6.9 build crossroads advice

[topgunn]

My 6.9 block is being bored 30 over with the rest of work. My biggest question is about 6.9 or 7.3 heads? I have a set of 1989 ,7.3 already rebuilt that I was going to use with ARP studs, however through (miscommunication or? ) I was given info on studs on ebay. These turned out to be 7/16 not 1/2 for 7.3 heads. So now will the 6.9 heads do as good of a job or should I get the correct studs and get the block drilled and tapped. I have to have the 6.9 heads rebuilt, so the cost of rebuilding heads or 1/2 studs and drill & tap are close. Also is the cost of coating pistons ($320 for top,sides and under) worth it? As always thanks for any input.

 

[argve]

Seeing as how the heads are already waiting. I would just have her drilled and tapped.

 

[sassyrel]

someone said tho travis----that the area around the studs--that if you do that to a 6.9--things will pull loose as there wont be enough meat left---

 

[topgunn]

That is my concern. The machinist said that he was leary about opening them up to the 1/2. Any pros and cons on this? Also can the 7.3 heads be used with the 7/16th studs. Are the 7.3 heads any better?
Thanks for input.

 

[funnyman06]

I recently read somewhere that the 7.3L head actually flowed better than the 6.9L. That was one of the major changes they made when they upgraded the engine.

 

[icanfixall]

Your machinst is correct about drilling out the 7/16 to 1/2 thread. You will only have about 1/2 the thickness of the threads cut into the head. They will pull out. The only thing that possible is to drill & tap to 9/16. Then you will have a complete thread but... This has never been done on this board or block. The head will need to be drilled out to over the 9/16 inch stud. I dought there is enough room for that. Just use the 6/16 studs with the 6.9 heads. You can use the 7.3 heads but they will not do anything better for you that the 6.9 heads. Maybe trading those rebuilt heads for the work on the 6.9 heads. Alabama Cylinder Heads sells rebuilt heads for around $285.00 and new 7.3 heads for well over $300.00. Bare heads cost around $330.00 too. The cost of coating the pistons sounds high but it does really work well. I used Tech Line Coating CBX and its fine but can only be done by a pro. They wont sell to you and me. Its very hazardous material.

 

[argve]

I guess you could have sleeves pressed into the 7.3 bolt holes so that you could run the smaller bolt diameter.... I can't think of any reason it would not work.

 

[david85]

The castings for the 6.9 and 7.3 are VERY similar, and I doubt there is any performance difference other than the injector depth or MAYBE the precups.

Do not attempt to drill 7/16 out to 1/2 thread. You will rip the threads out and then you will be screwed. The 1/2" studs for the 7.3L are twice the price anyway, so that will easily bite a huge chunk out of your budget so just go with 6.9 heads.

From what I understand, the 6.9 heads are less prone to cracking anyway of you get the 2nd generation version.

 

[FordGuy100]

I would have the machinist look at 9/16" to be honest. See if there is enough room to cut them out. First contact ARP though, try to find out if they sell 9/16" studs with the right thread pitch and length (and right material). They should have some crazy clamping force. See what he thinks if there is enough room.

 

[Agnem]

This is probably a crazy idea, but would thread inserts work? In other words, could you install like a Helicoil into whatever size opening would be required, to give you a 1/2" thread?

 

[dyoung14]

i thought all a 7.3 is, is a bored out 6.9 with bigger head bolts, and minor other little things, so why cant one drill and tap the holes? or is the block different??

 

[Agnem]

Your COULD drill and tap the holes to 1/2" if it were a blank hole. However, with the hole already drilled and tapped for 7/16", there is already a significant loss of material. Just drill and tap anything for 7/16", and then do it again to make a 1/2" bolt fit, and you will see what happens.

 

[david85]

There isn't a big enough size difference between 7/16 and 1/2 bolt sizes. If you were to try and tap out a 7/16 thread to 1/2, the result would be 1/2 threads that are not full depth and therefore not strong enough. This is why the mention of 9/16 threads since there IS a big enough size difference between 9/16 and 7/16 to get a full thread depth on the larger size.

I don't know if there would be enough meat in the block to accept a hole that big though. Maybe.

I don't really see the point in going to much larger stem though. In reality, EGTs are probably going to be a bigger limiting factor than sheer boost pressure by the time you reach the limits of an ARP head stud intended for a 6.9.

Something else to remember, is that it is indeed possible to have too stiff a head bolt. The bolts need to have limited flexibility and be able to stretch tiny amounts during normal operation from thermal expansion and vibration. If you clamp the head down too hard, something else will give, possibly the threads in the block itself or the bolt stem will simply snap after a while. Cylinder heads are basically dancing around when the engine is running and the bolt needs to yield enough to allow this to happen.

 

[typ4]

This is probably a crazy idea, but would thread inserts work? In other words, could you install like a Helicoil into whatever size opening would be required, to give you a 1/2" thread?

NO


BTW the heads use the same locating dowel so I would use whichever ones were done.

 

[dyoung14]

ok thanks guys for clearing that up now i understand why you cant do it