a pair of older, cast iron heads down to the machine shop, they'll just machine them without ever measuring them. It becomes more of an issue with things like aluminum heads and especially overhead cams.
Most of the machining limits on engines are adhered to by machine shops for the sake of liability. If you took too much off of the heads, there's more of a chance of overheating and/or cracking. I did quite a few jobs that went clear past these defined limits, and was OK by the shop owner with a cash money, no receipt deal. And any good machinist is going to stand behind his work anyway.
From what I've seen of the castings, machine work, and actual metal (really high nickel and tin content) that go into any International engines, they have always been built hell for stout; that's why I've owned 4 of em.

Just my $.02