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Cylinder Head Fastener Torque (Read 119 times)
DragBikeMike
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Cylinder Head Fastener Torque
12/13/19 at 12:31:50
 
Back in October, my cylinder base gasket failed.  It appeared to be sucked-in as a result of my installation of a check valve in the engine breather system.  The check valve is supposed to result in a slight vacuum in the crankcase during engine operation.
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DragBikeMike
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Re: Cylinder Head Fastener Torque
Reply #1 - 12/13/19 at 12:32:42
 
When I disassembled the engine to replace the base gasket, I noticed that the head nuts did not seem to be very tight.  The nuts are threaded onto four long studs.  Typically, when a long stud is stressed properly, it takes quite a bit of nut rotation to relieve the stress.  My head nuts seemed to loosen completely with only about ¼ turn of counterclockwise rotation.

I last installed and tightened the head nuts when I did my compression upgrade.  For that upgrade, I machined .130” off the top of my cylinder to raise compression and tighten quench.  To prevent the cylinder head acorn nuts from bottoming out on the studs, I installed an additional copper washer under each cylinder head nut.  The copper washers are .090” thick, so now I had .180” worth of copper under each cylinder head nut.

There are comments on this forum regarding the application of sealant on the copper washers.   They are known to leak oil.  That seemed reasonable to me, so I coated the copper washers with case sealant.  I used Permatex Optimum Grey High Temp Gasket Maker #27036.
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DragBikeMike
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Re: Cylinder Head Fastener Torque
Reply #2 - 12/13/19 at 12:33:27
 
Fastener Torque is a measure of the friction resulting from the contact area between the internal threads in the nut, the external threads on the stud, the face of the nut, and the surface that the nut contacts and rotates against.  The factory spec for the head nuts is 21-24 ft-lbs (252 – 288 in-lbs), lubricated with oil.

Applying sealant to the washers affects the friction developed while tightening the nuts.  More friction, less rotation required to reach the specified torque value.  Less rotation, less stud-stretch.  Less stud- stretch, less fastener stress.
 
Conversely, less friction will result in more fastener stress.

I decided to do some tests to see if the sealant reduced or increased fastener stress.  Was it a good lubricant, or a poor lubricant?
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DragBikeMike
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Re: Cylinder Head Fastener Torque
Reply #3 - 12/13/19 at 12:34:24
 
I purchased a cylinder stud.  It’s a typical reduced body stud.  The threaded portions are M9x1.25.  The reduced section is .258” diameter.  Pretty skinny if you ask me.  It must be some excellent material.
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DragBikeMike
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Re: Cylinder Head Fastener Torque
Reply #4 - 12/13/19 at 12:35:10
 
I machined the ends of the stud so they were square & flat.  I wanted good surfaces to use to measure the stud length.
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Re: Cylinder Head Fastener Torque
Reply #5 - 12/13/19 at 12:35:57
 
Then I set the stud up in an old cylinder for some stress tests.
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Re: Cylinder Head Fastener Torque
Reply #6 - 12/13/19 at 12:40:02
 
Without tightening the nuts, the free length of the stud was 7.911”.

Then, I installed a copper washer under the nut, lubricated the washer, threads, and the face of the nut with clean engine oil, and tightened it in four increments.  First 70 in-lbs, then 150 in-lbs, then 220 in-lbs, and finally 280 in-lbs.  Tightened to 280 in-lbs, I measured the stud.

Lubricated with engine oil, the stud stretched .019”.  It was 7.911” unstressed, and at 280 in-lbs torque it measured 7.930”.

Then I repeated the test using the Permatex sealant.  I lubricated the threads with clean oil, and applied sealant to the faces of the washer and nut.  At 280 in-lbs torque, the stud measured 7.920”.  That’s only .009” stretch, less than half of what it stretched when I used oil.  That didn’t look good to me.

After getting the same results several times, I decided to test some other sealants.  Here are the results of the tests.

Engine Oil:                                                            .019”
Permatex Optimum Grey #27036:                          .009”      
Permatex Optimum Black #27037:                            .009”
DAP Silicone Gasket Sealant:                                .010”
VersaChem Type 2 Gasket Sealant:                          .015”
Loctite 242 Blue Thread Sealant:                                  .024”
Permatex High Temp PTFE Thread Sealant #PX59214:      .021”
Loctite High Temp PTFE Thread Sealant #592:            .023”
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Re: Cylinder Head Fastener Torque
Reply #7 - 12/13/19 at 12:41:11
 
I decided to see what it would take to stretch the stud to .019” with the silicone sealant applied to the washers.  I maxed out my torque wrench at 600 in-lbs and still didn’t achieve .019” stretch.  At 600 in-lbs (50 ft-lbs), the stud only stretched .017”.  When I disassembled the fastener and cleaned things up, this is what the copper washer looked like.  You can see the gall-welded aluminum on the bottom of the copper washer.
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Re: Cylinder Head Fastener Torque
Reply #8 - 12/13/19 at 12:41:59
 
Note how the top of the copper washer was also galling and transferring copper to the face of the nut.
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Re: Cylinder Head Fastener Torque
Reply #9 - 12/13/19 at 12:43:38
 
It’s evident that silicone sealant has a dramatic effect on fastener torque.  It is not a good lubricant.  I guess if it was a lubricant it would say so on the tube.

I decided that I would not be reassembling the engine with those copper washers.  Stacking up the washers just introduces more places for oil to leak.  It also fills the assembly with more soft copper to deform and work harden as the engine repeatedly heats up and cools down.  Instead, I manufactured steel washers (.220” thick) to replace the copper washers.
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Re: Cylinder Head Fastener Torque
Reply #10 - 12/13/19 at 12:44:51
 
I sealed the steel washers with Loctite High Temp PTFE Thread Sealant 592 and lubricated the threads with clean engine oil.  When I torqued up the cylinder head nuts, they pulled very smooth and there was at least 90° of nut rotation from a just snug 30 in-lbs to 250 in-lbs.
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Re: Cylinder Head Fastener Torque
Reply #11 - 12/13/19 at 12:46:45
 
After 250 miles of operation there is no leakage from any head-nut or washer.  Time will tell how it holds up, but I think the prospects are good.

My tests using oil (as recommended by the OEM manual) show that the test stud stretched about .019” when tightened to the high end of the factory spec.  That works out to about 79.4 ksi stress, or 4128 lb-force.  Although the four studs are different lengths, if torqued to the same value they should all end up stressed equally.

That may sound like a lot of force, but if you consider the force resulting from peak combustion pressure, 4100 lbs per stud provides a reasonable margin.
 
I searched the net looking for a value for peak combustion pressure in a spark ignition (SI) engine.  I found values as low as 725 psi to values as high as 1500 psi at full power.  If you use 1000 psi and calculate the force over a surface 3.7” diameter (the LS bore is 3.7”), you arrive at 10,750 lbs-force trying to blow the head & cylinder off the engine.  That works out to 2687 lbs per stud.  Stressing each stud to 4128 lbs provides a 1.5 factor of safety.
 
In contrast, if the stud is only stretched .009”, as was the case with silicone sealant, the stud is stressed to 37.6 ksi.  That results in 1955 lbs force.  Not even enough to restrain the force developed by peak combustion pressure at full power (estimated).  No wonder my gasket gave up.
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Re: Cylinder Head Fastener Torque
Reply #12 - 12/13/19 at 12:47:55
 
I don’t recommend using silicone sealant on the copper washers.  You will not get the stud stretched properly.

I suggest you install new copper washers lubricated with a liberal coating of clean engine oil, or anneal the old washers and install with a suitable sealant.  I’m likin this stuff.

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Re: Cylinder Head Fastener Torque
Reply #13 - 12/13/19 at 15:17:48
 
Mike ,nice work , I haven't done much research on compression pressures, what I did find was that they where about 100 psi for each point of static compression ( stock Savage with 8.5: 1 would be about 850 psi)   which seem to be close to what you found 725psi (7:1?) , 1500psi ( 15:1 dynamic ?)  .  with your reduction in chamber size and the results of your compression test ,I think your compression to be on the high side (11:1?),so I looked at using the Max 1500 rather than 1000psi  for your example ,rather than 10,750 psi  it ends up 16,125 psi , However I think you made the mistake of dividing your number by 4 to arrive at 2687 lb/ft per stud . I divided my 16,125 by six (total head studs) and arrived at the same 2687 and thus the same 1.5 factor of safety, even at 1500psi. I think your motor may see this high a pressure because we haven't taken into account how higher combustion temperatures affect pressure.
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Re: Cylinder Head Fastener Torque
Reply #14 - 12/13/19 at 17:47:27
 
Stupid question. Are those acorn nuts sitting on top of the studs on top of the shortened cylinder? Can the stud be booming out in the acorn nut?
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