Connecting rod bolt stretch question

[Hipopinto]

Hey all I have yet another question on my build

I have a very high quality set of connecting rods for my current build. These are American made 300m material dyers top rods brand with 7/16 ARP LA 625 D bolts

I’m using the supplied thick brown lubricant on the bolts

I’m noting some differences while assembling

Torque range on the card is 70-80 lbs but to get these to stretch to .0054-.0058 that range is all over the place

I have the bolt measurements recorded and I have not overstretched the bolts

Some rods stretch at about 70 lbs some stretch at 85 lbs I’m using a clicker MAC torque wrench to start then I pull them and measure

Am I making a mistake in this method?

Everything I read says stretch is far more accurate for fasteners but why do we measure stretch on rods but only torque on main and head bolts/studs?

I just want this little guy to live a long life while making a tremendous amount of power

Any feedback would be awesome guys

Thanks again
Dave

[blykins]

How many times have you stretched each bolt?  You will need to do it 2-3 times each bolt, each time putting lube on the threads and under the heads. 

You will ultimately see some variation, so if you've cycled them a couple of times, just do whatever it takes to get to the correct stretch.  On the higher end stuff I do, most of the bolts are torque-angle, and to get them to stretch the correct amount, sometimes you have to go 1-2° past spec. 

[allrightmike]

I had a bolt stretch vs torque value issue on a set of Eagle rods with premium ARP bolts. One of 16 bolts felt soft and came up to stretch at around 20 ft. lb. to low, even felt soft. I had pre torqued the bolts several times previous to assy. and re lubed each time with proper lube. If your bolts in question are not way out of range and do not feel soft through the wrench I wouldn't worry about it. I did not use that bolt.

[allrightmike]

Another thought, pretty much impossible to measure stretch on main bolts and head bolts because of the blind holes.

[Joe-JDC]

Another way that has worked with success, is to measure bolt stretch, and see what the torque was required to get there, and then torque all the bolts to that torque.  We survived two world wars, multiple regional wars, built cars for over a hundred years, and now all of a sudden, we need to stretch our bolts for them to survive?  Joe-JDC

[blykins]

Another way that has worked with success, is to measure bolt stretch, and see what the torque was required to get there, and then torque all the bolts to that torque.  We survived two world wars, multiple regional wars, built cars for over a hundred years, and now all of a sudden, we need to stretch our bolts for them to survive?  Joe-JDC

I'm right there with you, Joe.  I usually try to follow the suggested torque method by the manufacturer, but if one of the methods is torque or torque/angle, I'll go straight to that.

[WConley]

Most of the OEM's are using rundown guns that measure torque and angle.  We started doing that at Dearborn Engine in the early 90's. 

Often they are used with torque-to-yield fasteners, but they work just fine with high quality standard pieces.  If you're beyond the specified angle and the torque hasn't come up to the prescribed range, something is definitely wrong!

I remember doing a quality audit at the plant, and I watched a bunch of engines get kicked off the line for failing cylinder head bolt torque.  Up comes the foreman, who looks around but doesn't see me.  He grabs a big breaker bar, gives all of the cylinder head bolts a good tug, and sends the engines on down the line 

So much for high tech fastener monitoring 

[Hipopinto]

That is precisely my thoughts

A few of these stretched in perfectly

Others are requiring a lot more torque to make it happen

So my question is at what point is torque correct or should stretch be the go to?

Also at what point in the automotive time line did we start measuring stretch? Did they do it in 1966?

I’m just curious as to how the stuff was done to compete in LeMans and Daytona back then

I just want this to hold together and not self destruct maybe I’m paranoid

Thanks
Dave

[blykins]

Stretch is the most accurate way of checking something and when a company gives me a stretch number, then I'll use it.  If it takes more than 70 to stretch it, then it takes more than 70. 

I have a digital torque wrench that measures a myriad of different things.  When I have a torque-angle spec, i.e. 30 lb-ft + 60°, it will read me the amount of total lb-ft it took to get to spec.  That will vary 5-10 lb-ft sometimes. 

With that being said, if you don't like the way something feels, then toss that bolt and buy another one.  ARP makes most of the rod bolts out there, whether it be an entry level Scat rod bolt or an L19 Oliver rod bolt.  They are available individually and I'll buy extras when I'm doing a $$$ refresh.

[BattlestarGalactic]

Bolts made by the thousands and then heat treated will have variances depending on the accuracy of the manufacturing of the barstock material and the final heat treating process.  Nothing is perfect when made in those large quantities.

I grew up with an old school torque wrench, and still have it.  Agreed, it all works fine for the most part but stretch is just a high tech way to make things more accurate.  Technology is great, but can drive you nuts in the use of it.

[GJCAT427]

Gotta agree with Larry Most repair shops don`t have the luxury or tools to do torque to yield. so out come the good ole torque wrench.

[allrightmike]

Is it accurate to say that torque values are established to provide a desired amount of bolt stretch? Is bolt stretch what keeps a fastener from loosening?



















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[SSdynosaur]

As a user, my first experience with anything HiPerf from Ford that had a documented stretch spec. was when Ford released the "SPS" rod bolts for the NASCAR connecting rods. Jack Roush gave me a set along with a small strip of paper imprinted with the stretch spec. My next question was how do I do this since stretch gauges were, definitely not a common item. His reply, "torque them 70 lb-ft with red lock-tite and don't concern yourself about the stretch spec..

[WConley]

Bolts made by the thousands and then heat treated will have variances depending on the accuracy of the manufacturing of the barstock material and the final heat treating process.  Nothing is perfect when made in those large quantities.

I grew up with an old school torque wrench, and still have it.  Agreed, it all works fine for the most part but stretch is just a high tech way to make things more accurate.  Technology is great, but can drive you nuts in the use of it.

Stretch is the most accurate measure of clamp load.  Torque is an indirect measure, since it depends upon the integrity of the threads and the thread friction.  If you have a burr on a thread, or you're using the wrong (or no) lube, the torque value will be way off.  Likewise, if a thread somewhere on the bolt or hole (nut) is starting to shear, the torque value will go soft.  Incorrect heat treating on the bolt does the same thing.

For super high performance fasteners, you want both stretch and torque to be in a known range.  That's belt and suspenders - good threads and good material properties on your bolt.  Stretch gauges have been around for a long time, but were limited to high end stuff.  I think the DC bolt rundown guns came out in the 80's for production use, but didn't come into widespread use in OEM production until the 90's.  Before that it was air guns and finish torquing with clicker wrenches.

[cjshaker]

I'm not denying that stretch is the best method, but has anyone experienced, or even seen a failed engine due to a failed rod bolt? Even the stock stuff? I suppose it's happened, but I've never seen it with anything, big or small. That's good enough for me, although I'll replace any stock bottom end hardware these days for any performance engine. 40-60+ years of use is way beyond my comfort zone.

Using the wrong lube is a bonehead user mistake. A light burr on a thread should go away after a few torque cycles, which should always be done just to "set" the threads for a good torque reading. Any bad burring should be chased or replaced.

I've had bolts (not rod bolts in my cases) that suddenly went 'soft' when tightening. At that point you have to stop and carefully check the threads for pulling out, or toss the bolt if the threads are ok.