Shear Stress and Head Fasteners

[dsm-onster]

Well, I have not brushed up on my materials and statics courses, But I was thinking about this the other day. . .

Most would assume that the shear stress applied to a head fastener such as L19 or typical ARP head studs is quite miniscule. And I agree.

But considering the higher the carbon content in the steel the better the tensile stress resistance, wouldn't there be a point where such minor shear stress would cause a head fastener to fail? I do know the more carbon content, the weaker the tensile stress is neccesary to degrade the metal. As I see it, we can't assume that the forces applied on the head during combustion do nothing to push the head in the same planar direction as the deck. So am I just out in left field, or is there another good reason to O-ring the block/head other than shielding the head gasket from combustion? Having the head more anchored with respect to force/motion along the same plane as the deck, wouldn't that allow the much higher carbon steel to do it's job against tensile stress without the worry of the loading in shear being too much for that same high carbon steel?

Is that the REAL reason engine builders O-ring blocks?

 

[greengoblin]

Don't the head alignment dowel already do that?

 

[Strm Trpr]

The head studs shouldn't be loaded in shear.

 

[BigBeans]

It's always been my belief that O-rings are completely about sealing. The reduction in shear stress might be a side effect, but i tend to think you're out in left field on this one. We don't use them as frequently in the Mitsu world because we have nice, meaty cylinder walls so unless it's a very high power application, a standard MLS can usually get the job done. When you look at high boost setups in other makes (BMW is the first to come to mind), head gasket failures happen very frequently, usually without fastener stretch/head lift.

When you look at the very thin walls of say, an S52 (E36 M3 engine) you can see why any serious build has an O-ring. It's not an option to run even what we consider lower boost levels (mid 20s) reliably without an O-ring, and it has nothing to do with shear stress. If anything, BMW engines experience less overall shear because unlike ours, they are not bi-metal engines. Block and head are Al, so they expend at the same rate. Yet with less shear, they are more dependent on O-rings.

FWIW, i had an MX bike that only used an O-ring. No HG material whatsoever. The head and "jug" (block i guess) were machined, and there was a replaceable O-ring made from a special rubber-like compound that you just popped in the groove.

 

[dsm-onster]

But I am discussing detonation events. . . where two flame fronts collide explosively and off center of the head bolts. Pushing THIS motor to the edge of it's head sealing capabilities.

But you raise a good question, that is really what I am asking. Is it absolutely just pressure against the cylinder wall that causes a composite gasket to fail even with L19s, as would be speculated with other platforms that perform below this one in sealing? Why wouldn't it be the mirror of a case of the piston rocking from the intake to the exhaust with detonation? In relative terms, what is different from the aluminum piston crown vs. the aluminum cylinder dish?

The head studs shouldn't be loaded in shear.

Have you seen damage from high power detonation? The pistons are rocked so hard against the cylinder walls that they can dig and score the cylinders. When looking at a combustion chamber, the piston crown is the bottom half. What ever happens with the bottom half I believe mirrors with the top half.

There is a torsional load because the head bolts are never perfectly built. And you can see torsional movement because of the effect of detonation described above. Whatever makes the piston turn about the wrist pin so forceful as to damage the cylinder wall and piston, the same force is being applied in the mirror direction on the head dish. I am speculating that this does cause a shear load.

We build head fastening solutions to combat detonation. Ideally there would be no shear force. Because we want all the explosion to propogate in the direction perpendicular to the crank arm. But, it takes a bit experimentation to find that point, which means you're going to get some detonation along the way. . . I just can't see the head not wanting to lift on one side over the other in high power detonation. The sum of the vector still has two parts. And one of those parts MUST shear the head fasteners. . . But is it enough? High carbon steel will shear easily but tensile strength is through the roof. . . The trade offs of higher tensile strength is worth WHAT quantitatively? That is what I am asking. Because higher carbon content causes shear strength to go through the floor. There MUST be some shear stress, as the piston crown would never contact the cylinder wall during detonation.

I keep blowing my head gaskets under my intake manifold. Not between cylinders. Not on the ends. But following the same trend of torsional movement that the pistons make upon detonation.

Don't the head alignment dowel already do that?

I would submit that they are possibly not enough for the loads seen at 600whp, speculatively. Considering the damage to pistons. Which in itself doesn't constitute a trend for the masses, just what I've always seen.

 

[donniekak]

Don't the head alignment dowel already do that?

Yes they do. The studs should not see any shear force. Arp specifically states that their fasteners aren't to see a shear load, that is for dowels.

Shear forces on headstuds due to detonation are a non issue. I have been building engines, and reading engine building publication, and websites for years, and have never heard of it being an issue in any engine. Now the repeated shear of dissimilar metals heat cycling can damage a gasket.

 

[dsm-onster]

But at what point are shear forces strong enough to overcome the two not-too-thick and not-too-deeply-penetrating dowels. . . ????

I agree that there was a concession there by the engineers. Who were working with a rather strong metal in the 6bolt headbolts, but what was their agreed limit. I suspect we reach it frequently with guys swapping in arp hardware hoping for over 600whp and no gasket pop with minor detonation. But further, add the much lower shear stress of a steel like what is used with L19 studs. Much more shear stress than engineered in the stock dowels plus high carbon steel makes for a mess on the track, as I see it. . . All speculation. But that is why I'm asking if there can be a limit in either direction (high-carbon fastening hardware or dowel sthrength)seen in the sirius motor with a stock block other than forged rods/pistons.

O-ringing a block makes 4 more real big dowels . So another question is, would larger than stock dowels make a difference? Has anyone experimented with this? Or have they just jumped to o-ringing the block and gained more pressure resistance directly onto the gasket in addition. <== a combination of benefits is what makes an option a clear decision. But what part of that is because of pressure directly on the gasket, and what part of that is because of shear load turning the head off the block? . . . Probably something unanswerable currently. . . .But 'knowing your enemy' is why I ask.

 

[VETTE_50_TH]

Two very smart people gave you an answer. If you dont want to accept it, then dont. Whats your point out of this anyway.

 

[dsm-onster]

My point is understanding whether or not there is a significant shear load. And determining if it is better to simply reinforce the dowel setup or actually o-ring. . . Yes, I understand they are very sharp guys. Thanks for the help with . . . that .

 

[needsy]

This seems unlikely to me. I don't see how much lateral force could be transmitted from combustion into the head. There isn't that much surface area for the force to effect in that direction. I have a really hard time believing what force is translated into shear is enough to overcome the dowels. Have you ever heard of such a failure? In my opinion, you are worrying over nothing.

 

[dsm-onster]

Well, the reason why I worry is that I see my pistons strike the exhaust side of the cylinder where the exhaust side of the pistons get damaged at the skirt. I see the intake side of the ringland divisions get cracked. I see the piston crown at the intake score the cylinder. I see the intake side pop at the gasket. More stress is placed on one side. And I am worried about the head twisting up in that mirror direction as a consequence. . . Which is happening, because the gasket blow out on that side. So I suppose you can say that I HAVE indeed see the failure firsthand frequently, every time my head gasket lets go.

. . . Yes. I probably am 'worrying over nothing'. But I wanted some feedback other than just 'O-ring the block'. Though I know it will ultimately solve my problem. Why? . . . Which I concede is typically of no concern to the average successful gearhead looking for X goal and has a list of proven techniques to choose from to achieve it.

 

[donniekak]

Most people don't even o-ring these blocks the right way. The o-ring is placed in the block, and a receiver groove is cut into the head, in a proper setup. Even without the dowels, the sheer amount of friction is a lot to overcome to shear the fasteners. The head is in essence sitting on the block, with 100,000+ lbs sitting on top of it.

 

[dsm-onster]

Great point! Friction force alone amounts to alot. Especially with a 'sticky' composite gasket. i completely overlooked that aspect of fastening a head to a block.

 

[greengoblin]

Have you ever seen a sheared head stud? I know I personally have not. Like every one said above the dowels are more than adequate for locating the head under 99.9% of situations.

 

[dsm-onster]

Good point too! I am under the impression that the ductile nature of steel could cause it not to show much stress, though in reality high carbon steel can take minimal shear stress before moving. Not neccesarily breaking. . . But still. You are right. There arn't ANY examples of shear stress breaking in two a head stud or bolt.

 

[91stocker]

I see what your saying. I can see a reason for questioning. The combustion chamber does not receive just a vertical impact. The cc as a somewhat hemispherical shape is going to take abuse on all 180 degree. 2cyl's worth of side pressure at the same time has to be pretty substantial. It pushing in all directions at the same time might cancel that out though.

Standard grade 2 12mm bolt has a shear strength of like 1500lbs. Standard grade 8 bolts have like 4700lbs of shear strength. Quote from another site "the ultimate shear strength of a fastener is typically about 60% of its ultimate tension strength." Arp's have to be quite a bit higher then those. If all head bolts,gasket, and dowels work together. That seems pretty extreme, i think the head and block would crush flat before the head sheared off haha.

Plus the friction like mentioned. Anyone see the mythbusters that tested friction with overlapping phone book pages, pretty amazing the force it could stand.

 

[bastarddsm]

How are you experiencing these problems? Stock head gaskets and stock bolts are not really an issue till more than 500hp.

 

[dsm-onster]

Yes I have more under the hood than in my profile as of now. I will update when i have solid, clean numbers.

 

[bastarddsm]

Yes I have more under the hood than in my profile as of now. I will update when i have solid, clean numbers.

Well, that better be north of 700whp to justify the things your asking about.

 

[greengoblin]

I'd think stress corrosion(rust pitting) from improper handling would be more like to happen and cause failure in L-19 and H-11. Even then I have never seen it happen in our application.

Kevin

 

[dsm-onster]

Thanks, I think I'm just going to switch over the L19s.

Well, that better be north of 700whp to justify the things your asking about.

I still have arp head studs. . . Well on my second set. . . Folks lose composite head gaskets all the time at 550-600whp with that setup. I am when I see a minor string of detonation. And where the gasket keeps letting go and noticing the condition of my old 1g pistons that I cracked and others pistons with the same damage; that is why I was wondering about how the stress is being transfered through the head. You can see plain as day that with one single event of detonation, exposion is does not equally spread the force thoughout the combustion chamber. I know you must have seen that apparent in cast piston damage somewhere along the way.

I am concerned that going to L19s will not stop the busting of the gasket at the same spot. Because based on the above, I had felt there MUST be at least SOME shear. But apparently, all are saying here that there is no sheer activity and thus the higher carbon L19s will hold the head down tighter. So I should see an improvement with simply swapping to them. . . I know Paul Volk here really likes the O-ring + simple ARP combination and he's been working with some incredible torque numbers recently. And he was having zero issues at this power level with that form of head fastening. . .

I was just kicking around a thought. Accepting that I would need more than 700whp to ask these questions would mean that I accept that my initial speculation only would occur after that point. And I was seeing some evidence that led me to believe it was happening right now and at lower power levels actually. And that's why I was posing the question here. Other than 'just do it this way', which by the way is ok to say because ultimately we all know what works just not why many of the times; the best rebuttal has been the friction between the block and head is significant and should cancel out any vector that would cause shear stress, if even possible.

 

[bjones18]

As pointed out earlier, shear is taken by the head-block friction interface. For the bolts to see shear, they would have to have a "press fit" OD clearance. Our heads could slide ~1mm or so before the head studs would touch.

Cracking on intake-side piston crowns may be due to larger intake valve reliefs cuts. Power-side cylinder wall scraping might be affected by the crank "swinging" the rods, applying a side-load to the wrist pin.