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We are at an interesting point with the TR2 engine build.

Conrods are MaxPeeding fitted with ARP 2000 bolts

The supplied spec says use 45 ft lbs torque and ARP thread lube.  
This suggests it gives 0.005”- 0.006” bolt stretch

 

 CC3CCB8E-0560-4065-BA54-7FE4CCD97251.jpeg.f503e801c5295b275d3e33c7a985adf8.jpeg800F277D-08B1-4B3F-8562-34C66F77A465.jpeg.5ad009ecf31092db3bae54dca3e17533.jpegF2CA5F1C-8156-43AC-BBBD-403DEE113CC4.jpeg.4175d005abc0b49d232d90e7ef97957d.jpeg           

Have checked torque wrench accuracy and it is fine.

Torqued the bolts and we are getting only 0.003”-0.004” stretch.

Should the torque be increased with hope of getting the greater stretch or left as recommended?  
 

I have read Bob’s actions with these rods and bolts in his excellent rebuild thread and he found an angular torque did not provide the torque recommendation.  
 
Thanks for any advice

Peter W

PS. Photos are for principal of how the measurement is done.  Not the measurements finally observed.
 

 

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Release and torque them again, … and do the same thing again. About 25 years since I last fitted the ARPs but I seem to remember the bolts need pre stretching by multiple torque and resets before you get the correct stretch figures. Check out the ARP threads for better info than my memory.

Mick Richards

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2 minutes ago, ntc said:

Suggest you check ARPs website first 

Thanks.  We will do as suggested before any spanner twisting.

Peter W

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Hi Peter,
I do not know if these bolts are plastically deformed when the specified torque is reached. If they are, you are measuring the sum of plastic and elastic yield. You are probably aware of this, but worth noting. As a result, the measured elongation with the bolts installed cannot be used to calculate the actual bolt stress using Hooke’ s law. This can only be done in the elastic range of the bolt material.

Cheers,

Waldi

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Here you go Peter.

I often refer to hot rod engine builders who achieve great results and are microscopically interested in being able to achieve consistent results. This is from 2013 I believe, I would be confident there hasn't been any worthwhile change in methodology since then.

https://hotrodenginetech.com/how-to-tighten-stretch-a-rod-bolt/

This is how the rod bolts are stretched, the bolts need to achieve the stretch specified to achieve the clamp ARP (or the conrod manufacturer separately ) have established as required.

ARP Rod Bolt Stretch Gauge How-to

The ARP Rod Bolt Stretch Gauge makes checking rod bolt stretch quick and easy as shown. And you can use the finger ring to guide the gauge deeper into the block to easily check bolts that are farther down as seen on the lower right.

Most engine builders concede that torquing rod bolts during final assembly is no longer an accepted practice. While no doubt adequate for general engine rebuilding where loading rarely approaches that of competition levels, basic rod bolt torquing simply cannot account for the many variables that affect the ultimate integrity of the fastener and rod cap relationship. Closer examination is necessary to fully understand the factors involved and the reasons why any particular method offers superior service and durability. Two methods are commonly used to tighten rod bolts and rod bolt nuts, but only one excels for application to racing and high performance engine assembly.

Common Methods of Tightening Rod Bolts

Torquing a bolt or nut to recommended specs with a torque wrench

Tightening a bolt or nut to the manufacturer's recommended stretch factor

Among these, tightening a bolt or nut to the recommended rod bolt stretch factor is the preferred method for all racing and high performance engines.  It has long been recognized that improperly tightened fasteners will not apply the required preload to the application. If rod bolt toque is insufficient for the application it will not apply the proper clamping force and the assembly is subject to failure due to excessive movement.  If the bolt is over-torqued it exceeds the design stretch limit and is subject to failure by surpassing its maximum yield point. All fasteners require a specified amount of preload to achieve the optimum design clamping force. Correct preload is only achieved by stretching the bolt the proper amount as determined by its metallurgical properties and extensive testing. In the practice of race engine assembly few fasteners can be properly checked for stretch because it is not possible to measure the overall length of blind hole fasteners as they are tightened. Rod bolts are a fortunate exception. They provide access to both ends of the fastener for easy measurement.

ARP Ultra-Torque fastener assembly lubricant is specially formulated to duplicate all ARP fastener preload specifications, thus providing consistent bolt torque and rod bolt stretch recommendations.

Factors Affecting Fastener Performance

Manufacturers determine a bolt’s ideal stretch factor through known metallurgical characteristics and extensive testing to establish a “Yield” point or dimensional range where the bolt becomes permanently elongated and will not return to its original length. The “Yield” point is primarily affected by the parent material of the bolt, bolt length and shank diameter, ductility, heat treatment and other factors that contribute to the bolt’s performance characteristics. Once the yield point of a bolt has been identified manufacturers typically peg the recommended stretch at 75-80% of the known yield point. This provides the maximum clamping force known to exceed the known or anticipated load factor of the engine’s rotating assembly. According to ARP a bolt stretched to .001-inch or more of its original static length has been compromised and must be replaced. This minimal amount of excessive stretch serves to underscore the importance of stretch and the loss of clamping force that occurs beyond the design limit.  In the absence of proper stretch every cycle of the rod and piston assembly will witness a slight separation between the rod and the rod cap. The separation reverses as the reciprocating forces alternate causing an ongoing cycle of stretching and relaxing that ultimately fatigues the bolt. To preventing these separation cycles the bolt’s preload or clamping force must exceed the forces exerted by the engine’s rotating assembly.

You can apply ARP Ultra Torque Lube to the bolt threads and the bottom of the head or to the mating surface on the rod cap itself.

Cap screw type rod bolts that come in your connecting rods are chosen by the manufacturer to meet the specific requirements of the rod’s intended application. Their selection is no accident as there are at least six or more primary rod bolt materials all with different specifications according to use. Their tensile strength ranges from about 210,000 psi up to 280,000 psi. Each will have its own recommended stretch factor.  As recommended by leading manufacturer ARP (Automotive Racing Products), each rod bolt must be stretched to a recommended spec when the rods are installed in the engine. ARP’s rod bolt stretch gauge allows you to measure each bolt’s static (free) length and monitor its stretch as you tighten it.  As seen in the accompanying photos an ARP stretch gauge is particularly convenient because it incorporates an integral finger loop that makes it easy to grasp while inserting it into the bottom of the engine for quick stretch checks as you tighten each rod bolt. Follow the procedure as specified below to adjust rod bolt stretch to the recommended range as specified by the manufacturer.

The spindle points on the rod bolt stretch gauge fit precisely into the dimples at the center on each end of the rod bolt; providing consistent measurement points as you tighten the fastener.

Install the rod cap and insert the rod bolts with the recommended lube on the bolt threads and the bolt seating surface.

Use a nut driver or the appropriate ratchet and socket to snug the bolts and gently seat the rod cap.

Loosen both bolts to ensure there is no preload.

Zero the stretch gauge and measure each bolt independently to determine and record its free length.

Torque both bolts to an indicated 25-30 ft-lbs to ensure a small amount of preload.

Tighten one bolt to the manufacturers recommended torque and measure the stretch.

If necessary increase torque until you reach the recommended stretch.

Once the bolt is stretched to the recommended spec, loosen the opposing bolt and retighten carefully until the recommended stretch is achieved.

Check and record the torque value required to reach the recommended stretch.

If you can duplicate the torque value on at least two rods many engine builders will consider that a valid torque figure for the rest of the batch.

Other builders will measure and record every rod bolt to ensure maximum integrity of the fastener assemblies.

Notes:

The spindle points on the gauge must center in the dimples on each end of the rod bolt to achieve an accurate reading.

Be certain to re-zero the dial gauge for each bolt that you check. Consistency is critical. Apply the same amount of ARP Ultra-Torque lubricant to the threads and seating surfaces of each rod bolt.

Cleanliness is mandatory. The rod bolt, parting surfaces and threads must be absolutely clean to ensure consistent results.

Editors note: If you found this article helpful, please click HERE to let them know and to order you copy of their 2013 catalog.

SOURCE:

Automotive Racing Products
1863 Eastman Ave.
Ventura, CA 93003
805-339-2200
www.arp-bolts.com

 

Mick Richards

Edited by Motorsport Mickey
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Mick,

Thank you for the link and research.   I had trawled through countless pages on hot rod engine builders sites and seen snippets of the info.

Thanks again

Peter W

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Peter,

You have a PM message.

Mick Richards

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40 minutes ago, Motorsport Mickey said:

Peter,

You have a PM message.

Mick Richards

Thanks Mick

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Peter,

 For me the key thing is whether or not the recommended torque takes the bolt into the inelastic zone. If it does then once used, the bolt is scrap. 

Rgds Ian

Edited by Ian Vincent
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There is a narrow window between tight …but not enough…and running the risk of not enough clamping force being maintained on the con rod cap.

The other side of the window is a bolt that is over tightened into the overstretch condition meaning it’s untorqued length when stripped and measured has grown  by over 1 thou and it should be not used again.

In between is the Goldilocks zone ( just right) of a bolt torqued sufficiently to stretch to the proscribed amounts, causing it to apply a predetermined clamp to the con rod clamp. This bolt then shrinks back within the overall parameters when released and is still of reusable condition.

These bolts are viewed as consumables, in a competition environment they will be measured every time that are released and a log completed showing how the fastening eventually “relaxes” after …just one more use, which then means the bin and a replacement.

Mick Richards

 

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4 hours ago, Ian Vincent said:

Peter,

 For me the key thing is whether or not the recommended torque takes the bolt into the inelastic zone. If it does then once used, the bolt is scrap. 

Rgds Ian

Working on the torque figure given of 45 ft lbs and the required bolt stretch of 0.005-.0006" We found the 45 ft lbs has stretched the bolt by max 0.004"

So I would say the bolt has not been overtightened by the use of the suggested torque.

Peter W

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Neil,

These AREN'T stretch bolts.

These are bolts which have a calculated torque AND stretch factor used in their fastening process. The stretch factor is designed in, is temporary (as long as not exceeded) and measurable after being fitted, when the fastening is released and measured as long as the bolt returns to it's original length (with no more than a 1 thou increase in length) the bolt is fine and suitable for being used again. A bolt which stretches and when released doesn't have a tensional force pulling it back to it's original dimension for length DOESN'T apply the correct clamping force...it has relaxed and shouldn't be reused.

ARP who sell these bolts explain this is the case,

..."Once the yield point of a bolt has been identified manufacturers typically peg the recommended stretch at 75-80% of the known yield point. This provides the maximum clamping force known to exceed the known or anticipated load factor of the engine’s rotating assembly. According to ARP a bolt stretched to .001-inch or more of its original static length has been compromised and must be replaced. This minimal amount of excessive stretch serves to underscore the importance of stretch and the loss of clamping force that occurs beyond the design limit..."

and ARP would gain most from claiming that once stretched (during the fitting process on a once only basis) the bolts should be replaced, from customers replacing bolts barely used.

There are stretch bolts sold which should be discarded after an initial fitment and torquing to the applied figure, I seem to remember there were some specified by Triumph on the original TR4a for the con rod caps.

Mick Richards 

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1 hour ago, BlueTR3A-5EKT said:

Working on the torque figure given of 45 ft lbs and the required bolt stretch of 0.005-.0006" We found the 45 ft lbs has stretched the bolt by max 0.004"

So I would say the bolt has not been overtightened by the use of the suggested torque.

Peter W

So just to be clear Peter, the stretch of the bolt when torqued to 45 lb ft has grown by 0.004 when measured in the conrod clamped...Yes ? 

So you are continuing to increase the torque clamped as recommended ?

..."If necessary increase torque until you reach the recommended stretch.

Once the bolt is stretched to the recommended spec, loosen the opposing bolt and retighten carefully until the recommended stretch is achieved."..

This should then give a torque figure which when applied to ALL the ARP bolts in the conrods should achieve an equalised stretch figure for the bolts that matches that specified as required by ARP. 

When the ARP bolts are then relaxed and undone they should return to their original non stretched length within a maximum of 1 thou increase.

Mick Richards

Edited by Motorsport Mickey
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Posted (edited)

Yes to the 0.004” stretch at 45 ft lbs.

Yes the next step is to increase torque by a minimum and find the recommended stretch.

Thanks for the link to those instructions.

No the bolts have not extended in length after the 45 ft lbs of torque was applied and then relaxed.

 

Cheers

Peter W

 

Edited by BlueTR3A-5EKT
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Peter,

Typically very high strength steels don’t exhibit the same marked yield point that more normal steels do. The following  web site provides a bit more detail. 
https://aip.scitation.org/doi/pdf/10.1063/1.5038307

so if you stay in the elastic region, no problem, once you stray beyond into yield, that’s when you have to scrap the bolt. 

Rgds Ian

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