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Bleed Nipple Torque?


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#16 Tornado99

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Posted 23 February 2021 - 06:49 PM

Torque values for various fasteners are usually quoted for ‘clean dry threads’ - which is not the case when you tighten a bleed nipple lubricated with brake fluid - which is why I tighten by feel


Torque is used to achieve a certain tension on the fastener, but it is not a direct measure of tension.
While its true that a lubricated fastner will tension load more under same torque amount, this is true no matter if using a torque wrench to get there or your feel. The bolt or screw will still fail at same torque force whether or not it is lubed. What changes is how much tension gets applied to the surfaces being fastened, which is important for things like cylinder heads and such. What is needed is a reduction in torque on lubed fastners. There's likely an adjustment table somewhere.

#17 Revd

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Posted 23 February 2021 - 10:23 PM

 

Torque values for various fasteners are usually quoted for ‘clean dry threads’ - which is not the case when you tighten a bleed nipple lubricated with brake fluid - which is why I tighten by feel


Torque is used to achieve a certain tension on the fastener, but it is not a direct measure of tension.
While its true that a lubricated fastner will tension load more under same torque amount, this is true no matter if using a torque wrench to get there or your feel. The bolt or screw will still fail at same torque force whether or not it is lubed. What changes is how much tension gets applied to the surfaces being fastened, which is important for things like cylinder heads and such. What is needed is a reduction in torque on lubed fastners. There's likely an adjustment table somewhere.

 

That is the point is when tightening by feel, that you reduce the amount of torque applied.What you don't want to is exceed the elastic limit and reach the yield point where the fastener is permanently deformed.



#18 Pete649

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Posted 24 February 2021 - 05:45 PM

I wonder if when a manufacturer quotes a figure for bleed nipple torque they take into account the fact it is lubricated by the brake fluid.



#19 Cooperman

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Posted 24 February 2021 - 08:05 PM

To think that in the 1960's we used to rebuild Minis without any torque wrenches.
In fact, I have even seen engines assembled with just spanners. They all seemed to work fine.

#20 MiniMadRacer

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Posted 24 February 2021 - 08:23 PM

Torque wrenches were designed to stop us "animals" over tightening things and thereby actually weakening the clamping force in many cases... a lot to be said for mechanical sympathy



#21 Revd

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Posted 24 February 2021 - 10:16 PM

To think that in the 1960's we used to rebuild Minis without any torque wrenches.
In fact, I have even seen engines assembled with just spanners. They all seemed to work fine.

 

 

Torque wrenches were designed to stop us "animals" over tightening things and thereby actually weakening the clamping force in many cases... a lot to be said for mechanical sympathy

  

I have a video somewhere of the Tiger production at Triumph's Meriden works in the early 60's, the frames and engines are assembled, including critical parts like the flywheel, without recourse to torque wrenches.

 

My first experience of spannering as a teenager was on a BSA 250, you soon learnt not to be too heavy-handed or it would mean a bus ride to Jack Butler's Motorcycles in Leamington Spa for a replacement part or a helicoil. 



#22 Spider

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Posted 25 February 2021 - 02:58 AM

Not specifically mentioning Bleed Nipples here, Generally speaking regarding how tight Fasteners need to be, what's needed is not the Torque applied to it, but the amount of Stretch (or in some instances, compression) over the length of the Fastener between the 2 points where it's being held by, eg a head stud in the block is held one end in the block and the other by the nut. It's the Stretch of the fastener that does the job of holding - clamping - the parts together under the required pressure, just like a G Clamp.

 

The Torquing of Fasteners in General using a Torque Wrench, when you break it down, the Re-active Torque or effort needed on a fastener by Torque is a combination of stretching the fastener by holding one end fast, then pulling on the other end by dragging it up a ramp (ie, the Thread) and the Friction of both these Threads running against each other and between the back-side of the fastener head and the part that under it.

 

The Stretch can easily and accurately be calculated by the angle of the Ramp (the Pitch of the Thread), however, the Frictional values are quite variable and more so when dry of any lubricant. The Threads might be nicely rolled formed in materials that respond well to that process, or cut in to Cast Iron by Tapping. Each of these will have very different frictional values. Another factor here is the type of finish applied to the fastener (and how well that's been done), eg plain steel vs galvanised. The Finish and Hardness of the Surfaces of the underside of the Fastener and the Part can equally be quite variable, particularly when dry. Torquing and undoing then Retoquing again for about 5 or 6 cycles may 'bed' the parts to each other and so reduce the variation in fastener stretch, but it will still have a high variation between 2 identical fasteners.

 

According to ARP, and Unbrako Torquing Dry can have a variability as much as 50%. Most fastener manufacturers how have Torquing Charts are mostly Oiled numbers.

Coming back a bit to out Minis, all the Engine & Gearbox fasteners are Oiled numbers. The Suspension and Brakes I'm not 100% sure of, but they'd likely be Dry numbers. It would be handy if they included all this info with the various charts !



#23 Tornado99

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Posted 25 February 2021 - 08:37 AM

Was doing a torque critical job today, an alu cylinder head on a cast iron engine block of a classic brit bike (Norton Commando). Head had just benn rebuilt with helicoils into three of the head stud since original threads had pulled out.
Before torquing it all down, I confirmed my torque wrench was calibrated by clamping socket end into vise, pulling arm via a digital scale hooked at 1' from pivot point. There are vidoes on utube on how to adjust calibration if needed. I'd done that last year and all was still good today.
The manual that came with the wrench states to always lube fasteners for proper torque settings. Also to loosen a fastner then apply torque when checking a previously set fastner.

by the way, I also had to use the 90 degree angle trick on three nuts for this head due to clearance issues. I bought the MotionPro spanner/wrench clamp torque adpator and it worked well.




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