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Discussion Starter · #1 ·
Hi guys,

I need to remove the towing hitch for powder coating (it is very rusty). It attaches to the car with 3 bolts on each side that are angle torqued. In general bolts that have been angle torqued are not reused especially in critical places like engine head bolts or towing.

But the bolts have been discontinued (PN:30682101 "screw kit") and buying a new hitch is much more expensive.

So, what is the safe procedure here? I guess the same problem is faced by all the body shops that have to fix rear-ended cars with hitch installed.

Thanks
 

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There are 3 ways to specify tightening a bolt:

1) Torque
2) Torque + angle
3) Torque + angle yield

#3 requires replacing the bolt. #2 does not. Which bolts are #3 vs. #2, I don't know? Volvo as a practice seems to replace all bolts and nuts on the suspension and major engine components. VIDA even says to replace the nuts on the sway bar links, if you remove them. A new bolt/nut will give you a more accurate torque reading, especially if they are rusty, which is good.

Remove a bolt and find a grade 8 bolt that is the same size. The Volvo part number might indicate the size, in VIDA or online parts site.
 

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Remove a bolt and find a grade 8 bolt that is the same size. The Volvo part number might indicate the size, in VIDA or online parts site.
Agreed. Swapping to a different style bolt for the hitch is the same thing as replacing factory TTY head bolts with something such as ARP that go by straight torque instead of torque+angle.

Here's a link to the installation instructions for the Curt hitch that I recently put on my car. The bolts are M12-1.75 x 40 and are torqued to 79 ft-lbs.
https://assets.curtmfg.com/masterlibrary/12318/installsheet/CM_12318_INS.PDF
 

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If it were my car, I'd use grade 8 or find them in stainless if your worried about rust and tigthen the piss out of it.....Agreed with Exocet.

Vida will tell ya to use new fasteners most all of the time, re-using them would make them liable for whatever "could" happen...Most of the time re-using is fine unless they are rusted/pitted/compromised or angle torqued.
 

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If it were my car, I'd use grade 8 or find them in stainless if your worried about rust ...
Except stainless is typically grade 5 so not as strong as grade 8. Depending on what you plan on pulling I'ld go with the grade 8.
 

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Volvo specifies new hardware most of the time because they often apply dry thread locker to the bolt and want you to have new thread locker every time. If you use loctite and tighten the old bolts to spec you'll be fine. I can't imagine the trailer hitch bolts are torque to yield.

Also for you guys talking about bolt grades, the car is metric so I think you want to be recommending class 10.9:



This chart should have the metric bolts labeled "class" not "grade".
 

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I disagree. Lots of fasteners, like white engine head bolts, are torque plus angle and re-usable. Torque plus angle plus angle are TTY bolts in every case I have ever known. Many T+A bolts have their tightening prescribed that way only to give uniformity. Red Motor head bolts of the same part number and classed re-usable by Volvo if not exceeding a length spec were originally torque in steps and then became torque plus angle. Torque plus angle is also a machine way of tightening bolts. Torque bar driven by impact for the torque step then a splined sleeve drops and engages splines on outside of the socket and a position tracked high torque motor with gearbox tighten the bolt the angle. Every fastener assured of the same stretch since the torque for motion that varies with surface prep and fastener surface is not the guide for the clamping force desired. Of course it is never a bad idea to replace bolts, especially bolts in underbody exposed to road salt.
 

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Discussion Starter · #10 ·
Great link! Thanks!

Install instructions say "Tighten to 79 Nm (59 lbf.ft)." Then later "Wait at least five minutes, so that the joint has time to set. Then tighten the
screws (8) on both sides, a quarter turn (angle tighten 90°).". They also say "Torque tightened screws that have been removed must not be re-used.".

So is this "2) Torque + angle", right.
 

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I disagree. Lots of fasteners, like white engine head bolts, are torque plus angle and re-usable. Torque plus angle plus angle are TTY bolts in every case I have ever known. Many T+A bolts have their tightening prescribed that way only to give uniformity. Red Motor head bolts of the same part number and classed re-usable by Volvo if not exceeding a length spec were originally torque in steps and then became torque plus angle. Torque plus angle is also a machine way of tightening bolts. Torque bar driven by impact for the torque step then a splined sleeve drops and engages splines on outside of the socket and a position tracked high torque motor with gearbox tighten the bolt the angle. Every fastener assured of the same stretch since the torque for motion that varies with surface prep and fastener surface is not the guide for the clamping force desired. Of course it is never a bad idea to replace bolts, especially bolts in underbody exposed to road salt.
Go ahead and reuse the head bolts. Let us know how that works out.
 

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Once the bolt is plastically deformed as implied by an angle torque spec it is not really reusable... You cannot unyield a bolt.
 

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Once the bolt is plastically deformed as implied by an angle torque spec it is not really reusable... You cannot unyield a bolt.
Yes, all TTY bolts are plastically deformed and cannot be reused. TTY gives superior clamping force with a smaller bolt.

I would not reuse head bolts under any circumstances. Not worth the possible problems.

Maybe a Volvo tech could as answer the TTA vs TTY question. Beyond that, it would be a mechanical engineer question.
 

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If the engine overheats you replace head bolts. If the bolts come out with damage you replace. If the head gasket fails and there has been coolant in the threads you replace. I have had dozens of white motors apart and the head bolts were re-used and not one failure. I rebuilt the head and ringed a 2004 V70R I owned when it had 251K miles on it and reused the head bolts. That car has done 27K miles, two TX summers, and it has zero issues. Until VIDA went to the cartoon illustration version the repair manual book print file version was used and the paper book it came from had a spec for bolt length and bolts were re-used. You guys are misunderstanding that all torque plus angle is not done to elastic limits of bolts no more than the old torque only procedure did.
It is absolutely OK to replace them when they are not a specified replacement bolt and there is no circumstance indicating if you want to spend the money. Mark head bolts on a white engine. Don't use any moly paste or o her special lube on the bolts, just clean bolts and clean threads. torque them to 90 ft-lbs. Look at the marks made and see the different angles those bolts have turned through. When you put an aluminum head on an aluminum block you will have variations in the surface finish of head and the threads in block. torque to a higher number gets into the friction area and the torque is the product of the friction and not a reflection of the clamping force achieved due to stretch of the bolt. The desired stretch may be 0.26 mm and is not anywhere into the range permanent deformation of that long bolt. When you torque to a low torque you are compressing gasket if used and/or setting a low preload. Then you rotate the fastner through as angle that stretches bolt and compresses head and block a bit. The result is the same clamping force throughout. Some of those bolts were harder to turn through the same angle as the others but the inward travel as result of the thread pitch is the same and the tensile strength of the metal of the bolt allows elongation and assure the clamping. If you buy into all fasteners being taken into the plastic range then do you replace lug studs with every remove and re-install of a tire & wheel assembly? What about front spindles on the RWD cars using T+A on the nut for the hub? The rear CV axle stud sticking through wheel hub with T+A method on the nut? Do all of those parts gets changed when the shaft that elongates is a part of that assembly? Diesel engines for 40+ years in most cases use TTY bolts and have torque plus angle plus angle. When you tighten many of those bolts you have rising torque to move breaker bar and then the force lessens and some degrees before the final pull the force to move wrench goes WAY back up. You work harden those fasteners and they will not do that again. I again point you to automated assembly methods. You don't get good service life out of impact style tools. The multi-driver tools you can see in many vehicle assembly videos will have torque bars with the external splines on the socket and the sleeves that deploy over the socket to do the angles. A method that does not require tool replacement nearly as often. Since they assemble that way the torque that has been established as correct for that system became the torque listed in service info. Torque plus angle is just better in every case due to the reliability of clamping load.
 

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You can re-use head bolts at least once. I've built countless engines without issue. Now the head bolts are so cheap it's not even an option, just replace with new.

Absolutely do not use stainless in this application. Stainless steel is tough not strong.
 
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