Used Bolts

[Rick C.]

I think there was a thread recently where it was written that bolts in critical applications should not be reused in some circumstances, but I can't find the thread. Would someone review for me what those circumstances are? Has anyone done any actual tests or know of any actual tests that prove that a used bolt (or is it one that has been replated) will fail or loosen before a new bolt? Or is it all hearsay? I'd sure like to use some of my original chassis bolts.

Rick

[Terry M.]

Re: Used Bolts

Search this board for "Hydrogen embrittlement" -- without the quotes.

Then do the same with Google or whatever your favorite search engine is. Then YOU decide if the risk is worth it.

[Michael G.]

Re: Used Bolts

It really depends very much on the application, as well as what damage has been done to the bolt during its previous service. You could write a book on this (a very boring one) , but I'll try to address a few major considerations:

Bolts are essentially springs, designed to be stretched during installation, to apply a clamp-load to hold things together. The loading they see does not significantly fatigue them (in almost every case) so, in most circumstances, a given bolt's ability to deliver service load and resist loosening during second , third, or even fourth (or more) installations is not significantly affected by its previous life. Generally, a re-used bolt will require slightly less torque to achieve its intended clamp-load during subsequent installations (because the threads and under-head surface are polished during its first installation) but this shouldn't alter your torque spec. An undamaged bolt should, therefore, be reusable in most circumstances.

Three major exceptions to this are fairly easy to identify: 1) A bolt that has had very severe corrosion to its threads, or thread body, or head will be weaker than its design intent and should not be re-plated and used again. 2) A bolt that has been over-torqued or excessively loaded during installation, service, or removal will generally have yielded slightly in the threaded area between the head and the threads that were engaged in the internal thread. This can generally be identified using a "go" ring gage. If it doesn't spin freely over all the threads (after corrosion has been removed with a brass wire wheel) there's a good chance that the threads have been stretched beyond their elastic limit, resulting in permanent deformation. 3) obvious wear marks on thread bodies will reduce the cross-sctional area of the shank, resulting in reduced strength. All three of these types of used bolts will possibly fail under service load, so if there's any question, be safe, throw it away.

Its also a good idea to understand that the presence of oil and other lubricants on either internal or external threads will significantly alter assembly specs. Additionally a part stripped of factory plating and not re-plated will not deliver the intended service load under factory torque specifications.

I'd also be very careful about electroplating high-harness (>34 Rc) critical fasteners yourself. High harness, highly loaded fasteners, tend to break in service due to hydrogen embrittlement if they are not properly baked after electroplating.

As I said above, there's a lot more to it, so if you have a particular issue I'd be glad to help. Mike

[Gene M.]

Re: Used Bolts

Rick,
Terry and Mike are correct to warn about "Hydrogen embrittlement" as a result of replating. In most all cases the high grade 8 bolts use a black oxide or phosphate to avoid the brittleness issues with electroplating in the GM original applications.

The grade 5 and lesser are often zinc plated and some also chromated for added protection. These bolts in the normal applications should be safe to replate zinc. Many platers do not relieve for "Hydrogen embrittlement", even if you ask for it.

[Joe L.]

Re: Used Bolts

Many platers do not relieve for "Hydrogen embrittlement", even if you ask for it.

Gene------


YES!

[Alan S.]

Re: Used Bolts

Hi Michael & Gene,
Thanks very much for the information.
It's strange how reading about things we're interested in isn't boring.
I had never made the 'connection' (pun intended) between the location of phosphated bolts and and the location of grade 8 bolts.
Regards,
alan

[Kurt B.]

Re: Used Bolts

Michael,
Very informative post.
Can you elaborate more on using oil or lubricants when assembling things?
I always used such and thought I was doing a good thing by doing so.
Am I wrong in using oil on wheel studs and then using a troque wrench to torque to the specified torque?
I know nothing about this and would like to learn.
Should bolts and nuts be assembled DRY without lubricant ?
Thank you in advance,
Kurt
#26406

[Edward J.]

Re: Used Bolts

Kurt, No lube on wheel lugs or studs ,threads should be clean as the threads on lug nuts. when lubed the chances are that the wheel will not hold the true torque.

[Richard F.]

Re: Used Bolts

Shouldn't connecting rod bolts get special mention? My own rule has been to never use one that has already been torqued. That is partly because I don't want to be picking up pieces of one off of the street, and also because my engines were not frequently disassembled. These bolts can get pretty expensive if you replace them very often.

[Michael G.]

Re: Used Bolts

Regarding lubricants: Lubricants applied to threads (or under the head) of fasteners significantly alter the frictional coefficient of the surfaces involved. Since the friction between such surfaces determines the relationship between the amount of torque applied to the fasteners and the actual clamp-load created in the fastened joint, if you apply lubricant where none was present during factory assembly, the production torque specification will not deliver the intended clamp load in the joint. In most bolts with older-type surface finishes, (zinc plate, for instance) the lubricant will increase the clamp load over what was expected, so you could overload and break a bolt, or damage the joint by applying too much load. In some slippery finishes, (such as cadmium) it won't make a big difference, however, even experts can't necessarily predict the actual change in friction without a test. As a rule don't ever add lubricants, unless the original design called for it.

In regard to connecting rod bolts: while they are probably reusable, get new ones, the risk is way too great not to spend the money.

Mike

[Kurt B.]

Re: Used Bolts

Thanks Michael.
Looks like I was doing more harm than good by lubricating bolts and fasteners.
Kurt

[Joe C.]

Re: Used Bolts

I think there was a thread recently where it was written that bolts in critical applications should not be reused in some circumstances, but I can't find the thread. Would someone review for me what those circumstances are? Has anyone done any actual tests or know of any actual tests that prove that a used bolt (or is it one that has been replated) will fail or loosen before a new bolt? Or is it all hearsay? I'd sure like to use some of my original chassis bolts.

Rick

I would not recommend re-using old fasteners on items subject to stress/strain cycles, heat, and pressure.

Fasteners, especially if tightened beyond their yield strength, will plastically deform. Once this happens, they will no longer provide the design clamping force for the application. It is impossible to know, for sure, whether or not a fastener has been subjected to plastic deformation. If a bolt will not hold a specified torque value, that is a sure sign that the bolt has been strained, but most times it will still hold specified torque.

There is another class of fasteners, known as "torque-to-yield" (TTY), which are designed to deform (stretch) upon tightening. The use of a strain gage is required to measure the proper installed deformation of these specialized fasteners.

The cleanliness of the threads, both on the fastener and within the subject part, including whether or not these threads have been strained ("stripped"), as well as the type of lubricant used on the threads and under the fastener heads/washers will determine torque values when assembling. Moly grease on threads and under heads provides significantly more lubricity than motor oil, thread locker or thread sealant. Thread sealant with teflon seems to provide lubricity approaching that of high pressure lubes, such as assembly lube or others containing molybdenum disulfide.

I would absolutely NOT recommend re-using old fasteners in the following locations:

Head to block
Flywheel/flex plate to crankshaft
Pressure plate/torque converter to flywheel/flex plate.
Oil pump housing to main cap
Main caps to block
Connecting rod bolts

In "critical" chassis locations, many fasteners are rubber isolated and/or damped by the springs, and so not subject to direct shock loads.

Some engine builders/machinists find it necessary to remove the black oxide plating from critical fastener such as head bolts, because they believe that the coating alters the friction coefficient of the threads. I have not found evidence of the necessity of this, however.

[Michael H.]

Re: Used Bolts

As I said above, there's a lot more to it, so if you have a particular issue I'd be glad to help. Mike

Thanks Michael. Very informative.

[Rick C.]

Re: Used Bolts

Thanks to all for a lot of great information.

If you should ask a plater to relieve for hydrogen embrittlement, is there any way of knowing whether or not it was actually done?

Rick

[Joe L.]

Re: Used Bolts

Thanks to all for a lot of great information.

If you should ask a plater to relieve for hydrogen embrittlement, is there any way of knowing whether or not it was actually done?

Rick

Rick-----


No way that I know of. Sometimes, cad or zinc plated parts will slightly yellow as a result of the heating process, but it's pretty subtle and often it's not even noticeable, at all.