850 subframe bushing bolts

2forty · Post by **2forty** » 13 Feb 2018, 20:36

I am going to install the subframe poly bushings. It says to use "new" subframe bushing bolts. I presume they must be a stretch bolt but at over $7 per bolt I question how many people are replacing them? So if you have installed the poly bushings did you replace the bolts or use the old ones. Also did you notice a difference after installing the poly bushings.
Thanks!
2forty

Post by **oragex** » 13 Feb 2018, 21:31

Most bolts on these cars are stretch type. When you read on the specs "torque to xxlbs then turn xxdeg", it means it's a stretch bolt.

What is important to remember is that you will not be able to torque a used bolt to the original specs. This is what stretch means. The moment the bolt is turned xxdeg the metal goes from elastic deformation to plastic deformation and remains permanently into the plastic deformation behavior. When the metal is in plastic deformation mode, if the maximum torque is applied the metal will elongate and the torque applied will diminish until the metal snaps. In the graph, notice how the possible stress (call it torque) cannot be increased anymore once in the plastic region. Basically, when the bolt goes into the plastic region, if you keep torquing it will feel like it takes less and less force to torque until the bolt will snap. In other words, once a stretch bolt has been turned the xxdeg, it cannot be tightened again to the maximum torque or with the rotation of xxdeg.

You will need to torque the bolt somehow below the specified torque, and even forget about the rotation. How much below? Impossible to tell without the graph for that specific metal/thickness. And the other question is, is it safe to torque the bolt below the specified torque?

Post by **Clemens** » 14 Feb 2018, 02:02

I guess the Volvo Manual will alwayas suggest to use new bolts.
If they are indeed stretch bolts i wouldn't want to be cheap here. The idea of subframe bushings coming loose isn't very appealing to me...

Post by **abscate** » 14 Feb 2018, 05:15

Guidance from the internet experts on this topic is usually wrong but MVS is a place where it is usually spot on.

Get new bolts AND a good torque wrench with mid range or more torque spec.

For non safety related stretch bolts, like an axle bolt, you could reuse if you punch mark and inspect at daily, week, then monthly intervals

For subframe, ball joints, control arms.....new bolts

Post by **tryingbe** » 14 Feb 2018, 05:47

I didn't replace mine when I did the subframe bushings.

2forty · Post by **2forty** » 14 Feb 2018, 08:04

Thanks for the replies. I will order the new bolts.