The Stovebolt Site Forums The Shop Area Engine Shop Reassembling engine - question on bolts

I decided to paint the engine before putting it back in. Would you suggest replacing the bolts that hold the intake & exhaust manifold on? If so do they need to be a certain grade? I've already ordered new bolts for the pressure plate and flywheel. Any others I should replace or just clean them up and reuse?

I like to replace the bolts for the manifolds. They have been heated & stressed many times. I use grade 8 bolts for all replace ments.
George

If you go up on the grade of a bolt, also be sure to increase the torque you use to tighten them. Grade 8 bolts tightened to Grade 5 torque specs will back out due to insufficient stretch to keep them tight. Hope you don't strip the threads in the head!
Jerry

I've always run mine under the wire wheel, lubricated them to eliminate any false tension/reading on the torque wrench. Never had a problem but there is a first time for everything.

I'm not a mechanical engineer, but as I think back a couple of eons ago to the materials class I once had to take, a steel grade 5 and grade 8 bolt should have the same modulus of elasticity - i.e. the strain on the bolt for a given stress is the same as long as no permanent deformation occurs (fastener is said to be in the "elastic" region of the stress/strain curve). Therefore, either bolt will "stretch" the same amount for a given load until the point it is permanently deformed.

A grade 8 bolt is considered stronger, since it takes more stress to permanently deform it (fastener yields and enters the "plastic" region of the stress/strain curve).

I also remember sitting through a presentation once by some fastener engineering guys. I seem to recall them saying that for most joints, the recommended fastener torque is typically a function of the desired clamp load, which in turn drives the fastener selection. (For example, an interfaces that requires a clamp load which exceeds the capability of a grade 5 fastener, would require a grade 8 fastener.) Therefore, I would be hesitant to change the recommended torque of a joint without good reason.

All the above, of course, assumes we are not talking about a torque to yield design, which I doubt is the case here.

Hopefully, some ME is reading this, and can spank me and set me on the straight and narrow if I have run off the road here...

Best Regards...

Edit: After rereading your post, I can't help but wonder if the issue of the grade 8 bolts backing out where the grade 5 bolts do not could maybe be related to the softer grade 5 bolt head "catching" on a surface, when the harder grade 8 bolt does not ?

OK. This is a first for me. I may get the chance to actually get two people mad at me in the same thread...


I totally agree that clean threads are essential to consistent torque. However, it is important to know that lubricating them can have a large effect on the clamp load for a given torque, which may have less than desirable consequences.

Thinking back again to that fastener presentation, I remember a slide that was pretty interesting. It said that for a given torqued fastener, approximately 50% of the torque was used to overcome friction under the bolt head, roughly 40% was used up in friction of the threads, leaving only about 10% to provide clamp load. (Threaded fasteners are actually pretty inefficient, if you think about it).

The friction on either the bolt head or threads will be significantly changed by adding (or not adding) lubricant. In the "with lube" scenario, this means that as less of the torque goes into overcoming friction, more goes into stretching the fastener, resulting in higher than anticipated clamp loads. For some joints, this may not be a problem. For critical ones, it may be a pretty big deal. Of course, determining which joint is critical, and which isn't, is not always very straightforward.

For this reason, most manufacturers or assembly guides will specify either wet (lubricated) or dry torque specifications. Some instructions even specify the type of lubricant to use on the fastener if the torque spec is "wet".

Fortunately, there is a lot of "over design" in older vehicles, so they are pretty tolerant. If something has been working successfully for someone for a long time, it probably will continue to. However, I think it is also good to know some of what is going on behind the scenes, just in case things go south.

I know that I was once totally guilty of this when it came to lug nuts. My dad always put a couple drops of oil on each stud before installing the lug nuts. Torque for these is typically specified as "dry", so for most of my formative years, I was probably over-torqueing my wheels. Even now, when I know better, I feel like his ghost is standing behind me with an oil can every time I rotate my tires.

Once again, please take the above in the spirit that it was given. I am not trying to criticize, just inform.

Best Regards...

p.s. Yes. I like to replace the manifold bolts, too. Most are pretty rusty, or beat up when they come out.

If it really needs to stay tight, use studs, not bolts. Instead of putting the rotating force on the somewhat brittle cast iron threads, a straight-out pull is much less likely to strip them and the tightening torque to stretch the fastener is put on a fine thread at the end of the stud. That's why the drag racers use studs on the cylinder heads on their blower engines.
Jerry

Using studs with NC threads in the head and NF threads under the brass nuts make for a cleaner assembly that is easy to service, rather than having to use a torch.

Ed

Grade 8 is probably overkill and yes, you run the risk of cracking one or both of the castings. The intake and exhaust assembly does not need to be cranked down. The inside clamp bolts are 15-20 foot pounds, the end clamps are 25-30. Really it is just a clamping action as the intake rings and the end dowel pins on the head provide the horizontal and vertical stability. The bolts provide the just clamping/holding pressure, not to hold the assembly in place. And no, it is snot a good idea to replace the dowel pin on the head with a bolt. You could easily crack the exhaust manifold. Trust the engineering on this one, its been proven for nearly 100 years. The only reason to replace the dowel is if you are running tube headers. So if the stock hardware is not badly pitted, they are generally reusable. Otherwise, I would use grade 5 to get the proper stretch. I would also use a Rimflex gasket. They are a one piece gasket and work/seal way way better then the stock type gaskets.

Wow I miss shop class. Lots of knowledge being shared. Believe that grade 5 would be the best choice as well. I lean towards replacing hardware when reassembling parts, to me it's just a good practice after having broken several used bolts.

Sure do appreciate all the great information. Such a great forum, even a basic question gets really good in depth responses!

Mike

Unfortunately Rimflex is not made for everything, making by hand the old way does not work.

Ed

That would be me, but I believe you're still on the road here. Good info.
To follow your point about lube, my modern truck has 2-piece lug nuts and specifies a drop of oil on the interface between the two parts and NOT on the threads. That additional step of the drop of oil adds yet another step of cleaning the wheels of the 8 stripes of oil after a few miles.
To add another thing to the puzzle, fine threaded fasteners require a different torque to obtain the same clamping force as coarse threaded ones. But as was said, there's a lot of tolerance on these old trucks.
As far as reusing fasteners, there's nothing worse than that sickening feeling when you're halfway to the torque spec and the needle doesn't move on the wrench, indicating stripped threads either on the bolt or worse yet, the threaded hole in the block. Worst case, break out the helicoil kit. Another really bad one is when the needle drops to zero and the head pops off the bolt. If you're lucky, disassemble and spin the stub out unless it's a tight fit, then go to one of the methods to extract it (check the tech tips for lots of good suggestions).
Moral of the story is to inspect your fasteners for rusted threads and general condition and if in doubt, replace them.

What trips my trigger is the guy who is skilled at getting fasteners, (especially wheel nuts) within just a few ft/lbs of the break/strip point without actually breaking anything, and then sends his customer out the door to lose a wheel after a few miles. One thing that happens pretty regularly is that the lugs get somewhat different torques and the tightest one is doing most of the work of holding the wheel on. Once that one breaks or strips, then the next tightest one takes over. When that scenario repeats 5 or 6 times, there goes a wheel! Properly-torqued lugs share the strain equally, which is why I would never let my students install lug nuts with an impact wrench- - - -snug them all, and then break out the torque wrench. Over-torqued and stretched fasteners can't be fixed by loosening and retightening correctly- - - -the damage is already done.
Jerry

Jerry, that's very true. The tire monkeys with their high powered impact wrenches set to as high as they can go are at fault. Getting the jobs out the door is more important than doing the job right. Even torque sticks are not very accurate, if they even bother to use them. A hand torque wrench is really the only way to get it right, and try and find one that's been calibrated in it's entire life.

Mine gets a certified calibration annually, but I get called upon to be an expert witness on occasion, after inspecting the aftermath of some of those lost wheel accidents. Most legal beagles don't want me anywhere near a witness stand after reading the reports I write!
Jerry

LOL, Especially the offending shop's lawyers.

What a great and important topic with loads of spot on information concerning the wonderful world of torque. One thing I would like to expand upon is about the clean threads on the bolt AND INSIDE THE TAPPED HOLE is extremely important... meaning that chasing those internal threads is a must. About 40% of the effort turning the torque wrench is consumed by friction across the threads and another 50% is consumed by friction under the bolt head as tightening occurs. That only leaves about 10% of your effort available to stretch that bolt to provide your clamping force. That is on a good day. Dirty thread interfaces rob more effort from your 10% so those foot pounds you dialed in are not making it to clamp your load. Your wrench clicks or" beeps" but your preload you need is not there. Pretty important for all those head bolts as you want the same clamping load over your head.
Also when I am done using a "clicker" type torque wrench I dial it back to zero as you don't want it sitting in your tool box with load on the internal spring... putting it out of calibration for next time.
I agree in that I cringe when I see somebody hit lug nuts full on with an impact gun putting full torque on each lug nut. You need to creep up on clamping that wheel. Half torque then full torque. Same goes for cylinder heads.

I snapped this photo several years ago on the way to work. It is at a point on a road that transitions from an expressway into a normal street, so as traffic was backed up at the light, I was able to get out my phone and safely take a pic.

I show it to my students when we cover fasteners and torque.

You will note that the object wedged under the guard rail is an HD truck brake drum.

In addition to wondering how it came to end up there, I can't help but wonder what happened to the ten lug nuts and wheel(s) that had to come off first.

Some say it must have fell off of a scrap truck, but the inner drum surfaces were shiny and clean the first day I saw it. They slowly started to rust over the next week or two as it sat there. After about three weeks, it was gone. I assume some scrapper jumped out at the light and pried it out of there.

Best Regards….

Edit: p.s. Could not agree more concerning the importance of torque wrench calibration. Having spent most of my career in engineering or test labs, the annual calibration interval for tools like these was the norm.

For stuff at home, there are local labs that can provide this service for a reasonable fee. Very inexpensive piece of mind.

Once a threading tap gets too dull to cut a clean thread, hit the tips of the cutting edges with a grinding wheel to blunt them a little and make a thread chaser. Trying to chase threads with a new, sharp tap is a good way to oversize the female thread and create a sloppy fit. I usually keep a set of worn taps in a separate tool box drawer, modified into thread chasers to avoid that problem. Dipping the shank of the tap into some cold gun bluing solution to blacken them makes it easy to tell a chaser from a regular tap at a glance.
Jerry

Totally agree. But, if you happen find yourself without a worn tap to fit your application, Snap-on, Lyle, Craftsman, and others offer kits like these.

They even include thread files which are handy for things like cleaning up the threads of the stubs of full floating axles, after some idiot has used a chisel to tighten the bearing nuts...

Best Regards....