What's new
What's new

Best compromise for less than ideal main bearing bolt design?

SynViks

Plastic
Joined
Aug 18, 2022
We're building an engine and just got it back from automotive machining. This engine normally has four bolt (10.9 M12 coarse, 95 ft-lb) main caps (no deep skirt or cross bolted mains), but we've had this block machined to fit up an aftermarket cast iron bedplate that uses ARP 2000 studs (9/16 coarse, 200 ft-lb).

Here's a picture of the stock main bearing cap registers from a youtube video:
2024-02-03 10_48_46-(68) 7.3 Build_ Cam Bearings, frost plugs - YouTube — Mozilla Firefox.png
The holes closest to the bearing cradle are deeper and have a deep counterbore, the holes farther from the cradle are shallower and have a shallower counterbore. The bolts are different length to suit the different depth the holes such that the inner and outer holes have the same number of engaged threads. The threads of the inner holes will pull on the block from deeper in the main bearing web, while the outer holes will pull on the block closer to but below the bearing cap register surface.

And here's a of the block we've had machined to accept the aftermarket bedplate kit:
PXL_20240203_021556740.jpg
The depth of the holes is the same as stock, but there is no counterbore and the threads go all the way to the top. The studs are different lengths to suit the different depths of the holes, but the inner studs will have more engaged threads vs the outer studs. The inner and outer holes will both pull on the block at the top of the bearing cap register surface.

The big concern we see with no counterbore on these mating surfaces is the threads closest to the surface will see the most engagement, ergo the thin wall area near the the cradle (now thinner after expanding the holes) will see tension it wasn't designed to handle and the top of the register will see more localized distortion when torqued down since the material closer to the surface will be pulled the most, vs material deeper in the block where there's more meat to spread out the deformation. Attached pic gives a visual representation of what I'm getting at in the second point:
2024-02-03 11_31_25-(68) Powerstroke 6.0 - Head Gasket Failures and Head Bolt Clamping Force -...png

The bedplate kit instructions makes no mention of drilling a counterbore, and even if we did, that would make the already thin wall thickness near the bearing tang notch even thinner to the point that the auto-machinist we've been working with specifically advised against it since, in his opinion, it would inevitably lead to a crack.

The auto-machinist didn't seem aware of the whole "first thread sees the most load" concept but said, in the end, this kit's design is sub-optimal and the best compromise would be to grind away the threads on the ARP stud such that engagement happens deeper in the hole (like a stock engine) and that a little more average wall thickness is retained near the bearing cradle vs counterboring. The big downside I see with this is the tension on the studs will be disproportionately placed on the minor diameter of the previously threaded areas vs the shank (which has a slightly larger diameter than the thread minor diameter).

This isn't ideal in terms of fatigue and ultimate strength of the bolt but I'm guessing the bolts are big and strong enough such that this wouldn't matter in practice, especially since the bedplate distributes more of the crankshaft forces to all the main bolts vs the stock four bolt caps. Additionally, as can be seen in this picture, the stud bottoms out with threads exposed on the bottom and the top (since the nut only threads on the very end) so the minor diameter will see disproportionate stress anyway:
PXL_20240203_021222898.jpg

Another potential issue is this will change the deformation of the bearing bore and could intentionally put it out of tolerance (since it was line bored/honed with the studs torqued down as is) but I don't know if that's really likely to result in a substantive change.

I'm still waiting on a response from the manufacturer of the bedplate kit concerning this issue. Unless they have compelling argument against doing so, I think grinding away the threads on the studs will be the solution we go with. Measure how deep the threads start in the stock block, calculate the "engaged threads per diameter" ratio it has, grind off threads on the studs such that it replicates or maybe exceeds that ratio a little and pulls on the block deeper in the main bearing web?

Before doing anything permanent with these high dollar items we wanted to get some other eyes and opinions on this thing, make sure nothing is being overlooked. If any of you with knowledge/experience of bolted joints/casting deformation/thread loading could chime in with thoughts we'd love to hear them. Thanks!


This thread is running long so a little addendum here with some things I've seen mentioned in relation to this issue:
  • I've seen it recommended to drop a ball bearing in the hole to replicate a dog point stud and bottom it out to load the bottom threads such that the load is distributed evenly when torqued, like in this picture:
    aq.JPG
    I'm not sure that helps in practice as, when the stud is pulled by the crank pushing on the bedplate, the top threads on the bearing cap register will still see the most of that force? Plus there are oil channels under some of the bolts such that it wouldn't work anyway. If I wanted to go this route I'd have to grind threads off the end of the bolt.
  • https://www.practicalmachinist.com/forum/threads/studs-stronger-that-screws.325801/#post-2841995 <- in this post it's recommended to use a high strength threadlocker to get "full engagement strength", but again I'm not sure how that'll address our issue and it will likely result in a permanent install which isn't ideal.
  • We talked a little with an ARP tech and they said something like "the stud loads all threads in the hole evenly", but I'm not sure the person I was speaking with understood what I was explaining. Do studs load female threads differently vs bolts? Or does the "first thread loaded the most, second thread less than first, tapers off until it becomes negligible past 6 or 8 threads" idea still apply?
 
Last edited:
You post is a bit wordy so I didn't fully digest it all, but I don't agree the first thread sees the most load. It could, but it also couldn't. It will depend on the very finite details of the machining as to where the load falls. In other words, in a classroom discussion it might see the most load, but in reality it could see no load or little load if the bottom threads are tighter, for example.

Going up to a 9/16" bolt from 12MM is a big jump...nothing wrong with that if the block happens to have enough meat to support it. Often they don't.

As for ARP, they are fine company making fine products, but I have found they tend to make studs and bolts for a given application then apply those also as 'the closest thing we have' to a bunch of other stuff. Or, the hot-rod companies applying their bolts do it. Either way, I see lots of ARP stuff that is not applied optimally. People think ARP solves all their problems when all ARP does is solve the problem of sub-par fasteners. I don't see a lot of broken fasteners, and when I do it's almost always rod bolts. I also can tell you ARP will throw up all over any suggestion you modify their hardware via grinding or cutting, etc.

You might have said 'why' but I didn't see where you did.....why don't you counterbore the threaded holes if you are concerned about it?

What raises my concern a lot more than the c'bore is that you are going from a 95lbs torque spec to a 200lbs spec. That's a shit load of extra force, and I'm suspicious of the block being able to take it.

Since you don't say what block this is or why you feel the need to use the bedplate with bigger bolts. Do these engines have a known issue with puking out the crankshaft?

And...why would the bedplate be cast iron of all things? Most hot rod stuff uses steel or in some cases aluminum. But cast iron? Hmmm...
 
Last edited:
Threads in the block ground, milled or cut with a tap. Same with the studs?
Do you know the difference in your modeling of what loads where?
Bolts and studs are very stiff springs.
The real world is different than that in a CAD.
CAD and FEA are all good for a starting point but the devil is in the details the computer does not know about.
I disagree with your stud thread loading pics.
 
Last edited:
You might have said 'why' but I didn't see where you did.....why don't you counterbore the threaded holes if you are concerned about it?
Here's the relevant portion:
The bedplate kit instructions makes no mention of drilling a counterbore, and even if we did, that would make the already thin wall thickness near the bearing notch even thinner to the point that the auto-machinist we've been working with specifically advised against it since, in his opinion, it would inevitably lead to a crack.

Apologies for the wordiness. I try to give a lot of context when I ask questions so people can provide informed answers... often leads to too much context haha.
What raises my concern a lot more than the c'bore is that you are going from a 95lbs torque spec to a 200lbs spec. That's a shit load of extra force, and I'm suspicious of the block being able to take it. Since you don't say what block this is or why you feel the need to use the crossplate with bigger bolts. Do these engines have a known issue with puking out the crankshaft?
When you say your suspicious, do you mean be able to take the initial torquing of the studs or the load imparted by the crank? There's a few reasons we went with the bedplate:
  • The engine has a known issue of main webs cracking from the camshaft bores, along core-shifted parting lines (where the web is thinnest), up to the outer bearing cap register bolt holes: ezgif-2-4e8ba475e5.jpg
  • The caps are retained by four bolts and the register; fretting of the caps/cap walk happens.
  • Production variance resulted in some blocks being more rigid than others. If you have the time, equipment, and a bunch of blocks you can find the rigid ones that have less of the above issues and resist twisting under load (we do not have that capability). Filling the waterjackets with "concrete" has also been done to reduce twisting.
The engine we are building with see higher cylinder pressures than stock so the bedplate (which acts as a single bearing cap and also bolts to the oil pan rails on the side of the block) was chosen to make reduce the odds of the issues above manifesting. The bedplate kit was designed in the context of pulling competitions so that's why it has the oversized hardware that isn't maybe ideal for the application; "this off the shelf ARP stud will make it stronger" type thinking is my guess.

It will depend on the very finite details of the machining as to where the load falls. In other words, in a classroom discussion it might see the most load, but in reality it could see no load or little load if the bottom threads are tighter, for example.
By tighter I assume you mean if the threads at the bottom have less clearance? I get that but these threads were cut with a tap; I assume the clearance should average out the same through the threaded hole?

And...why would the bedplate be cast iron of all things? Most hot rod stuff uses steel or in some cases aluminum. But cast iron? Hmmm..
Good question! I assume it's cost related given this is a big heavy piece. I know one company makes one milled out of chromoly but it's *very expensive*.
 
Last edited:
Threads in the block ground, milled or cut with a tap. Same with the studs?
Do you know the difference in your modeling of what loads where?
Bolts and studs are very stiff springs.
The real world is different than that in a CAD.
CAD and FEA are all good for a starting point but the devil is in the details the computer does not know about.
I disagree with your stud thread loading pics.
The 9/16 UNC threads in the block were started with OSG straight flute starting and cut with a OSG Hypro 2948601 spiral. I think the shop used a giant radial drill for the drilling and tapping ops. I'm pretty sure ARP forms threads on their studs.

The picture isn't specific to the situation, its from Mahle and concerns head gasket clamping. It was meant to illustrate our thoughts on the block deformation where the studs are pulling, but perhaps using that caused more confusion. We haven't done any modeling specific to this situation.
 
Last edited:
I’m thinking the elimination of the counter bore is what will come around to bite you in the ass. I have to believe the extra length of the original capscrews was a direct result of the having the original joint come loose. The extra expense to counterbore and add longer capscrews had a reason. Most joints come loose because the capscrews are too short.

Normally you would design a joint so the free length of the capscrew is about 2.5x its diameter wherever possible. Every joint will have different requirements. Free length is the length of the screw from the bottom of the bolt/nur head to the first engaged thread.
A 1” free length of a grade 8 capscrew will stretch about 0.004” at rated torque. The material under the head will relax about 0.001” Leaving you with 75% of the original clamp load. Any movement in service can easily take your clamp load to 0.

Your 212#’ torque is assuming a fastener coefficient of .2 which is very high but may be ok if your stud is an interference fit. If not think .1. If there is oil on the screw think less than .1. No mention of the nut and washer so no comment other that a flanged nut will throw off your torque tension and reduce the clamp load.

threads To the top of the hole would erupt the surface and screw up the joint. A a counter sink to the right depth solves this.

loctite does not keep joints from coming loose, it keeps screws from falling out. Also screws up the torque tension relationship. It shouldn’t be used in critical structural joints.
 
I’m thinking the elimination of the counter bore is what will come around to bite you in the ass. I have to believe the extra length of the original capscrews was a direct result of the having the original joint come loose. The extra expense to counterbore and add longer capscrews had a reason. Most joints come loose because the capscrews are too short
To be clear the depth of the M12 holes on the bearing register of an unmodified block are the same depth as the expanded 9/16 holes that have been cut into the block we are working with. The only difference, aside from thread size, is that the modified block has threads to the top of the hole (though they are chamfered as pictured).

Regarding the ARP studs: kit instructions say the coarse threads going into the block are coated with engine oil and installed "hand tight". For the fine threads at the other end, a supplied anti-seize lubricant must be used on the ground washers, nuts, and upper stud threads when tightening (substituting will throw off the bolt tension).
 
We're building an engine and just got it back from automotive machining. This engine normally has four bolt (10.9 M12 coarse, 95 ft-lb) main caps (no deep skirt or cross bolted mains), but we've had this block machined to fit up an aftermarket cast iron bedplate that uses ARP 2000 studs (9/16 coarse, 200 ft-lb).

Here's a picture of the stock main bearing cap registers from a youtube video:
View attachment 426521
The holes closest to the bearing cradle are deeper and have a deep counterbore, the holes farther from the cradle are shallower and have a shallower counterbore. The bolts are different length to suit the different depth the holes such that the inner and outer holes have the same number of engaged threads. The threads of the inner holes will pull on the block from deeper in the main bearing web, while the outer holes will pull on the block closer to but below the bearing cap register surface.

And here's a of the block we've had machined to accept the aftermarket bedplate kit:
View attachment 426522
The depth of the holes is the same as stock, but there is no counterbore and the threads go all the way to the top. The studs are different lengths to suit the different depths of the holes, but the inner studs will have more engaged threads vs the outer studs. The inner and outer holes will both pull on the block at the top of the bearing cap register surface.

The big concern we see with no counterbore on these mating surfaces is the threads closest to the surface will see the most engagement, ergo the thin wall area near the the cradle (now thinner after expanding the holes) will see tension it wasn't designed to handle and the top of the register will see more localized distortion when torqued down since the material closer to the surface will be pulled the most, vs material deeper in the block where there's more meat to spread out the deformation. Attached pic gives a visual representation of what I'm getting at in the second point:
View attachment 426533

The bedplate kit instructions makes no mention of drilling a counterbore, and even if we did, that would make the already thin wall thickness near the bearing tang notch even thinner to the point that the auto-machinist we've been working with specifically advised against it since, in his opinion, it would inevitably lead to a crack.

The auto-machinist didn't seem aware of the whole "first thread sees the most load" concept but said, in the end, this kit's design is sub-optimal and the best compromise would be to grind away the threads on the ARP stud such that engagement happens deeper in the hole (like a stock engine) and that a little more average wall thickness is retained near the bearing cradle vs counterboring. The big downside I see with this is the tension on the studs will be disproportionately placed on the minor diameter of the previously threaded areas vs the shank (which has a slightly larger diameter than the thread minor diameter).

This isn't ideal in terms of fatigue and ultimate strength of the bolt but I'm guessing the bolts are big and strong enough such that this wouldn't matter in practice, especially since the bedplate distributes more of the crankshaft forces to all the main bolts vs the stock four bolt caps. Additionally, as can be seen in this picture, the stud bottoms out with threads exposed on the bottom and the top (since the nut only threads on the very end) so the minor diameter will see disproportionate stress anyway:
View attachment 426523

Another potential issue is this will change the deformation of the bearing bore and could intentionally put it out of tolerance (since it was line bored/honed with the studs torqued down as is) but I don't know if that's really likely to result in a substantive change.

I'm still waiting on a response from the manufacturer of the bedplate kit concerning this issue. Unless they have compelling argument against doing so, I think grinding away the threads on the studs will be the solution we go with. Measure how deep the threads start in the stock block, calculate the "engaged threads per diameter" ratio it has, grind off threads on the studs such that it replicates or maybe exceeds that ratio a little and pulls on the block deeper in the main bearing web?

Before doing anything permanent with these high dollar items we wanted to get some other eyes and opinions on this thing, make sure nothing is being overlooked. If any of you with knowledge/experience of bolted joints/casting deformation/thread loading could chime in with thoughts we'd love to hear them. Thanks!


This thread is running long so a little addendum here with some things I've seen mentioned in relation to this issue:
  • I've seen it recommended to drop a ball bearing in the hole to replicate a dog point stud and bottom it out to load the bottom threads such that the load is distributed evenly when torqued, like in this picture:
    View attachment 426565
    I'm not sure that helps in practice as, when the stud is pulled by the crank pushing on the bedplate, the top threads on the bearing cap register will still see the most of that force? Plus there are oil channels under some of the bolts such that it wouldn't work anyway. If I wanted to go this route I'd have to grind threads off the end of the bolt.
  • https://www.practicalmachinist.com/forum/threads/studs-stronger-that-screws.325801/#post-2841995 <- in this post it's recommended to use a high strength threadlocker to get "full engagement strength", but again I'm not sure how that'll address our issue and it will likely result in a permanent install which isn't ideal.
  • We talked a little with an ARP tech and they said something like "the stud loads all threads in the hole evenly", but I'm not sure the person I was speaking with understood what I was explaining. Do studs load female threads differently vs bolts? Or does the "first thread loaded the most, second thread less than first, tapers off until it becomes negligible past 6 or 8 threads" idea still apply?
Rule: Failure occurs where local stress exceeds local strengh.
I am not seeing a strong coorolation between the cracked block and the fastening system. I do see a decreased capacity in the system by the modifications. If you want studs, the block engagement threads should be aproximately the same length as the orginal cap screw threads. The threads on the other end of the studs can be whatever the application requires. The diameter of the stud between the threads should be equal to or slightly smaller than the root diameter of the block threads. Never mind about the block holes being threaded and not counter bored. Install the studs in the block by hand to bottom out and unscrew a half thread or so, no lock tite ever. Increasing the diameter of the fastener probably decreased the fastening capacity of the joint. The change resulted in a loss of capacity on the threaded hole in the block. The tensile stress is stored in the cap screw or stud and not the cast iron block. The fracture in the block appears to be a single load brittle unrelated to fastening system.
 
Increasing the diameter of the fastener probably decreased the fastening capacity of the joint. The change resulted in a loss of capacity on the threaded hole in the block. The tensile stress is stored in the cap screw or stud and not the cast iron block. The fracture in the block appears to be a single load brittle unrelated to fastening system.
Agreed that increasing the fastener diameter is probably not the best choice. If we were designing a bedplate kit we would have retained the stock thread size.

Regarding the fractured block in the picture, you're correct that it doesn't relate to the fastening system. That was shown in response to the question of why we went with a bedplate for this engine instead of using the stock bearing caps (basically the bearing webs are on the thin side and a bed plate takes the stress off the webs and adds stiffness to the crankcase).

The potential for a crack, that the automachinist was concerned about if the 9/16 coarse holes are counter-bored, would occur in this location:
PXL_20240203_021222898~2.jpg
 
I'm no expert in such things, but it seems to me that a counter-bore is needed to ensure that there is adequate meat where the actual load transfer from steel bolt to cast iron body occurs. Bottoming the bolt at the bottom of the threaded hole would also cause the stress to peak deep in the iron, well away from the very thin web, which seems like a good idea. Assuming that there is actually a lot of metal in all directions down there.
 
To be clear the depth of the M12 holes on the bearing register of an unmodified block are the same depth as the expanded 9/16 holes that have been cut into the block we are working with. The only difference, aside from thread size, is that the modified block has threads to the top of the hole (though they are chamfered as pictured).

Regarding the ARP studs: kit instructions say the coarse threads going into the block are coated with engine oil and installed "hand tight". For the fine threads at the other end, a supplied anti-seize lubricant must be used on the ground washers, nuts, and upper stud threads when tightening (substituting will throw off the bolt tension).
That is what I understood regarding the counterbore. This means you are reducing the overall stretch in the fastener by a ratio of the cap height divided by counterbore depth + cap height. I don’t think you realize how important stretch is to maintaining a joint.

your description of the stud with oil and antiseize and 200#’ tells me that you will either yield the stud or break it.
 
That is what I understood regarding the counterbore. This means you are reducing the overall stretch in the fastener by a ratio of the cap height divided by counterbore depth + cap height. I don’t think you realize how important stretch is to maintaining a joint.

your description of the stud with oil and antiseize and 200#’ tells me that you will either yield the stud or break it.

The top of the bedplate is farther from the top of the cap registers than the top of the caps are from the registers. With regards to the torque, the kit manufacturer states that the torque values were arrived at through testing so I'd be very surprised if the studs were yielding. I'd expect the block to give first.

In any case the bed plate was torqued to the block during line bore and none broke.

Garwood: it's a International 7.3
 
I actually would not be too concerned with the counterbore causing a crack in the bearing tab area. I don't think that's a high stress area, and even if it did crack into the tab notch, so what? The crack would probably stop there and life goes on. I tend to doubt a crack there would turn into a continued crack. All of that is my guess, mind you.

You could (if the block weren't already machined) stay with the M12 fasteners and sleeve the bedplate down to accommodate them.


As for the torque increase from 95 to 200 lbs, my concern is that the threads - which are anchored in the already too thin block - won't hold or will cause a lot of distortion in the casting. I also wonder why 200lbs is the value...I haven't checked but that seems like a lot for a 9/16" fine thread.

I remember Robert Landy, back when he ran DLI, back when DLI was still in business, telling me on a 426 Hemi to ignore all the ARP and even OEM figures and to leave the lifter valley head studs 'loose'. I don't recall the values, but it was something like instead of the factory 55lbs or ARP's 75lbs, use 35lbs. That was because the stud - if torqued to spec - would distort the head in that area and cause a head gasket failure. Their actual experimenting, and experience, showed that the lower value did the trick on all counts.
 
The counterbore is to move the end of the thread on the stud below the joint line to increase the strength of the joint .
Has anyone mentioned the following, I haven't seen it. - Shorten the 9/16 threads on the stud end that screws into the block, starting at the block mating surface so that the overall length and the deep end is left alone but there is an unthreaded portion closest to the mating surface equal to what would be the desired counterbore depth? I hope that sentence is clear. I mean turn it down to minor diameter starting in the center (edit - beginning at the start of the opposite thread, I was thinking the center of the stud would already be at the minor dia.) and feeding toward the end by 1/2" or however long you would otherwise make the counterbore depth. This would move the first engaged thread (1/2") deeper into the block, would present a longer length of fastener for stretch, and would leave the full 9/16 tapped threads in the block to support the tang area that cracks. I'm not big on modifying fasteners, perhaps there's a suitable stud of this configuration available.
 
Last edited:
loctite does not keep joints from coming loose, it keeps screws from falling out. Also screws up the torque tension relationship. It shouldn’t be used in critical structural joints.

Loctite doesn't 'screw up the torque relationship'

It doe's change the torque req'd depending on which loctite you use.

I have a document from one of the largest companies in the US that has approved torques for dry/oil/red loctite/blue loctite applied to threads. Safety is their NO 1 priority (no it's not Boeing :sneaky: ) so they did the work req'd to clarify the effect of different liquids used on threads.
 
OK, fastener question. The OP is using studs with coarse threads on one end, and fine on the other. When figuring stress in the fastener, do you use the area from the coarse thread and the tension assumed from the fine thread nut to get the stress? Or is the desired stress sufficiently below yield (or ultimate) that the difference in area between coarse and fine threads is insignificant?
 
Agreed that increasing the fastener diameter is probably not the best choice. If we were designing a bedplate kit we would have retained the stock thread size.

Regarding the fractured block in the picture, you're correct that it doesn't relate to the fastening system. That was shown in response to the question of why we went with a bedplate for this engine instead of using the stock bearing caps (basically the bearing webs are on the thin side and a bed plate takes the stress off the webs and adds stiffness to the crankcase).

The potential for a crack, that the automachinist was concerned about if the 9/16 coarse holes are counter-bored, would occur in this location:
View attachment 426573
Maybe a small amount of stress there, not enough to concern. Provided the threaded portion of the stud is about the same as the orginal cap screw. SAE specifications state the length of the threaded portion of the threads for each size cap screw/bolt. The thickness of the block's web has the least reistance to failure. If you consider a SAE nut for the size of the stud and measure acoss the flats, the thickness of the web should be a bit thicker that the nut across parrell flats, at the threaded section. If you are using a nut on the other end of the stud, stress systems for bolted joints apply. Remember bolts have nuts and cap screws screw in to threaded holes in machines and components. SAE bolts and their nuts are manufactured from the same alloy. Studs and the machine components are frequently made from different materials and have different requirements for installation. therefore increasing the strength of one and not the other, may be counterproductive.
 
Last edited:








 
Back
Top