I went through the same dilemma. When I measured my original bolts they all were the slightly different length. I noticed that the bottom of each bolt is cut unevenly so an accurate measurement is not possible. I assume they did this because these rods use a nut instead of threading the cap. Makes a stretch measurement nearly impossible so you can't really judge the condition of the bolt. I opted to buy new just in case. Perhaps ARP might be able to help with a non-OEM bolt if you got a hold of one of their techs. Then a proper stretch-measure install would also be possible.
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connecting rod bolts
- Thread starter NewHavenMike
- Start date
Well, I went to the supply house to return the con rod bolts I had. They had the right ones in stock and the picker for the order just picked the wrong part.. I swapped them for the ones I needed. I brought one of the connecting rods with me to be sure they would fit.
These parts seem to get mixed up quite easily, going by the 3-4 places Ive called. Also, everyone Ive dealt with from older people to the younger crowd, no one seems to know what the correct hardware looks like so its hard(pretty much impossible) for them to match up parts to a part number.. The guy had a bag of similar bolts and gave me a bolt that was a lot different, but said it was the part number I wanted. The other 4 bolts were the. A lot of this stuff is all mixed up and the numbers arent matching.
The bolts the guy sent to me apparently were $40 each. the XS400 ones are around $9 each.
These parts seem to get mixed up quite easily, going by the 3-4 places Ive called. Also, everyone Ive dealt with from older people to the younger crowd, no one seems to know what the correct hardware looks like so its hard(pretty much impossible) for them to match up parts to a part number.. The guy had a bag of similar bolts and gave me a bolt that was a lot different, but said it was the part number I wanted. The other 4 bolts were the. A lot of this stuff is all mixed up and the numbers arent matching.
The bolts the guy sent to me apparently were $40 each. the XS400 ones are around $9 each.
Are the ARP bolts suitable Replacements?
Well ARP have bolts that withstand 180 000-200 000 psi that fit the measurements (M8 45mm) the pitch is a bit different (1.25 instead of 0,75) and the shop manual calls for a torque of 27 ft-lbs, theirs are rated at 24ft-lbs.
I did send ARP an email...the only thing that makes me a little nervous is the torque difference...should it be a Concern? I wouldn'the want to blow my engine going 60mph....
Other option is NOS bolts but on some of the pictures of those I seem to see wear...if I buy used bolts without really knowing...it kinda defeats the purpose...
Ah shit, almost wishing I didn'the take them apart now...but I wanted the check the crank bearings for excessive wear.
And did anyone change-out the crankcase bearings and conrod bearings ? At this point I figure I might as well change everything as this is turning into a pricey rebuild...
I did send ARP an email...the only thing that makes me a little nervous is the torque difference...should it be a Concern? I wouldn'the want to blow my engine going 60mph....
Other option is NOS bolts but on some of the pictures of those I seem to see wear...if I buy used bolts without really knowing...it kinda defeats the purpose...
Ah shit, almost wishing I didn'the take them apart now...but I wanted the check the crank bearings for excessive wear.
And did anyone change-out the crankcase bearings and conrod bearings ? At this point I figure I might as well change everything as this is turning into a pricey rebuild...
I changed the con rod bearings and left the crank bearings alone. I used new original size bearings on the con rods too. My engine only had 4000 miles on it when I tore it down though. On the ARP bolts, I'd go with whatever ARP says.
All I can say is that the manual says nothing about replacing the con rod bolts and in other places is very clear about not re-using parts if it is not recommended. If it were me, if the torque to spec without an unusual amount of cranking on the nut (indicating stretch), I would leave it alone. If you had a length you could check it, but I don't really see the need.
HoughMade, What manual do you have?
I have the Haynes manual that I assembled my engine with. It gave a pretty clear warning that the rod bolts were "of the stretch type and not to be reused".
Some people seem to have gotten away with it though. I would assume that it depends on how many miles the bottom end has.
Also, the rod bolts are pretty much impossible to measure since the threaded end is uneven. I went though this already with a guy selling NOS parts.
I have the Haynes manual that I assembled my engine with. It gave a pretty clear warning that the rod bolts were "of the stretch type and not to be reused".
Some people seem to have gotten away with it though. I would assume that it depends on how many miles the bottom end has.
Also, the rod bolts are pretty much impossible to measure since the threaded end is uneven. I went though this already with a guy selling NOS parts.
The factory manual available on this site.
I wouldn't use stretched bolts either. What makes you think they have stretched?
I wouldn't use stretched bolts either. What makes you think they have stretched?
From what i've read about conrod bolts, on other sites mostly for car engine, these bolts stretch on the factory installation torque...
So apparently they would already be stretched as they are now....
Which is why once you remove them you would have to replace them cause another torque would break them or weaken them....
So apparently they would already be stretched as they are now....
Which is why once you remove them you would have to replace them cause another torque would break them or weaken them....
Where did you source the Bearings?
Bellerophon
XS400 Member
I know this thread is almost 4 years old, so sorry mods if this is breaking forum etiquette.
However, this is the first thread that comes up when you search "conrod bolts" and there seems to be a lot of misinformation here so I thought I would add some technical info to help clear things up, especially as conrod bolts are pretty much impossible to get hold of these days.
First off, the Haynes manual says the bolts "may" have stretched or weakened, not will have stretched or weakened. Second, this is only mentioned in the Haynes manual, not the manufacturer's service manual. I will leave you to make your own inference on that matter.
So, what is a Torque to Yield Bolt? - Wikipedia gives a very good explanation here . It is basically saying that when a TTY fastener's clamping force is higher than the applied load, the strain (how much the bolt is stretching every time it is subjected to a load) is lower than when the clamping force is lower than the applied load.
To be clear
- Yield Strength is a stress limit of how much load a material can withstand without permanently deforming. Therefore, by definition, a Torque to Yield bolt should not exceed its yield strength and should not have stretched! (see below about torque specs) If it has, it means it has strain hardened.
- Strain hardening - If you apply a load that causes a stress higher than the yield strength it will plastically deform. If this stress was lower than the material's tensile strength, then when this load it released it will elastically deform to its new length. It will still stretch elastically just like before, except now it needs a higher load to stretch the same amount. The yield strength has been increased and is now closer to tensile strength.
- Tensile Strength is a stress limit of the maximum load a material can withstand, in most hardened steels this is approximately 20% higher than the Yield Strength. If you reach this its game over!
- Fatigue strength - When a material is subjected to a load over and over again, it is called cyclic loading and this can dramatically reduce the materials strength and lifespan. The higher the number of cycles the lower the fatigue strength becomes, it will typically start to decrease around 1,000 cycles and will level off around 10,000,000 cycles. A failure from fatigue will present itself as a crack, once this crack is large enough the object will fail (usually catastrophically) by fracturing. The cyclic loading does not cause the bolt to stretch over time! The aim of a TTY fastener is to increase the bolt's lifespan by reducing the amplitude of these cycles and increasing the fatigue strength.
Why should they be replaced?
If the bolt has strain hardened then when you replace it you will not be getting the correct amount of tension (stretch) in the bolt if you are applying the same torque. If on the other hand you are using a torqing method other than a wrench (like angle torquing) then everytime that fastener is retightened you are plastically deforming it and getting closer to its tensile strength.
Also, as a bolt is being fastened it experiences other secondary stresses such as torsion. If micro-cracks are in the bolt, these secondary stresses can cause premature failure.
What will happen if you don't replace them?
I would say this will depend on three main variables - how many cycles the engine has already been through, has the bolt strain hardened and what fastening torque you use.
Applying a lower torque than specified - This will not subject the fastener to a high clamping load and therefore may save it if microscopic cracks are present, however, it will reduce the future life of the bolt as the cyclic loading will have a higher amplitude.
Applying the specified torque - If the bolt has indeed stretched and strain hardened this would not create the desired tension in the bolt and lower the future life span of the bolt
Applying a higher torque - If the bolt has microscopic cracks you could be running the risk of fracturing the bolt as you tighten it or not long after you start running the engine.
Also, to really fry your brain, we should remember that torque wrenches are only accurate to +/-4%. The XS400's conrod bolt torque is specified to 33-38Nm which means that it could be tightened to anywhere between 31.68Nm and 39.52Nm, this is a total tolerance of +/-11% ! On top of this you will have the variable of friction causing an even higher discrepancy, which for a dry fastener can far exceed the innacuracies of the torque wrench (don't forget that moly-disulphide grease!). So I imagine it can be quite easy to exceed the bolt's yeild strength if the aim is to hit an accurate value just below it.
If you are able to get hold of new conrod bolts, then by all means buy and use them and share with us your source (I hear ARP are the guys to go to).
If anyone else has anything techincal to offer, or wants to correct me, please do.
Lasty - has anyone here, who has rebuilt their engine had it blow up?
However, this is the first thread that comes up when you search "conrod bolts" and there seems to be a lot of misinformation here so I thought I would add some technical info to help clear things up, especially as conrod bolts are pretty much impossible to get hold of these days.
First off, the Haynes manual says the bolts "may" have stretched or weakened, not will have stretched or weakened. Second, this is only mentioned in the Haynes manual, not the manufacturer's service manual. I will leave you to make your own inference on that matter.
So, what is a Torque to Yield Bolt? - Wikipedia gives a very good explanation here . It is basically saying that when a TTY fastener's clamping force is higher than the applied load, the strain (how much the bolt is stretching every time it is subjected to a load) is lower than when the clamping force is lower than the applied load.
To be clear
- Yield Strength is a stress limit of how much load a material can withstand without permanently deforming. Therefore, by definition, a Torque to Yield bolt should not exceed its yield strength and should not have stretched! (see below about torque specs) If it has, it means it has strain hardened.
- Strain hardening - If you apply a load that causes a stress higher than the yield strength it will plastically deform. If this stress was lower than the material's tensile strength, then when this load it released it will elastically deform to its new length. It will still stretch elastically just like before, except now it needs a higher load to stretch the same amount. The yield strength has been increased and is now closer to tensile strength.
- Tensile Strength is a stress limit of the maximum load a material can withstand, in most hardened steels this is approximately 20% higher than the Yield Strength. If you reach this its game over!
- Fatigue strength - When a material is subjected to a load over and over again, it is called cyclic loading and this can dramatically reduce the materials strength and lifespan. The higher the number of cycles the lower the fatigue strength becomes, it will typically start to decrease around 1,000 cycles and will level off around 10,000,000 cycles. A failure from fatigue will present itself as a crack, once this crack is large enough the object will fail (usually catastrophically) by fracturing. The cyclic loading does not cause the bolt to stretch over time! The aim of a TTY fastener is to increase the bolt's lifespan by reducing the amplitude of these cycles and increasing the fatigue strength.
Why should they be replaced?
If the bolt has strain hardened then when you replace it you will not be getting the correct amount of tension (stretch) in the bolt if you are applying the same torque. If on the other hand you are using a torqing method other than a wrench (like angle torquing) then everytime that fastener is retightened you are plastically deforming it and getting closer to its tensile strength.
Also, as a bolt is being fastened it experiences other secondary stresses such as torsion. If micro-cracks are in the bolt, these secondary stresses can cause premature failure.
What will happen if you don't replace them?
I would say this will depend on three main variables - how many cycles the engine has already been through, has the bolt strain hardened and what fastening torque you use.
Applying a lower torque than specified - This will not subject the fastener to a high clamping load and therefore may save it if microscopic cracks are present, however, it will reduce the future life of the bolt as the cyclic loading will have a higher amplitude.
Applying the specified torque - If the bolt has indeed stretched and strain hardened this would not create the desired tension in the bolt and lower the future life span of the bolt
Applying a higher torque - If the bolt has microscopic cracks you could be running the risk of fracturing the bolt as you tighten it or not long after you start running the engine.
Also, to really fry your brain, we should remember that torque wrenches are only accurate to +/-4%. The XS400's conrod bolt torque is specified to 33-38Nm which means that it could be tightened to anywhere between 31.68Nm and 39.52Nm, this is a total tolerance of +/-11% ! On top of this you will have the variable of friction causing an even higher discrepancy, which for a dry fastener can far exceed the innacuracies of the torque wrench (don't forget that moly-disulphide grease!). So I imagine it can be quite easy to exceed the bolt's yeild strength if the aim is to hit an accurate value just below it.
If you are able to get hold of new conrod bolts, then by all means buy and use them and share with us your source (I hear ARP are the guys to go to).
If anyone else has anything techincal to offer, or wants to correct me, please do.
Lasty - has anyone here, who has rebuilt their engine had it blow up?
