ARP head studs

So if you don't mind me asking, what are we gaining by using ARP studs on a Model-A engine?

I guess my view is with specifically made Model-A/B Grade 8 studs being under $40 (-which includes the hardened nuts), the 7/16-14 threads cut into the cast iron cylinder case are by far the weakest link in the entire scenario. Maybe someone needs to post a pull-out value of a 7/16-14 thread in cast iron, -and then compare those numbers to Grade 5 to Grade 8 (8740) commercially available studs, -vs. the ARP 8740 Moly fastener.

FWIW, if someone is going to measure the pull-out strength, the print calls for the threaded hole to be 3/4" deep and to be threaded 7/16-14 USS tap with .392 +/- pitch diameter.
 
I can't speak for others, but my interest in ARP is specifically about their claim that with their studs and lube (Ultra-Torque), you can substantially cut preload scatter.

As you and I discussed yesterday, a Model A head doesn't require preload anywhere near the yield point of a Grade 8 stud. So when we're torquing to 55 (or 60 or 65), we're doing so for a number of reasons that aren't strictly about engineering:
- to compensate for a non-flat deck
- because that's (roughly) how they did it at the factory
- out of the proverbial abundance of caution

The problem is that there's only so many times you can put 8,000 or 10,000 lbs of clamp load on the block before the threads start to pull out. The further problem is that the preload you're applying isn't consistent:
- if you torque 10 studs to 55 lbs, even with oil on the nut threads, the actual preload can vary by +/- 8%
- if someone is re-using fasteners, the preload will go either up or down consistently* every time you reuse that stud/nut

* - (whether it goes up or down seems to depend on the nature of the operation - if the fine threads are being progressively distorted, clamp load goes down; if they're being merely burnished, clamp load goes up)

Either scenario creates a risk that some of your block threads will experience a clamp load much higher than you intended, which degrades them and increases your risk of head gasket failure.

ARP's claim is that, with their studs and lube, you can hit exactly the same preload every time, both across multiple fasteners and using the same fastener multiple times. If that turned out to be true, it might be really good to use ARP for Model A blocks because you would have confidence that the preload you were applying was consistently within the block's ability to take without distorting. We talk sometimes about having a responsibility as a caretaker for these cars – it feels to me like, if a vendor is proposing that their product reduces the risk of accidentally over-torquing the block, that's something we should at least take a look at as part of the mission to preserve these blocks.
 
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As a side note, I did some back-of-the-envelope math on torque. This is purely speculation, not advice and certainly not an expert opinion.

Maximum compression ratio likely to be used on hobbyist Model A head: 7:1

Peak combustion pressure, rule of thumb, 100x CR: 100 x 7 = 700 psi per cylinder

Maximum cylinder bore ~4 inches across, so area = 4(pi) sq. in.

700 psi x 4(pi) sq. in. = 2800(pi) lbs peak lift force on the head

Rule of thumb, you need clamp load of 3x peak lift force to prevent head lift-off, so 3x 2800(pi) = 8400(pi) or about 26,400 lbs of clamp force

Each cylinder is surrounded by five head studs, so the clamp force per stud needs to be 26,400 / 5 = ~5300 lbs.

If that math is right (and I have no idea whether it is right), the minimum required torque on a flat head, with a flat deck and good lubrication, could be as low as 30 ft-lbs. I mention it not to argue that we should be torquing to 30 ft-lbs, but rather that it might be the case that over-torquing studs is the rule rather than the exception. We don't know! I wouldn't be surprised if the factory put a higher clamp load on the studs than was strictly needed. But if you keep doing that for 90 years, what does that do to the block? I think it's worth investigating.
 
Brent has asked the right question, for a stock motor no gain, in my opinion.

As for the others who have the tendency to increase power output it is optional.

If you look at the grade 8 specifications, for torque, it is usually limited to about 50 or 60 ft/lbs. (7/16" fastener)
I have had grade 8 bolts yield at 55 ft/lbs. Once this happens, the bolt is junk.

Broken off studs in a block? I hate having to dig them out, most are machined and the threads saved in the block, not always. Trust me, I get them when others have tried and given up. Why did the stud break off in the first place?

In critical assemblies, better fasteners are necessary.

As for the strength of threads in the block, on performance motors, I test every one.
I have a couple ARP studs just for this purpose, consistency makes a difference.

John
 
johnneilson said:
As for the strength of threads in the block, on performance motors, I test every one.
I have a couple ARP studs just for this purpose, consistency makes a difference.
Click to expand...

This is exactly the thing I'm worried about: how to design a testing protocol that reliably duplicates the forces applied in a real cylinder head install, but doesn't exceed those forces by so much that it damages the block threads in the process of testing.

This is why I'm looking at preload scatter. If the test protocol has high scatter, the test might lead to both low preload values (too low to simulate the real force on the engine block) and high preload values (so high as to damage the block).
 
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alexiskai said:
This is exactly the thing I'm worried about: how to design a testing protocol that reliably duplicates the forces applied in a real cylinder head install, but doesn't exceed those forces by so much that it damages the block threads in the process of testing.

This is why I'm looking at preload scatter. If the test protocol has high scatter, the test might lead to both low preload values (too low to simulate the real force on the engine block) and high preload values (so high as to damage the block).
Click to expand...

Yes, this poses quite the predicament, if you apply torque high enough to start the deformation, you will more than likely cause a catastrophic failure. Then what do you do? I do the testing on the bench, if one fails, it is easy to go into the machines. If doing this in the car, not so easy, especially near the rear of the motor. Guess which ones are going to fail.

The issues of deck rigidity, head rigidity and the gasket compression all come into play. The worst situation is when two head compressive type gaskets are used, this is just inviting trouble.

John
 
alexiskai said:
As a side note, I did some back-of-the-envelope math on torque. This is purely speculation, not advice and certainly not an expert opinion.

Maximum compression ratio likely to be used on hobbyist Model A head: 7:1

Peak combustion pressure, rule of thumb, 100x CR: 100 x 7 = 700 psi per cylinder

Maximum cylinder bore ~4 inches across, so area = 4(pi) sq. in.

700 psi x 4(pi) sq. in. = 2800(pi) lbs peak lift force on the head

Rule of thumb, you need clamp load of 3x peak lift force to prevent head lift-off, so 3x 2800(pi) = 8400(pi) or about 26,400 lbs of clamp force

Each cylinder is surrounded by five head studs, so the clamp force per stud needs to be 26,400 / 5 = ~5300 lbs.

If that math is right (and I have no idea whether it is right), the minimum required torque on a flat head, with a flat deck and good lubrication, could be as low as 30 ft-lbs. I mention it not to argue that we should be torquing to 30 ft-lbs, but rather that it might be the case that over-torquing studs is the rule rather than the exception. We don't know! I wouldn't be surprised if the factory put a higher clamp load on the studs than was strictly needed. But if you keep doing that for 90 years, what does that do to the block? I think it's worth investigating.
Click to expand...

Colin, two things that should factor into this is; Detonation and the forces created by this trying to push a piston against inertia and rotating mass, ...and the premise that in normal application, the piston is moving downward and is not rigid or fixed. Therefore the lifting pressure or forces against the bottom side of the head may not even be as great as your calculations under the second scenario.
 
I use the ARP studs in my engine. The thing I like is the allen head socket in the top end. It's a lot easier to pull the head of you remove the nuts and then unscrew the studs. With this setup, I can remove and install the head without removing the fan and water pump.
 
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