Tq or stretch for ARP rod bolts?

[3literpwr]

Im about to start assembling my 3.0 and noticed that ARP recommends a stretch method for installation, but also shows they can be torqued to 50 ft lbs... Seeing as I don't have a stretch gauge, would it be advisable to just do the tq method or buy the tool?

Thanks

[Henry Schmidt]

Although the stretch method is more accurate, torquing the bolts will be just fine.
This is not rocket science.

[3literpwr]

Thanks Henry...

[304065]

[AlfonsoR]

It is true that it is not Rocket Science, but it is critical to do it right. The reason the stretch method is recommended is because it can give a more accurate measurement of fastener preload.

The torque method is an indirect method of measurement as it measures force needed to overcome a certain amount of friction. By default, the torque method assumes that you have the same amount of friction in each rod fastener. Looking at the fastener closer, you have two joint surfaces that are responsible for the friction, the nut face to rod or rod cap and the nut thread to the stud thread. There are several factors that contribute to each fastener, even a high quality fastener like the ARP's, having a difference in the amount of friction needed to achieve a desired preload. Are those difference significant, probably not. But that is the question.

The stretch measurement method is a direct measurement. It assumes that material in each fastener is the same, and that therefore a certain amount of stretch will equal a certain amount of preload on the basis of material strength and cross sectional area of the smallest diameter of the fastener. Generally, it is accepted as a safer assumption, as compared to assuming the the friction in the fasteners to be the same.

Some people use a combination of both methods just so they can sleep sound at night and not have to worry that they missed something.

Professionals like Henry have done so many engines that they have probably developed a "feel" for putting things together. They can tell right away if it is right or wrong. For most of us that do one or two in our lifetime, we tend to go overboard to ensure that we did it correctly

If you are going to use the torque method, follow ARP's instructions to a Tee, i.e. burnish the threads. Make sure you use the ARP lubricant as indicated on the threads and face of the nut and keep everything super clean for assembly.

[boyt911sc]

Originally Posted by Henry Schmidt

Although the stretch method is more accurate, torquing the bolts will be just fine.
This is not rocket science.

The last three (3) engines that I rebuilt with ARP con bolts were measured and torqued to 50 ft-lbs. The stretch values obtained after applying the prescribed torque were so close or almost identical to the theoretical stretch recommended values. It is critical that you use a calibrated torque wrench and follow the manufacturer's instruction for lubrication. And you'll be good.

Tony

[brighton911]

I installed ARP rod bolts in my 3.2 using a stretch guage. The 3.2 rod bolts are smaller than yours (9 mm) so the torque value is stated at 40 lbs. Using the stretch guage, I found the torque values ranged from 39 to 42 (using ARP bolt lubricant) to get the correct stretch. So, 40 ft lbs would have been very close.

[3literpwr]

I have plenty of indicators here as I was working in a tool and die shop whilst in college... I should be able to mock a simple stretch gauge up just to test and see... Thanks for the input guys!

[Determined]

As mentioned, both are fine but make certain it's done correctly. Personally, I prefer a stretch gauge but I've done it both ways with success. Enjoy!

[JFairman]

I'll add this in detail fwiw.
Mine had Raceware rod bolts and nuts in it when I got it. They're reusable too so I did. They cost more than ARP rod bolts but I don't know if they're any better or different.

I took my 1987 911 turbo connecting rods to the best automotive machine shop in Lake Park, Florida to be resized and have the new Mahle small end bushings I bought here pressed in and honed to size.
Before doing that I called Raceware tech support and asked what torque the nuts should be tightened to. The guy said to 52 ft lbs in 3 steps: 20, 35, and then 52 ft lbs with 20w 50 non synthetic oil on the threads.
I told him i have some ARP fastener assembly lube. He saidnot to use it because it's too slippery and the torque value would be lower and he didn't say exactly what it would be... another words keep it simple and just use the motor oil.

The machine shop removed the bolts, machined a small amount off the mating surface of the rods and caps, reinstalled the bolts and torqued the nuts to 52 ft lbs with motor oil on the threads and then honed the big end hole so it would be round again.
Before they did that they measured the big ends to see if they needed resizing and they had become oval shaped by around .001" so they did need resizing.

When I installed them I did it like Raceware said to do.. with the 20w 50 non synthetic motor oil on the threads and in 3 steps: 20 ft lbs, then 35 ft lbs, and then 52 ft lbs. I used a 3/8 drive snap on click type torque wrench.

[AlfonsoR]

Yeah, the lubricant and its slippery-ness is factored into the equation to determine the proper torque value. Follow the fastener manufacturer's recommendations for lubricant as well as torque procedure.

[Walt Fricke]

To me, the time to use just torque is when you have pulled a rod or rods but not split the case, so you can't use a stretch gauge.

Stretch gauge fixtures are inexpensive, and so are dial indicators which are accurate enough for the purpose.

So when you have the crank out, and are using high quality bolts which can be reused if not permanently stretched beyond a spec, why not take the extra trouble? It means you are not at the mercy of the accuracy of your torque wrench, and without the measuring (which, to do accurately, you pretty much need the fixture and indicator anyway)you can't reuse the bolts because you won't know if they are too stretched.

But it seems clear that the sky doesn't fall if you are careful with the torque method. But we aren't trying to beat the flat rate time, etc.

[Jeff Alton]

With ARP bolts we have seen more than spec torque to get spec stretch. However the delta with the 10mm bolts is small. The delta was larger with the 9mm bolts. That said, I have not heard stories of folks with ARP rod bolt issues regardless of which method they used....

For piece of mind in the shop we use the stretch method as it only takes a few mins more...

[bh912e]

Not that I would ever suggest dealers/manufactures repair expertise, however I do not know one manufacture that has a stretch guage on their required tool list. Countless engines are rebuilt daily that live past their power train warranties.....rod bolt failure is rare. Far more engines are wasted due to owners not following maint. Intervals and or not checking their oil level.

[Henry Schmidt]

Interesting discussion but the truth is (like Jeff said) rod bolts rarely fail in Porsche engines without exceeding their design parameters. IE: over-rev or bearing failure.
Every fastener has a pre-load range that fits the specification of that fastener.
It goes without saying that any measuring tool you employ should be calibrated regularly but if your device is accurate either method will get you in the range.

[Lapkritis]

Torque method on every engine that I've used their hardware on.

[slow car]

What is the torque setting for new ARP 9mm rod bolts?3.2,
no instructions came with my bolts?

[zcoker]

My 2 cents. ARP/Raceware rod bolts are specifically machined during production on both ends for a stretch gauge, specifically for the stretch gauge fixtures to obtain the recommend stretch value of the fastener. You may "wing it" with a torque wrench and get into the range (or over the range). But that's guessing and hoping, subjective at best. Doing it right with no doubt implies the use of a stretch gauge.

[Gordo2]

Originally Posted by slow car

What is the torque setting for new ARP 9mm rod bolts?3.2,
no instructions came with my bolts?

Your ARP rod bolts for a 3.2L engine should be ARP part # 204-6005.

You can download the instructions here: The Official ARP Web Site | Kits

Or reference them with my installation notes here: Checking Connecting Rod Bearing Clearances With Plastigage

Gordo

[3literpwr]

I used the torque method for checking my bearing clearances with my calibrated gauge style tq wrench... It seemed to work just fine, but I still plan to mock up a dial indicator just to verify... Bearing clearances were .0348 to .0466

Thanks for the assistance guys...