who knows about stretch bolts??

[1jzracing]

i want to find out what people know and who has experience or knowledge about the mechanical charactoristics of a stretch bolt as it is used such as 1j - 2j bottom ends

I know head bolts are a use once item but in all the manuals have no reference to replace the bottom end bolts every time only to measure the thickness about 20mm from the head and if they are under a certain diameter they should be replaced

but this point is threaded!!! ... bit hard to get a mic on a thread

they specify stretch bolt settings so i did a little experiment putting a witness mark on a rod bolt, pre torquing and tightened it twice to see if it returned to the same spot ..... it didnt!! ... so they do indeed stretch further everytime they are used

can they be safly used again i guess is the burning question!!!

PS i dont want hairy fairy opinions just FACTS please

[3sgte]

Ther is some information at ARP:
http://www.arp-bolts.com/Tech/TechInstall.html

As an ideal practice, for rod bolts they seem to recommend direct measurement of stretch.
(If I am reading between the lines correctly)

[love ke70]

do not reuse.

was reading this just this morning
http://www.nzgaskets.co.nz/publicat.htm#TTY%20BOLTS

[1jzracing]

Ther is some information at ARP:
http://www.arp-bolts.com/Tech/TechInstall.html

As an ideal practice, for rod bolts they seem to recommend direct measurement of stretch.
(If I am reading between the lines correctly)

ive read this article before and as far as i can make out they are talking about bolts that are still in the elastic range and can be used over and over, which means measuring the stretch for the correct tesion makes real good sence

but stretch bolts such as toyota uses actually go past the elastic stage and into yeald where the bolt wont return to its original length ....sounds risky but they are all going that way europeans have been doing it for years so measuring it wont be much use as we still dont know how much perminant damage has been done to the bolts structure....

[love ke70]

cant you just go to ya local nut and bolt place and pick up some newies? im sure you could, i know i will be

[oldcorollas]

On the other hand, if a fastener is over torqued and becomes stretched too much – you have exceeded the yield strength and it’s ruined. If the fastener is longer than manufactured – even if it is only .001,˝ it is in a partially failed condition. Therefore, ARP has engineered its fasteners with the ductility to stretch a given amount and rebound for proper clamping.

exactly.

ARP make their stuf to be used within the elestic deformation region, which makes it reusable.
how pissed would you be if your expensive bolts had to be chucked away each time?
also, how pissed would you be if they failed from a missed gear .
by making them this way, they have extra capacity in terms of how much they can be loaded.
by measuring the stretch, you are making sure that it stays within it's elastic deformation region.

heres a kinda odd looking, but suiting our purpose, diagram, linked from this website which has a tutorial of sorts on stress-strain. stress (load) is vertical and train (increase in length) is the horizontal axis
http://www.shodor.org/~jingersoll/we...ial/node4.html


so, ARP bolts fall into the first region, where no plastic or permanent deformation occurs.

"torque to yield" bolts are manufacture in such a way is to plasticaly deform when loaded to the specified torque. this is often done by machining a "neck" of thinner diameter, which will have higher stress (force/area) than the rest of the bolt, and will thus deform first... "safely"

on the diagram, what happens is.... you torque up to the point of yield, and then a little more. as you can see, the first region past elastic is a "yield" region. this means that if you overload the bolt, it will plastically deform (stretch) enough to reduce the load to the level that it can hold. if you torque it a little more, it will stretch further, but the load it holds will be almost the same. this is of course assuming that the length between the load generating device (ie the ens of a bolt) are in the same position and thus the load can be reduced.
if you applied a fixed load, such as in a tensle tester, it would quickly fail after this point.

so.... when you reuse a torque to yield bolt, what happens??
each time (which could happen the irst time) you caue the bolt to yield more, ie more plastic deformation... ie, it gets longer. this will also cause the necked region (machined thinner) to reduce it's cross-sectional area!!! since the same amount of material is now longer... it MUST be thinner!!!

now... you want to exert a certain force on the bolt (ie clamping force), but now the bolt is thinner.... the STRESS (force/area) now has to be higher than before to get the same clamping force, so if you torque to spec again, you are actually loading it higher!!! so then it will yield even more to reduce the stress...which in turn reduces area more.... it's a bit of a circle, since the clamping force is fixed.

eventually, we run out of easy yield, and the bolt starts to work harden, which increases it's strength, but decreases it's ability to plastically deform. if you now torque to yield, you may load the bolt so much (with the same clamping force) that it exceeds it's ultimate tensile strength (UTS) and then it breaks.

so.... it all depends on the diference between the yield stress (where plastic deformation starts) and the torque to yield stress, and how much plastic deformation can occur before failure!!!!

so basically, whilst you might be able to get away with it, if the bolts experience an overload in service, if it is their first use, they may just stretch and reduce clamping force.
if it is their second or third use (or they were overtightened initially) they may actually fail instead of stretching... which is much more expensive

the reason why it's used? i think it gives more consistent clamping force. as long as the bolts are made well (ie deform at same strength) then a little les or a little more torqueing on the assembly line will result in the same clamping force... so it's a little easier to manufacture and it IS mroe consistent. if you have a non-yield bolt, you need to be more careful when initially torqueing, because that is what sets the clamping force... and then frixction under the bolt head etc plays a bigger part in the final force... for torque to yield, you torque to a certain point... and then go further... so if you were say.. 1/16 or 1/8 of a turn incorect cos of bolt head friction, you still will yield the bolt and get the right clamping force

that help?

[oldcorollas]

they specify stretch bolt settings so i did a little experiment putting a witness mark on a rod bolt, pre torquing and tightened it twice to see if it returned to the same spot ..... it didnt!! ... so they do indeed stretch further everytime they are used

can they be safly used again i guess is the burning question!!!

good thinkin '99 !!

so you have confirmed they are in the stretch region..

what you could do for an experiement is... to kill a bolt (dunno what it would do to the rod/cap etc)..
but you need to do it scientifically

ie, take note of the number of turns AND the load on the torque wrench. for a little while, the load on the torque wrench will stay fairly similar, but then increase to a point, and then fail (with continuing turning of the bolt head).
if you also pull it out and chck the length of the bolt, you can proably plot the length vs stress (on the torque wrench) and make a diagram like shown above
if you could measure the length of the bolt in situ, it would be more accurate, but the elastic region is usually recovered by about the same amount... so you will have a curve of length increase vs torque that wll be similar to the plastic deformation region of the diagram...

this will tell you a coupel of things....
how much more is too much!
what load the bolts will fail at (effectively your safety margin in service)
how much more can it be tightened before there is a large change in the clamping force vs length.
as for what is safe? hard to say

edit: and you could also measure the decrease in cross-sectional area at the point of necking (if it is obvious) and divide the torque by the reduction in area... to better reflect the strain of the bolt material.

it does sound odd that the point of necking would be within the thread... thats a recipe for disaster!! it doesn't have a slightly necked region above the thread?

[1jzracing]

cheers i know the basic principles and theories and its all logical, we even did a tensile bolt test at tafe many years ago

i guess the big question im trying to find out is how FAR! into the pink do manufacturers go .... so is it 10% so bolts can can be used safly a few times so twice shouild be totally safe or is it like 50% where twice is pushing the limits

Toyota does give a spec on the diameter so the waisting affect you discribed can be checked but the ones i measured were 0.15mm over the specs but who knows how far over they were to start with??

as a side note new ones cost me about $400 !!!!! for all the BE and mains on a 1j

stay tuned ill do another test and tighten one a few times measuring each time

edit: my torque wrench is a nice blue point so it just "clicks" at a preset value so torque loads cant be plotted... i can measure installed length using 2 ball bearings and my 3" mic so i will plot b4 and after length and diameter each time

[kingmick]

easy answer! no strech to yield bolts cant be reused, arp stretch studs are fine as long as you use a accrate gauge.

p.s 1j i put a proper sportsedan in my avtar for you.

[oldcorollas]

i suppose the most acurate way to do it would be to get a number of bolts and have them properly tensile tested.. to failure...

ie, unused bolt.
retightened once,
retightened twice etc

that way, the results will be more accurate, you can load with a constant strain and se how the stress changes vs strain, as opposed to measuring strain vs stress..

hopefully, the results will all fall along the same curve

edit, and then also, if you can plot the strain of the starting point (ie length of the bolt) for various re-usings... then when you take a bolt out and measure, you can see if it has been overloaded in service, and see how close to failure it is...

[1jzracing]

easy answer! no strech to yield bolts cant be reused, arp stretch studs are fine as long as you use a accrate gauge.

p.s 1j i put a proper sportsedan in my avtar for you.

Hey nice car i have seen an article in zoom about it - it was turbo v6 but they gone 350 chev now .... my aviator is toyota powered and best of all its mine!

well i have the comprehensive bolt analysis data!!!
take it from the results but i recon they cant be argued

toyota said dont use them aftere they fall below 8mm in diameter and from the test this looks to be exactly where the stretch and waisting starts to ramp away a little bit but this was after being done up 15 times!!!

so as long as you dont use them any more than 15 times i recon they are pretty safe

[oldcorollas]

so that was torqueing to spec each time??


so after 15 times, the area of the bolt had reduced by 5%, and so the stress on the bolt had increased 5%... and then after that, work hardening occurred...

excellent results

it wil be interesting to compare this data with bolts you take out of the car after racing then you can see how much life is reduced by the inservice use!

[1jzracing]

that was torque setting it to 21ft/lb then forther 90degrees each time as per workshop manual

each time for the first 5 or so tightenes there was barly 0.1mm diameter change from there it slowly started to increase

in reality you would only use them twice or 3 times at the most but i think the results show that there is little question on their reliability.... of course we couldnt measure the clamping force but i would bet its pretty consistant over the first few times

[kingmick]

i think you will find they loose clamping load on the second torque. you wouldnt want to much combustion pressure as even with new bolts you can get serious stretch{lift the heads}.
ill use the 7m as a bad example.

[1jzracing]

I think head studs are a different story all together i wouldnt try to use them again as i think they take them a lot further than the rod and main bolts so they are a once only item

the test above was for a conrod and i would assume the mains would be similar as they both refer to them in the manual as reusable as long as they meet the minimum diameters

there is no mention of reusing head bolts it states replace them!

as a side note i have used them again on a bmw and they worked fine it was just a road engine but they didnt snap or blow gaskets i drove it for years