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20NM + 90 degrees = ???
bikecarbfred - 7/2/18 at 07:45 PM

I just torqued up the camshaft sprocket to 20NM + 90 degrees.

Can someone tell me is this around 25NM in total as I would like to check it with torque wrench as I am not sure if it has loosed while I was rotating the camshaft around.


O don't fancy doing them again as their stretch bolts.

I't's just for a quick check to give me a piece of mind.

Angel Acevedo - 7/2/18 at 08:16 PM

As it says..
You torque to 20 NM...
Then the preload is the 90° Which will be a fixed distance given by 1/4 of the thread pitch.
This will be independent from Torque reading on Torque wrench...
Actually you may want to do this last "torquing" with a solid wrench or breaker bar.
HTH.

ETA: Reading again, If you went to 20 NM and donīt know if you torqued it to +90°, I think itīs safe to go back and re-torque.
As if you didnīt apply full torque, bolts wonīt be stretched to yield yet.
Again.
Hope this helps.

[Edited on 2/7/2018 by Angel Acevedo]

bikecarbfred - 7/2/18 at 09:15 PM

Friend, I do know what 90 degrees is.

What I was trying to say was after I had torqued it up 20NM + 90 , I then with a breaker bar was rotating the camshaft clockwise.

The valve springs, spring up quick, the breaker bar that I am holding tight jolts with it and I am wondering if the bolt lost some of it's tensions.

That is why I thought if I knew what 20NM +90 = in NM, I can give it a quick check with the wrench.

obfripper - 7/2/18 at 09:49 PM

quote:
Originally posted by bikecarbfred
I just torqued up the camshaft sprocket to 20NM + 90 degrees.

Can someone tell me is this around 25NM in total as I would like to check it with torque wrench as I am not sure if it has loosed while I was rotating the camshaft around.


It's not quite that simple, the 20Nm is bringing the bolt under (a repeatable amount of) tension, then the 90° stretches the bolt by 1/4 of the thread pitch; ie for m8x1.25 it would be 0.31mm stretch.
Some of this stretch will be elastic deformation, and some will be plastic deformation depending upon bolt grade. The plastic deformation permanently stretches the bolt and thins the section outside of the engaged threads, hence the recommendation to not reuse stretch bolts.

If the bolt is marked with a grade, the bolt will remain in elastic deformation up to the max recommended torque for that grade, therefore this will be the max torque you could safely apply on an installed stretch bolt; ie for a m8x1.25 10.9 grade bolt this would be 35Nm. Beyond this point there are too many variables affecting wether the bolt will torque/stretch/snap, hence a torque+angle technique is used for repeatable results close or into plastic deformation zone.
If the bolt is installed oiled or with a proprietary lubricant the max torque will be lower and dependant on the lubricant used - in this case only the pre-torque value (20Nm) will definately be safe.

If you're still not sure, post up the bolt grade and thread size, and i can give you a safe figure to check for dry installation.

I always use a fluid marker (and multiple colours) when rebuilding engines to mark every stage when tightening so no mistakes can be made, and if you get waylaid by someone it only takes a glance to pickup where you left off.

Having just seen your second reply, it is unlikely that the kickback from the valve springs has slackened the bolt. It would take a very solid jolt against a fixed object for this to happen, your hand/arm will have cushioned the jolt sufficiently.

Dave

bikecarbfred - 8/2/18 at 07:29 AM

@ Dave - I wish I had kept the arm loose while the jolt happened.

Hex Bolt is M8 : 10.9 grade

Edit: Used oil on the thread and a little on the underside of the head.

[Edited on 8/2/18 by bikecarbfred]

obfripper - 8/2/18 at 11:02 AM

The only rule of thumb i know for lightly oiled threads (plain engine oil not ep/gear oil) is to reduce applied torque by 10%, so 31Nm should be safe to test if the bolt has slackened.

Dave

bikecarbfred - 8/2/18 at 11:03 AM

Yup 10% less. I'll check it at 30NM should be good enough

bikecarbfred - 8/2/18 at 11:04 AM

Thanks Dave