Reusing Nuts'n'Bolts
Discussion
I think I kinda know the answers I'm going to get to this, but anyway...
Rebuilding my Alfa v6. Should I invest in all new rod bolts, bearing cap nuts, etc, or can I reuse the old ones? If I should replace the bolts, what about head studs etc?
Most of them are done up to a specified torque plus a number of degrees.
I ask because the factory rebuild manual makes no mention of it, and indeed suggests dummy builds with everything torqued up to take measurements; so presumably the bolts cannot be the use once, stretch type. Various online build threads and so on also appear to reuse them.
Replacing bolts isn't so bad, but I'm less sure about studs - do people normally replace these? If so, how do you get the buggers out?
And if not, why would you replace bolts but not studs?
Thanks in advance...
Rebuilding my Alfa v6. Should I invest in all new rod bolts, bearing cap nuts, etc, or can I reuse the old ones? If I should replace the bolts, what about head studs etc?
Most of them are done up to a specified torque plus a number of degrees.
I ask because the factory rebuild manual makes no mention of it, and indeed suggests dummy builds with everything torqued up to take measurements; so presumably the bolts cannot be the use once, stretch type. Various online build threads and so on also appear to reuse them.
Replacing bolts isn't so bad, but I'm less sure about studs - do people normally replace these? If so, how do you get the buggers out?
And if not, why would you replace bolts but not studs?Thanks in advance...
I think a lot of the bolts and stuff are generally one-hit wonders nowadays and need replacing. When I was rebuilding the engine on my bike the book said they needed replacing as torquing them down stretches them. However, when selecting bearing shells and plastigauging them you need to torque them down and loosen them afterwards so not sure what that's all about. I suppose there is a good reason for it though or they wouldn't say it, it's not like they're going to make a mega fortune out of replacement bolts is it?
Generally it depends if the fasteners are "torqued to yield" or not. It's easy to tell the difference:
1) if the tightening spec is a pure torque (i.e 80Nm) then that fasterner is been used purely in it's elastic region, and hence it will be fine to use it multiple times (assuming it passes an obvious visual inspection for thread form, shank or head damage etc!)
2) If the tightening spec is a torque plus an angle (i.e. 80Nm + 120deg) then it is most probably a fastener torqued to yield into it's plastic region. As a rule these bolts should not be reused (although, if you measure their length compared to a new bolt, and they are within a few % elongation then actually you can re-use them)
Generally bolts torqued to yield are used to limit the maximum compression load, for things like head gaskets etc.
1) if the tightening spec is a pure torque (i.e 80Nm) then that fasterner is been used purely in it's elastic region, and hence it will be fine to use it multiple times (assuming it passes an obvious visual inspection for thread form, shank or head damage etc!)
2) If the tightening spec is a torque plus an angle (i.e. 80Nm + 120deg) then it is most probably a fastener torqued to yield into it's plastic region. As a rule these bolts should not be reused (although, if you measure their length compared to a new bolt, and they are within a few % elongation then actually you can re-use them)
Generally bolts torqued to yield are used to limit the maximum compression load, for things like head gaskets etc.
Fireblade69 said:
That's what the bike bolts and little end bolts were, x-ft-lb + x degrees. I still don't get the torque it up, and then some more bit though.
The tensile load of a joint in compression with a bolted fastener will have 3 deffined characteristics for the tensile loading applied by the bolt load;1) initally, the joint will be much less stiff than the bolt, (especially if any form of gasket is present between the faces) During this zone, the total compressive load will only increase very slowly with the angle the bolt has been turned
2) the joint stiffness inceases and the tensile load then increases pretty linearly with bolt angle and / or torque
3) the bolt (or sometimes even the joint) reaches it's elastic limit. At this point further increases in angle do not increase tensile load linearly (and hence bolt torque also is non linear)
To ensure that joints are correctly loaded (not under loaded and not over loaded) modern mass produced engines use a yielded fastener system, where the fastener is deliberately tightened into it's plastic region. This means the joint cannot be under or over tightened. However, because rotational torque of the bolt is no longer linear in the plastic yielded region, the "angle tightening" method is used. The manufacturer will specify an angle through with the bolt must be turned that will guarentee to result in sufficient elongation of the bolt to push it into yield.
The inital "snug" torque is simply there to settle the joint initally and provide a reasonably repeatable starting point from which to start the angle tightening part.
note: elastic deformation = the fastener is able to return to it's original length when the load is removed and is not permanently elongated
Plastic deformation: the fastener is stretched beyond it's elastic limit, and will be left permanemtly elongated even when the load is removed (hence the requirement to replace used fasteners of this type)
For the torque + angle system the maximum compressive load in the joint is set by the fasteners cross sectional area and it's yeild strengh. this makes it impossible to over load the joint system.
(bolt torque vs compressive load is massively varriable depending on the thread and bolt head to system friction value, something that is very difficult to accurately control, whereas the "angle" tightening method is fixed to the bolts elongation just by the thread pitch (and hence closely controlled)
Edited by anonymous-user on Wednesday 5th January 00:02
Thanks.
That's all as I'd thought. Now, looking at the main bearing caps, they are bolted on using studs in the crankcase. These should be tightened to 25Nm + 79 degrees.
BUT the factory manual makes absolutely no mention of removing these studs, or indeed how to, in any of the strip down or reconditioning steps. Indeed, all of the pictures throughout show the studs in place. And 25Nm doesn't seem likely to take anything beyond its elastic limit, surely ?
Edit - I'm a clutz. Rod bolts are finally done up to 53-59 Nm, it's the mains which are tightened by angle as above.
That's all as I'd thought. Now, looking at the main bearing caps, they are bolted on using studs in the crankcase. These should be tightened to 25Nm + 79 degrees.
BUT the factory manual makes absolutely no mention of removing these studs, or indeed how to, in any of the strip down or reconditioning steps. Indeed, all of the pictures throughout show the studs in place. And 25Nm doesn't seem likely to take anything beyond its elastic limit, surely ?
Edit - I'm a clutz. Rod bolts are finally done up to 53-59 Nm, it's the mains which are tightened by angle as above.
Edited by The Black Flash on Wednesday 5th January 09:09
Remove the studs by running two nuts onto the exposed thread and tighten the nuts up to each other. You then wind out the stud using the lower nut as your purchase. A drop of oil around edge of the stud to help. If it feels too tight, sometimes you can just wind it in a small amount to free it up.
AFAIK there's no such thing as a "stretch" stud that's used with a separate nut so there is no need to remove or replace them. As to whether a bolt is a "stretch" bolt or not is not always easy to tell just from the recommended tightening method. Just because the last part of a sequence is an angle rather than a torque doesn't always tell you it's a single use item.
Bolts that are designed to be tightened past their elastic limit usually have a final angle turn of at least 180 degrees after the snugging torque has been applied. If in doubt play safe and replace but to be honest pretty much any bolt can be reused once before it's in any danger of stretching so far it's too weak for its intended purpose.
Personally I hate angle tightening methods and usually just work out a torque value to use from the bolt material spec and thread dimensions.
Bolts that are designed to be tightened past their elastic limit usually have a final angle turn of at least 180 degrees after the snugging torque has been applied. If in doubt play safe and replace but to be honest pretty much any bolt can be reused once before it's in any danger of stretching so far it's too weak for its intended purpose.
Personally I hate angle tightening methods and usually just work out a torque value to use from the bolt material spec and thread dimensions.
Thank you all.
I will leave the studs well alone then. I may give a dealer a ring tommorow to see about the bolts (not that I expect much, but it's worth a shot). If they're easy enough to get hold of, I'll replace them to be on the safe side. If it's a(nother) "send away to the USA and get fleeced" job, probably wont
I will leave the studs well alone then. I may give a dealer a ring tommorow to see about the bolts (not that I expect much, but it's worth a shot). If they're easy enough to get hold of, I'll replace them to be on the safe side. If it's a(nother) "send away to the USA and get fleeced" job, probably wont
Max_Torque said:
Pumaracing said:
and usually just work out a torque value to use from the bolt material spec and thread dimensions.
what friction co-efficient do you use to do this??Sometimes there's no other option such as when I first got involved with ZVHs many years ago (Zetec/CVH hybrids). The only way to fit the CVH head to the Zetec block was to use industry standard 12.9 grade 10mm x 1.5mm capheads rather than either the Zetec or CVH 10.9 grade stretch bolts which were the wrong length so you had to come up with a torque value and keep your fingers crossed. I settled on 75 ft lbs for those and never had a problem.
Big end bolts are usually either 8mm or 9mm with 1mm threads with the occasional oddball like the coarse threads in the Vauxhall XE engine so there are plenty of examples around to find a recommended torque value for a given material spec.
Final step is to do a trial run and see if the bolts will take the torque figure repeatably and keep tightening up to the same position with the heads marked with paint and such that you can't feel them "giving".
I also think the whole subject is nothing like as critical as people believe. If you take into account the variations in torque wrench calibration, whether people use oil on the threads or not and if so what type, how clean and burr free the threads are, whether you properly seat the threads by doing the bolt up and then loosening it off again several times or just fit once and forget then you start to realise that everyone actually tightens the same bolts up quite differently and yet most of the time they never fail.
Wheel bolts are probably the worst example. By the time those have got into the usual manky rusty state they all end up in you can have only a tiny fraction of the actual torque applied going into providing any clamping load, or at least compared with a nice new clean set of threads. When's the last time you saw one break or come loose though?
The usual non OE application for things like head or big end bolts is that of replacing a stock 10.9 grade bolt with a 12.9 grade one. Just add 15% to 17% to the OE torque value and she'll be right mate. Anything more fancy than that like an ARP bolt and they'll give you a torque figure to use anyway.
In general if you can find a torque value for a bolt of the same thread and a particular material tensile strength then for a higher tensile bolt you can raise the torque figure in proportion to the increase in tensile strength and theoretically that should load the new bolt to a similar percentage of its yield point.
Finally, stretch bolts are just a bulls
t way of getting you to buy new bolts at every rebuild to make the car company more money which never used to happen in engines pre about 1980. No respectable engineer would intentionally set out to use critical bolts past their yield point which can only ever weaken them so the only thing driving this is money.Look at ARP. They don't spec a single fastener to be intentionally used past its yield point. Every bolt has either an elongation setting using a bolt micrometer or a torque setting and you can refit them as often as you like.
Pumaracing said:
Finally, stretch bolts are just a bullst way of getting you to buy new bolts at every rebuild to make the car company more money which never used to happen in engines pre about 1980. No respectable engineer would intentionally set out to use critical bolts past their yield point which can only ever weaken them so the only thing driving this is money.
How much money do you actually think say FORD make from the sales of headbolts?? Its got nothing to do with "making money" and EVERYTHING to do with the process of repeatably and reliably mass producing engines from an efficient production line system (have a go yourself in your shed and see how many engines you can make in 1 Hr ;-)t way of getting you to buy new bolts at every rebuild to make the car company more money which never used to happen in engines pre about 1980. No respectable engineer would intentionally set out to use critical bolts past their yield point which can only ever weaken them so the only thing driving this is money.Stretch bolts are in fact excellent engineering, designed by clever (and very respectable) engineers who know that having a good product is one thing, and making it cheaply and efficiently quite another! (and the point of a stretch bolt is that although the bolt clamp load is outside the elastic region of the fastener, the cyclic joint loading is NOT greater than this force, hence your cylinder head DOESN'T fall off when you got to WOT..........)
(i agree, if you are just building a SINGLE, high performance engine, then yes, you can (with significant effort from measurement and trial assy) get better fastener "performance" from a well specified non "stretch" bolt.)
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