Big end stretch bolts

[trchris]

What's the view on re using the stretch bolts for the big end caps? Before lock part 1 I had the crankshaft reground new little ends and 89 pistons and liners, no crank balance reason? got the call your parts are ready NOW if you don't pick up today the doors are closed until further notice so no brainer pick up and rebuild. Coming back from Malvern stuck in traffic motorway started to get slight clutch judder so me thinks easier engine out to do clutch whilst out whip crank out and have it along with flywheel/clutch cover/bolts/ front pulley and bolt (no mechanical fan) and timing gear balanced. The stretch bolts were new when rebuilding have been in for 2200miles so replace or re fit?

Chris    

[harrytr5]

Replace if it was me

Regards Harry

[trchris]

That was my thought also but I thought Id ask

Chris

[RogerH]

Being devils advocate I must ask why?   Why is it necessary to replace after they have been used.

The bolts are designed to stretch and this locks them in place.

When you undo them they will shrink back to nominal size or there abouts.

If they didn't shrink back then they would NOT be unfit to use in its initial application as it would have exceeded it modulus.

Most metals can stretch and then stretch some more. Once you get to the critical point where they can stretch no more then they become useless.

So at what point do these bolts become useless.  I can't see TRiumph flying that close to the wind they the bolt has taken up all is stretch in one appilcation

Roger

[ntc]

Always replace std and good practice 

[trchris]

I've just had a interesting conversation with a retired college lecturer in mechanical engineering and his view was if a stretch bolt is exactly the same length as a unused stretch bolt then there would be no problem to reuse as he said EXACTLY the same length. So does any know the length of an unused con rod bolt for a 4a?

This is just his view everyone should make there own opinion to reuse or not 

Chris    

[RobH]

3 hours ago, RogerH said:

The bolts are designed to stretch and this locks them in place.

When you undo them they will shrink back to nominal size or there abouts.

Maybe this is a name thing but aren't "stretch bolts" the same as "torque-to-yield" , intended to be used so that they do permanently stretch? 

I understood that they are designed to be taken beyond the elastic limit and into the plastic region of the stress/strain curve. They are made from an alloy which has a curve which is flatter and longer than for normal fasteners, to allow this.   Having been elongated they should not be re-used, as re-torquing them will drive them an unknown distance further into the plastic region, towards the point of ultimate failure. 

The reason for using them is to achieve a consistent high clamping force, as it relies directly on the yield property of the metal whereas the clamping force from a given torque on a normal 'elastic' fastener depends on the condition and lubrication of the thread, so can vary from bolt to bolt. 

[BlueTR3A-5EKT]

£90 more buys you a balanced set of lighter con rods with ARP bolts.

https://www.maxpeedingrods.co.uk/product/high-performance-triumph-tr3-tr4-4340-en24-chrome-moly-forged-h-beam-connecting-rods-conrod-with-arp-bolts-x4pcs.html

 

Edited August 25, 2021 by BlueTR3A-5EKT

[Rob Salisbury]

Just my own experience but I've got a set of original big end bolts around 56 or so years old and they are all 2.086 inches long, maybe I've been lucky but I couldn't tell you how many times they've been re used since 1969 but it could well be on the way to 20 times, Vandervell VP2's were easy to come by in the early days and I used to change big ends regularly however I was careful to use a quality torque wrench every time, I only used a new set of bolts a few years ago when I installed one of the Pheonix forged cranks that Moss were selling in the early '90's, .... the same was true of the 2.5Pi saloon that I had for many years, I changed the big ends every 25000 miles and used the same bolts and have never lost a rod,   .... for a road going engine if, carefully torqued, it's probably not necessary, .... racing or rallying now that's a different matter altogether!!

Cheers Rob

[BlueTR3A-5EKT]

What is the part number stamped on the bolt head Rob?

105312 is non stretch type bolt which logically last for years

 

[Rob Salisbury]

Hi Pete, the number on these bolts that were in the car when I got it in '69 is 138528

Cheers Rob

[RogerH]

10 hours ago, RobH said:

Maybe this is a name thing but aren't "stretch bolts" the same as "torque-to-yield" , intended to be used so that they do permanently stretch? 

I understood that they are designed to be taken beyond the elastic limit and into the plastic region of the stress/strain curve. They are made from an alloy which has a curve which is flatter and longer than for normal fasteners, to allow this.   Having been elongated they should not be re-used, as re-torquing them will drive them an unknown distance further into the plastic region, towards the point of ultimate failure. 

The reason for using them is to achieve a consistent high clamping force, as it relies directly on the yield property of the metal whereas the clamping force from a given torque on a normal 'elastic' fastener depends on the condition and lubrication of the thread, so can vary from bolt to bolt. 

Hi Rob,

 thanks for that.  That makes sense.

 

Roger

[Ian Vincent]

Different steels have diferent stress/strain curves.  Mild steel exhibits a distinct flat point in the curve which is called the yield point where the strain increases for no increase in stress.  High yield and alloy steels tend not to exhibit a yield point they follow a straight line before starting to curve as the rate of strain increases relative to the stress before suddenly failing - i.e. breaking.

This is why the allowable stress in HYS is expressed as a percentage of Ultimate tensile Stress (UTS) rather than Yield Stress.

Provided stretch bolts haven't been taken beyond their elastic limit (the linear part of the stress/strain curve) there is no reason why they shouldn't be re-used.

Rgds Ian

[Z320]

15 hours ago, RogerH said:

Being devils advocate I must ask why? 

Roger, a LIKE from me for that...

Edited August 26, 2021 by Z320

[RobH]

38 minutes ago, Ian Vincent said:

Provided stretch bolts haven't been taken beyond their elastic limit (the linear part of the stress/strain curve) there is no reason why they shouldn't be re-used.

In which case they are not being used as stretch bolts.  As I said in my previous post I think this is a problem of the name being used. What is being described as a stretch bolt is not - it is an ordinary bolt whose elastic stretch is measured on tightening to determine the pre-load. That is a different thing entirely.  

Quote from a manufacturer:

Stretch bolts are fasteners that are used in heat-resistant constructions (for example, in boilers and turbines). ........This means that the elongation bolts are pulled into the plastic elongation area when tightened and therefore become slightly longer. These bolts can therefore be used once without the risk of breaking. The advantage of stretch bolts is, among other things, that they hold the attached materials together with the same force during temperature changes.

https://www.metavak.nl/en/products/stretch-bolt/

 

Edited August 26, 2021 by RobH

[trchris]

Thanks guys I appreciate all the comments and have learnt a bit about these " stretch bolts" as my conscience wouldn't allow me to enjoy the newly balanced bottom end when completed new bolts will be used

Chris   

[RogerH]

2 hours ago, Ian Vincent said:

Different steels have diferent stress/strain curves.  Mild steel exhibits a distinct flat point in the curve which is called the yield point where the strain increases for no increase in stress.  High yield and alloy steels tend not to exhibit a yield point they follow a straight line before starting to curve as the rate of strain increases relative to the stress before suddenly failing - i.e. breaking.

This is why the allowable stress in HYS is expressed as a percentage of Ultimate tensile Stress (UTS) rather than Yield Stress.

Provided stretch bolts haven't been taken beyond their elastic limit (the linear part of the stress/strain curve) there is no reason why they shouldn't be re-used.

Rgds Ian

Thanks Ian,

I think I am on steady ground.

Roger

[Rob Salisbury]

2 hours ago, trchris said:

Thanks guys I appreciate all the comments and have learnt a bit about these " stretch bolts" as my conscience wouldn't allow me to enjoy the newly balanced bottom end when completed new bolts will be used

Chris   

Hi Chris, for exactly the same reason I used new bolts for my new crank, however these old ones are just so substantial I seriously doubt 60 odd ft/lbs torque would get them out of bed let alone stretch!! .... out of interest they are redundant and if anyone has the means to test them through to destruction on a strain gauge please get in touch.

Cheers Rob 

[BlueTR3A-5EKT]

JThis is not a recommendation but an observation of what we used to do.

When we were young we used to fit ‘unbrako’ hex drive high tensile cap screws instead of the regular hexagon headed bolts.

We would tighten them to the correct torque and use ‘Loctite’ on the thread.   We have just removed a set from a very used sprint/ hill climb TR2 engine and they have been thrashing around in that engine for over 20 years.

 

Something like them is still around at 1/10 of the price of the genuine stretch type bolts.   

I use a set as slave bolts in the MaxPeeding conrods, rather than keep tightening the ARP items.  They are saved for final assembly when the stretch is checked.

https://www.kayfast.co.uk/view-product/UNF-SOCKET-CAP-SCREWS-HIGH-TENSILE-SELF-COLOUR

 

Edited August 26, 2021 by BlueTR3A-5EKT

[Motorsport Mickey]

"used to fit ‘unbrako’ hex drive high tensile cap screws instead of the regular hexagon headed bolts.

We would tighten them to the correct torque and use ‘Loctite’ on the thread."

 

+1 worked for me and also using Unbrakos on the flywheel to crankshaft fixing (I like my ankles).

Mick Richards 

[Andy Moltu]

Is it worth uprating the bolts on a standard rod, Not for me. The origninal are fit for purpose.

But taking the engine out to change the clutch is a bit of overkill and a lot of time.

If you are taking the engine out because you want to revisit balancingfair enough then I would go for the Maxpeeding rods are so cheap, strong and come with ARP bolts.