I am replacing the engine mount at the back of the engine which links The gearbox to the body as it is worn out, do I also need to replace the bolts or can I re use the old ones if they are OK ?

they are stretch bolts and should be renewed if removed, would it really hurt to re-use them, don't think so, its not like they are headbolts etc, up to you really.

 

if you want the numbers, its

 

1 x N90983002

2 x N10204808

12 hours ago, UrbanPanzer said:

they are stretch bolts and should be renewed if removed, would it really hurt to re-use them, don't think so, its not like they are headbolts etc, up to you really.

 

if you want the numbers, its

 

1 x N90983002

2 x N10204808

 

They may not be stretch bolts, I am of the opinion that most bolts are reusable unless they're designed specifically to stretch like cylinder head bolts.

Many bolts used which are exposed to the elements have a green anti corrosion coating, this coating is lost from the threads upon installation, simply applying a little anti seize compound before reinstalling replaces the coating and therefore renders the bolt reusable.

Stretch bolts will be used where axial loads are low and space is tight. By tightening to the yield point the clamping force of the bolt is maximised and allows the use of a smaller fastener for a given clamping force e.g. M8 instead of M10. This is useful when space is tight.

 

The downside is that the fastener is permanently stretched and on the downward side of the strength curve beyond yield. Reusing it will mean lower clamping force and a high chance of failure.

 

As @sepulchravementioned, one other reason to replace bolts is due to corrosion or damage to the coating. Do not grease them if you plan to reuse them as the reduced friction will result in overtightening when the specified torque is applied.

 

The remaining reason replacement is specified is for bolts with Stover lock nuts. These use reduced diameter or ovalled threads to lock them in place and will wear the bolt threads somewhat on removal. Reusing these nuts is a bad idea because they will be looser than new ones and may not stay in place. They can also result in overtightening because of the reduced friction that results from being stretched. Reusing the bolts is not recommended but you can probably get away with it if you use new nuts.

Whatever you do don't replace the rearmost, single one with anything other than the correct bolt.

There's very little clearance above the captive nut on the subframe before you get to the steering rack. Even a 4mm longer bolt than the correct 61mm one will write off the steering rack by denting the cylinder.

 

 

 

Well I try not to make a habit of posting incorrect info, especially when it's direct from the Skoda manual.

 

The bolts are torque tightened, then further angle tightened, this normally indicates that they are stretch bolts, the manual clearly states replace them............down to the OP as to which way he goes.

 

The corrosion coating will be a standard automotive Geomet finish used by pretty much all manufacturers

Edited 8 May, 20224 yr by UrbanPanzer

30 minutes ago, UrbanPanzer said:

The bolts are torque tightened, then further angle tightened, this normally indicates that they are stretch bolts,

Not always. It's also used where joint condition may be unreliable/unpredictable on standard bolts, e.g. rust, lubrication, dirt, etc.

1 hour ago, UrbanPanzer said:

Well I try not to make a habit of posting incorrect info, especially when it's direct from the Skoda manual.

 

The bolts are torque tightened, then further angle tightened, this normally indicates that they are stretch bolts, the manual clearly states replace them............down to the OP as to which way he goes.

 

The corrosion coating will be a standard automotive Geomet finish used by pretty much all manufacturers

 

There are three ways of tightening bolts:

 

Torque only.

 

Torque + angle

 

Torque + angle to yield.

 

Only the last method utilises stretch bolts which cannot be reused.

 

The reason it's done with angle at all is because of industrial power tools, not because of the fastener itself.

Norbar have some good background on torque as it applies to bolt tightening in their catalogue, starting at page 13:

https://viewer.ipaper.io/sna-europe/norbar/norbar-2022-torque-wrench-catalogue/?page=13

22 minutes ago, chimaera said:

Norbar have some good background on torque as it applies to bolt tightening in their catalogue, starting at page 13:

https://viewer.ipaper.io/sna-europe/norbar/norbar-2022-torque-wrench-catalogue/?page=13

 

I'm a practicing engineering professional, I make technical decisions based upon experience without referring to the Internet. I'm not asking you if some of these bolts are reusable, I'm telling you they are.

2 hours ago, sepulchrave said:

 

I'm a practicing engineering professional, I make technical decisions based upon experience without referring to the Internet. I'm not asking you if some of these bolts are reusable, I'm telling you they are.

I'm a practicing mechanical engineer myself (working for a torque tool manufacturer right now), and have also taught engineering at third level. From an educator's perspective, one of the things that's always useful is to find guides like that one that break down the engineering/scientific principles in an accessible way. You're not the only person who is posting on this thread, and certainly not the only one who will ever read it, so it's a bit self-centered to assume the article was linked for your perusal alone.

 

There's no difference in practice between torque + angle and "torque + angle to yield" in terms of the instruction to the operator or to a large degree how the fastener feels when you're tightening. VAG service manuals don't draw any distinction between TTY and standard fasteners either in terms of torque spec or instruction to replace or not. It may be the case in service documentation in other brands/fields, but not something I've ever seen in VAG or Porsche specs.

As to engineering practice, I like to look deeper into these things and see where there is leeway to adjust from the specification in the service manual. Service documentation is written for technicians to get the job done repeatably and well, and as such is always a good starting point, and generally I will stick to the manual and advise others to do the same. That said, there are situations where you need to deviate for some reason, maybe lack of resources, or you need to get the job done with what you have to hand because time's tight, and understanding how/when it's ok to deviate from the manual is useful.

 

In the case of this thread, it's useful to know why some bolts must be replaced, and why others could be reused with care, particularly for someone who comes along later and finds the thread, and maybe at some point in the past they've reused a bolt that should have been replaced, and knowing why helps/encourages them to go back and remedy it.

Look at the bolt shank, those that are designed to stretch once only to beyond the elastic limit will have a precision ground waisted shank like modern con rod bolts.

 

I can recall Ford garages adding a new sump plug o everyones service invoice because they claimed that the 20mm steel thread tightened into an alloy sump stretched its threads and could not be re-used 😆.

 

In general you never want to tighten a fastener to beyond the elastic limit, once it has reached the yeild point any service load will cause further plastic (& not elastic) deformation resulting in failure.

 

I rarely use a torque wrench but on occasions either using an old fastener, one that may have been defective or having no choice but to really pull up a joint for whatever reason or undoing a rusted fixing I can detect being on the cusp of the yield point, when this happens I always discard and replace the fastener.

 

One thing I have found when not having kept my hand in (due to owning a far too reliable MK1 Octavia) is that I started to break rusted fasteners when undoing them, when I was practiced I would always know how far I could go before stopping, applying more heat and/or lubricant, backing the nut back up the threads etc.

1 hour ago, chimaera said:

Service documentation is written for technicians to get the job done repeatably and well, and...

 

Profitably.

 

There, FYP.

 

There is no need to replace the bolts this thread is asking about, they can safely be reused unless they're actually damaged.

They're not stretch bolts so it simply isn't necessary.

I would re-use the original bolts if they clean up ok & the threads are good.

 

The repair manual would probably tell you that they should be renewed after removal because thats what Skoda want the technicians to do. It covers them if the new part fails & by charging ridiculous amounts for cheap bolts earns them more kerching!

 

If an independent garage was doing the work for you, they would probably clean the bolts on a wire wheel & re-use them.