Con rod bolt 500, torque
Discussion
pt
Were recommended by a Power company to not reuse the bolts, so I bought a new set. Torqued to 48ftlbs (65Nm) as per recommendation. They charged me VAT outside the EC, made a total of £235 + freight £61 + Norwegian VAT on top £74. This makes a total of £370 or £23 for each bolt...!! pt
They are no longer made by PRD AFAIK; the set I have, from TVR, are common to the speed 6 and AJP V8 engines - which makes sense - and are polished steel rather than the black of the PRD items. They have a thinner head and shoulder on them as well.
I have just received my stretch gauge from the States so I can do the job properly; I don't suppose anyone knows what the stretch is for these bolts?
>> Edited by victormeldrew on Monday 20th February 23:49
I have just received my stretch gauge from the States so I can do the job properly; I don't suppose anyone knows what the stretch is for these bolts?
>> Edited by victormeldrew on Monday 20th February 23:49
v8 racing said:
sorry victor never heard of stretch bolts for big ends? are you sure there stretch bolts, that dont make sense?
I'm going by what I've read about torquing rod bolts. All bolts stretch when you torque them down, not just "stretch" bolts that are designed to stretch once and not be reuseable. Normal bolts stretch and return to original length when undone, but if they stretch permanently more than 0.001" they are knackered. Measuring stretch is more accurate than measuring torque as that is a measure of friction, not clamping force. Some research:
Tom Smith ARP said:
Yes, you can quite reliably tighten rod bolts with a torque wrench, but this process assumes a few things. First, you must use the specified lubricant to give accurate torque readings. Second, you must use an accurate wrench; without insulting every home mechanic, not everyone has one. Third, a specific procedure in torquing the bolts is required with most rod bolts for an accurate job.
Mostly, though, the strongest reason for using a rod-bolt stretch gauge is simple from an engineering standpoint. At a certain amount of bolt stretch, a given alloy and a given bolt diameter will create a specific preload. Assuming that a certain torque on the nut will create that preload is not always precise. It may be accurate enough, or even very accurate, but the bolt threads, the nut face, and the cap’s nut surface all affect this friction. Each bolt is designed to stretch a specific amount when it sees a certain loading. With a torque wrench, you’re only measuring the friction, not the preload.
Car Shop said:
Proper clamping force is critical to the life of your connecting rods. Professional engine builders have used "bolt stretch" for years as a means to achieve proper clamping force, but stretch gauges have always been priced beyond the reach of most do-it-yourself engine builders. Now you can properly attach the bolts in your rods with this affordable stretch gauge from us. Don't guess or trust a torque wrench that may not be properly calibrated - stretch your bolts to the proper clamping load.
Oliver Racing Parts said:
For any fastener to supply clamp loads high enough to keep the parts bolted together, it must be stretched the proper amount. Torque does not measure bolt stretch, it measures friction. This is why we prefer the stretch method or the torque and angle method for tightening rod bolts.
To use the stretch method, measure and note the free length of each bolt before tightening with a stretch gauge or a micrometer with ball end attachments. Then, using the chart below, tighten the bolt until the proper stretch is achieved.
Theory of Rod Bolts and Other Prestressed Bolts
Engine Wives Tales Debunked said:Racing Articles
4.) Rod bolts break because they’re bad – Rod bolts are falsely accused of engine failure more often than any other part in the engine. 99% of the time a rod bolt breaks it’s not the bolt's fault, it’s the engine builder's fault – more directly the one who torqued the rod bolts. Not surprisingly, I’ve had a hard time getting people to agree – it’s a lot easier to say and believe that “It’s not my fault.” Rod bolts work by stretching, usually around .006”. They’re almost always less (say .004) when torqued with just a torque wrench. A stretch gauge is the only way to go.
HotRod.com said:
The bottom line here is that torque does not measure bolt stretch. It measures friction. For any fastener to supply clamp loads high enough to keep the parts bolted together, the fastener must be stretched the proper amount. That’s why rod bolts should not be torqued. Instead, they should be "stretched."
Tomorrows Technician said:Tomorrows Technician
With new metallurgy and engineering, if installed properly, rods just don't fail of their own accord anymore. However, if you are still using a torque wrench to tighten rod bolts, you need to stay in school. The proper way to tighten rod bolts is with a "rod bolt stretch gauge." Again, you need to follow the manufacturer-recommended tightening procedure.
Engine and fuel engineering forum
I thought this was best practice?
>> Edited by victormeldrew on Tuesday 21st February 20:03
I am not disagreeing with those guys, but how the heck do you no how far to stretch a bolt if its not a stretch bolt?? in 20 years of using arp bolts and the likes i have never read anywhere on there packaging or fitting instuctions to stretch them? if you try and try and stretch a non stretch bolt beyond the manufactures torque setting all you are going to end up doing is either sheering the bolt, ripping the threads of something or disstorting said item, lets face it the most important bolts on any car are the wheel nuts, if these fall off you are going no where or having a big accident! how many people you know stretch these!
Rob
You use ARP rod bolts?
Check out Rod Bolts Stretch and Torque Specs on their website. Notice the page title
Also see Using a Stretch Gauge
I'm not making this up, honest!
>> Edited by victormeldrew on Wednesday 22 February 22:53
You use ARP rod bolts?
Check out Rod Bolts Stretch and Torque Specs on their website. Notice the page title
Also see Using a Stretch Gauge
I'm not making this up, honest!
>> Edited by victormeldrew on Wednesday 22 February 22:53
v8 racing said:Is 40ft/lb for the black finish PRD bolts, or the shiny new same-as-AJP type ones? Being completely different friction coefficients I'd expect the torque figures to differ between them.
Yes we re-use them all the time, if i am totaly honest the only time we change is uprating to arp, we use a moly based thread lube and torque the tiv bolts to 40 ftib, obviously we do check the bolts to make sure no stretch or fatigue has taken place
>> Edited by victormeldrew on Wednesday 22 February 22:52
Right i see where you are coming from now sorry my mistake, i thought you were going to try and stretcth (torque) standard bolts by using a degree torque wrench, tightning bolts by a stretch gauge is fine, however they are not comonly used due to the fact that most bolts once in the specific item you are tightning down are very hard to measure!! i mean how the heck can you measure a head bolt!! personaly if you use plenty of lube the conventional torque settings are more than adequate! the 40ftlb is for the original bolts
I'm just a bit obsessed with doing the job as well as I can I guess! I saw the stretch gauge when I was buying a set of ARP thread cleaning taps to prep the block, and wondered what that was all about. So I read all about them and though, yeah, that sounds like the best way to do it, so I bought one. Now I have it I realize I don't actually know how much the TVR bolts will stretch at the correct preload. 
I'm going to torque up a selection of bolts, several times, and measure the stretch. Hopefully I will see some consistency, and be able to work out what the stretch is from that.
I'm going to torque up a selection of bolts, several times, and measure the stretch. Hopefully I will see some consistency, and be able to work out what the stretch is from that.
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