New Screws Really Needed When Replacing Rotors?
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Road Master
Joined: Oct 2008
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From: California
New Screws Really Needed When Replacing Rotors?
Going to install new HD floating rotors. Both the service manual and instruction sheet caution to use new screws when replacing the rotor.
Why is this? I could understand if these were tigh torque and soft metal, but we're only talking about max 24 ft lbs.
I know the screws are relatively cheap, but still, what's the need? Seems like more of a CYA for HD, plus a way to get yet more money out of us. Yes the rotors come with new screws, but for crying out loud, why couldn't they aT least provide chrome screws with chrome rotors!!!
Why is this? I could understand if these were tigh torque and soft metal, but we're only talking about max 24 ft lbs.
I know the screws are relatively cheap, but still, what's the need? Seems like more of a CYA for HD, plus a way to get yet more money out of us. Yes the rotors come with new screws, but for crying out loud, why couldn't they aT least provide chrome screws with chrome rotors!!!
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Road Master
Joined: Oct 2008
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From: California
Well I installed them tonight. Re-used the bolts too. There was a lot of dried red LocTite on them, but I used a wire brush to clean them up real well. The threads were all in good shape, so I think they'll be fine.
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I've seen way too many rear pulley bolts shear off the rear wheel to not use new ones whenever the pulley gets removed and replaced.
I apply that same caution to brake rotors.
You can't see the damage that occurs to a bolt once it has been stretched by torque. We actually stretch bolts when we torque them, where in fact a torque spec itself is an indirect measurement of desired bolt stretch. Some bolts are sized and designed to be torqued and retorqued many times where they are never stretched beyond thier elasticity when torqued to required specs, but sometimes the spec'd torque takes them to their design limits where they are not intended to be reused by design.
You will find this with many modern auto engine head and rod bolts where the MOCO insists you use new bolts each time.
Myself, I play it safe and follow the MOCO's specs.
There are lots of invisible stresses generated on rotor bolts....Brake rotors get toutured by heat and rotating torque...When the rotor gets scalding hot it increases in thickness and places additional stretch on the bolts at their heads, then those hot stretched bolts are subjected to tremendous shear stress as the brakes are applied. A dragging brake often goes unnoticed where the rotor never gets a chance to cool between braking events and it enters those cycles at elevated temps placing even more stress on the head of those rather small screws.
Think about how often you apply especially the front brake and then imagine how many heat/stress cycles described above occur in the life of those bolts and maybe then you will do like suggested and toss the old ones in the trash.
Like you said, they are cheap, they are included and you already have them out......they are one small step from the trash can, huh?
I apply that same caution to brake rotors.
You can't see the damage that occurs to a bolt once it has been stretched by torque. We actually stretch bolts when we torque them, where in fact a torque spec itself is an indirect measurement of desired bolt stretch. Some bolts are sized and designed to be torqued and retorqued many times where they are never stretched beyond thier elasticity when torqued to required specs, but sometimes the spec'd torque takes them to their design limits where they are not intended to be reused by design.
You will find this with many modern auto engine head and rod bolts where the MOCO insists you use new bolts each time.
Myself, I play it safe and follow the MOCO's specs.
There are lots of invisible stresses generated on rotor bolts....Brake rotors get toutured by heat and rotating torque...When the rotor gets scalding hot it increases in thickness and places additional stretch on the bolts at their heads, then those hot stretched bolts are subjected to tremendous shear stress as the brakes are applied. A dragging brake often goes unnoticed where the rotor never gets a chance to cool between braking events and it enters those cycles at elevated temps placing even more stress on the head of those rather small screws.
Think about how often you apply especially the front brake and then imagine how many heat/stress cycles described above occur in the life of those bolts and maybe then you will do like suggested and toss the old ones in the trash.
Like you said, they are cheap, they are included and you already have them out......they are one small step from the trash can, huh?
Last edited by oinker02; Oct 22, 2009 at 01:06 AM.
Road Captain
Joined: Mar 2007
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From: Southern Illinois
Oinker, Very well said. If you need any more incentive to buy new bolts just try to visualize what will happen when these bolts fail at speed while you are braking. It's an ugly picture.
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Road Master
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Mr. Willis was right!
Oinker is correct and provided a really good look at the stresses. But more importantly are the shearing stress applied when you use your brakes. The teeny bolts get used to this stress and how its aligned with the position of the bolt as fastened to the wheel.
When you reuse the bolt there is almost no way to make sure that bolt has exactly the same orientation in the wheel. So you change the sheering force direction. It's exactly like bending a nail back and forth - it will break eventually.
I was taught this eons ago in a high school metalshop class.
Thank you Mr. Willis.
...gene
