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Gon fix it with me hammer
 
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rod bolt torque wayne's book or Bruce

Hi guys,

was just about to start doing my rods, when i wanted to look up the torque values ...

Wayne's rebuild book said something like 17.5 lbs/ft
Bruce Anderson's book said 35 to 45 lbs/ft

something like , because forgot the books at home and i'm at work now...

it's quite a big difference, and nothing in the engine book errata post either.. so i'm confused now... btw. it's the stock bolts, no arp

i did search the forums , but no specific values came up , just discussions about using loctite or not , or lubing the bolts or not... closest i found was a reference for 944 rod bolts @ 75 lbs/ft

can anyone tell me the precise numbers ?
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Old 02-15-2005, 02:53 AM
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Wayne says 15 ft-lbs then 90 degrees.
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Old 02-15-2005, 08:42 AM
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does 90 degrees extra make it go to 35 to 45 ft-lbs?

it just seemed a bit weird..

and when i got 2 things contradicting... i try to find 3rd( or more) source to back up either one of them...
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Old 02-15-2005, 08:49 AM
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Quote:
Originally posted by svandamme
does 90 degrees extra make it go to 35 to 45 ft-lbs?

At least 35-45. As I recall it was a struggle to go 90 degrees. This is a bolt stretch issue I think. You need someone smarter than me to add more info.
Old 02-15-2005, 09:11 AM
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guess i'll just wing it then... threadlocker on those babies... i guess it can't go that much wrong , as long as they're all torqued the same...
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Old 02-15-2005, 09:34 AM
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Yes, the same torque is important. I believe it ir permanent loctite. You have new bolts right?
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Old 02-15-2005, 10:05 AM
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yep ... new everything
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Old 02-15-2005, 10:39 AM
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15# + 90 degrees is the more accurate method than 40# to get the intended stretch.
Old 02-15-2005, 05:26 PM
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well, i went for 37...with a 5 degree twist

the low minimum on my wrench was 15 ... and i know that torque wrenches aren't accurate on the low end of top end of their scale.. so figured 37 (midrange) would be more accurate...
and with the threadlocker ... i suppose it should suffice?

anyhoo.... one rod is still not on .... check this out :



no thread on the stinking nut...and i only had 12 of them
still got the old nuts... i know the bolts are stretching and are one time use only , does the same go for the old nuts? or should i just go out and find me a single nut?
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Last edited by svandamme; 02-16-2005 at 01:39 AM..
Old 02-16-2005, 01:35 AM
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LOL, we got a set of ARP bolts a few months ago that contained a threadless nut like that one.
Old 02-16-2005, 04:00 AM
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Don't guess or make up your own methods for torquing rod bolts. This is the most critical torque spec on the entire engine.

Either use bolt stretch, (the only right way to do it really) or 15lb/ft, then 90 degrees. (more accurate than torque alone.


When torquing using the stretch gauge, I've come across a number of bolts that reached torque spec WAY before the bolt reached proper stretch. Why? Friction. I swapped to a different nut on the same rod bolt and this time it stretched to the proper spec without issue. The low torque, plus 90 degrees is more accurate because it relies less on the right amount of friction.
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Old 02-16-2005, 11:44 AM
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And putting on Loctite changes the friction further... There was quite a debate a while ago on Loctite vs no Loctite, and I'm in the no Loctite camp.

My contention is that if the Loctite is preventing the nut from unthreading, the spring (i.e. bolt) has lost its preload, and is thus seeing the full force imported on the rod. The bolt will fatigue/yield quickly if this is the case and will fail.

If there's one tool important to building an engine, a proper torque wrench is one of them. And as Tyson mentioned, rod bolts are pretty darn important on an engine...

Your call.
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Old 02-16-2005, 11:50 AM
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Stijn,
It's bad luck but you picked the worst fastener to guess the torque at.
Get yourself an inexpensive 3/8" drive beam type torque wrench. In the US, Sears sells one for $23 that is accurate to 4%. The beam style is about 1/3 the price of the clicker, has equal or better accuracy and doesn't lose it's calibration over time.
I have a beam style like this that was 4 times the price and twice as accurate but it's a joke because you can't read the scale down to the stated accuracy.
-Chris
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Old 02-16-2005, 12:52 PM
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Chris: Didn't you build a bolt stretch checker? Did you give up on that?
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Old 02-16-2005, 12:54 PM
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Quote:
Originally posted by PBH
Chris: Didn't you build a bolt stretch checker? Did you give up on that?
Yes, I made one but it didn't work so I had to "mike" them all the hard way.
-Chris
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Old 02-16-2005, 12:57 PM
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The original post dosen't seem to present a clear understanding of what 90 degrees means. It means torque the rod bolt to 15 foot lbs THEN forget the torque wrench. THEN just turn the nut 90 degrees more, no matter how much the additional torque is. The additional torque may be anything from 20 foot lbs to 98 foot lbs, it dosen't matter, just turn the thing 90 degrees! Thats for the original factory torque to yield bolts. For Race ware or ARP follow their instructions. If you are even thinking of doing more than one of these, buy a stretch guage for the ARP or Raceware bolts. Or just keep using the factory bolts and buy a torque to angle guage.

The price of these instruments is so low that there is NO reasonable excuse not to do it the right way.

Last edited by snowman; 02-16-2005 at 07:41 PM..
Old 02-16-2005, 07:34 PM
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oh well...

at this point , i basically had enough of second guessing every little aspect of the rebuild , there's 10 opinions for each and every thingie... and none of them will guarantee anything...

i'll let you know how my guesstimate works out later on when my engine is running , broken in , and hopefully doesn't selfdestruct.
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Last edited by svandamme; 02-17-2005 at 01:29 AM..
Old 02-17-2005, 01:23 AM
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Let me start by agreeing with Tyson on the best method on torqueing the rod bolts.

But, given the (in)accuracy of torque wrenches, the use of lubrication or tread lockers by some and the (lack off) skill/sense of mechanics it is fairly certain that a significant percentage of rod bolts in (porsche) engines are not torqued/stretched to the perfect values. With this in mind I wonder how many engines actualy have failed because of inproper torqueing of rod bolts/failing of rod bolts. I did a search on the bbs and got very little results. If this value was that critical I would expect to see a lot more mishaps from failed rod bolts.

Although I fail te see the rationale in Stijns' approach by adding another 5 degrees, his final torque will not be much off the factory spec. all other variables may have a bigger impact. I therfore doubt if it will have any adverse effects on the life expectancy of his engine.

I always try to build an engine the best I can, so I stick to the book and try to use the right tools. I did mine as in the workshop manual, tighten to specified torque with an accurate torque wrench (belzer IZO-D) without using thread lockers. The next I'll probably do with the 15lb/ft 90 deg. method as this makes more sense to me.
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Old 02-17-2005, 03:38 AM
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rational is a functionality that was left out when my kernel was compiled.
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Old 02-17-2005, 03:40 AM
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apologies if ye all know this already, but as it hasn't actually been stated yet, so if you indulge me a bit here.............

I sell Torque analysing kit and torque wrenches, both manual and air as well as electric, to a variety of industries. Some of my customers are subcontractors to automotive industry, so whilst not being a scientist, I have garnered a fair bit of info over the years. It never fails to amaze me the number of 'professionals' who have no comprehension of torque when they call me up to help with a problem they have, say, on the production line.

Torque readings on ANY wrench, are, in fact, useless!! No, really !! If you follow this, you'll see why:
The basic tenet of tightening a fastener, is to clamp two pieces together. The strength of the finished assembly (like a big-end cap on a conrod, for example), is contingent entirely on how tightly clamped together those two pieces are.

Now, at the design stage of the rod, someone determined that it needed a certain amount of clamping force to keep the thing working. But, to measure the clamping force under the head of the bolt required a special measuring device - a load cell washer, connected to a display unit - or to measure the stretch requires bolt-length measuring equipment. The dictates of production machining, space etc, laid down, for example, that they use an M8 bolt and nut. To achieve the required clamping force required to keep the two parts together, the bolt manufacturer has a table you can look up to give you a suitably strong bolt. What happens when you use it is that you are going to stretch the bolt. This is fine in the R & D dept, but out on the floor it's not going to happen - it'll interfere with the ability to make and sell cars. Not forgetting of course, that Porsche build a 911 engine in under 90 minutes. Complete.

So how are the guys (and girls !) out on the production floor going to know if it's clamped tight enough? Well, that's where torque wrenches come in. The bolt mfr and engine mfr conduct tests (using the load cell above) and determine that in order to get X amount of clamping force, the recommended bolt had to be tightened to X Nm of torque. That's all it is. It's a broad correlation between how much torque gives how much clamping force.

So where does angle come in? Well, like all things, science and R & D moves ever onwards. Originally, bolts were tightened to X Nm, you ran the thing to bed it in, and then you re-tightened them a second time. This is because the parts bedded in, and bolts which were somewhat stretched on original installation, would now have relaxed a little, and would therefore now have lower clamping forces. This is not practical for internal engine parts, so bolt mfrs and engine builders have used materials to build modern bolts from materials which are stretched at point of use, i.e. on the production floor. These are stretched so that they still deliver the maximum clamping force AFTER bedding and running in, in other words, after they have relaxed. Think of all the hot/cold heat cycles and the effects of them on new parts.........

The measurement of this stretch is by simple length, but which again, is difficult to measure - remember, Porsche need X engines a day out of ya ! - so we can't be hanging around with mic's and stuff - besides, they're subjective..................so, again, the R & D dept gets together with the bolt mfr and they come up witha certain number of degrees of rotation after a minimum baseline torque has been set which CORRELATES to the clamping force they're trying to deliver.

That's why you can't re-use the old bolts - their elasticity is practically spent............if you re-tighten them as per originally fitted, they will either break during your engine build..............or very shortly thereafter................and you don't want to know about that.

Other factors: lubrication. Generally, torque figures are based on assembly of new, unlubricated clean threads. Anti-seize pastes, threadlockers etc etc all affect the final result. And usually on the high side, and can be by a considerable amount. Locking devices such as crush and nyloc bushings also greatly add to measured torque without achieving clamping force, so make sure you're replacing non-nyloc nuts with the same non-nyloc nuts for critical parts.

If this is telling you something you already know, apologies, but if not.........maybe it'll save you talking the engine down a second time.............

And an angle adaptor to go on to your torque wrench is cheap - say Eur65-70. Less than 1hrs mechanic's fees.............considering you might spend 40hrs doing the engine.............

Finally, if using your torque wrench, you shouldn't 'pause' the wrench before hitting your preset torque. If you do, all your calc's are for the bin. You should start the wrench in one position, and make one full smooth motion all the way 'til it clicks, or whatever it does (beeps, vibrates...etc etc)

My 0.02
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Old 02-17-2005, 05:08 AM
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