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Message started by DragBikeMike on 12/13/19 at 12:31:50

Title: Cylinder Head Fastener Torque
Post by DragBikeMike on 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.

Title: Re: Cylinder Head Fastener Torque
Post by DragBikeMike on 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.

Title: Re: Cylinder Head Fastener Torque
Post by DragBikeMike on 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?

Title: Re: Cylinder Head Fastener Torque
Post by DragBikeMike on 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.

Title: Re: Cylinder Head Fastener Torque
Post by DragBikeMike on 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.

Title: Re: Cylinder Head Fastener Torque
Post by DragBikeMike on 12/13/19 at 12:35:57

Then I set the stud up in an old cylinder for some stress tests.

Title: Re: Cylinder Head Fastener Torque
Post by DragBikeMike on 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”

Title: Re: Cylinder Head Fastener Torque
Post by DragBikeMike on 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.

Title: Re: Cylinder Head Fastener Torque
Post by DragBikeMike on 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.

Title: Re: Cylinder Head Fastener Torque
Post by DragBikeMike on 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.

Title: Re: Cylinder Head Fastener Torque
Post by DragBikeMike on 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.

Title: Re: Cylinder Head Fastener Torque
Post by DragBikeMike on 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.

Title: Re: Cylinder Head Fastener Torque
Post by DragBikeMike on 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.

Title: Re: Cylinder Head Fastener Torque
Post by batman on 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.

Title: Re: Cylinder Head Fastener Torque
Post by Armen on 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?

Title: Re: Cylinder Head Fastener Torque
Post by hotrod on 12/13/19 at 19:30:25

Is it safe to say that the base gasket failure had nothing to do with the crankcase check valve ? This opens the possibility of more check valve experiments without fear of hurting anything.

Title: Re: Cylinder Head Fastener Torque
Post by Armen on 12/14/19 at 11:18:09

This shows why a lot of manufacturers use 'torque angle' instead of a torque spec in Ft/Lbs. Maybe set up the stud with the suggested lube, torque to spec, measure the stretched stud, then put on whatever goop you like, and torque til the same length using a torque angle gauge, and use that angle as your new way of torquing the fasteners?

Title: Re: Cylinder Head Fastener Torque
Post by DragBikeMike on 12/14/19 at 12:21:26

Batman, I agree there are six studs restraining the cylinder head to the cylinder, but there are only four restraining the head and cylinder to the case. I think we can ignore those two 6mm studs on the side of the cylinder base adjacent to the cam chain channel. Either way, its pretty evident that stressing those studs up to only 37.6 ksi isn't going to cut it.

Armen, the acorn nut depth was on my mind when I did the quench project. That's why I stacked the copper washers. You will note from this post that I ditched the .090" copper washers and replaced with .220" steel washers. The steel washers are .130" thicker than stock. That compensates for the .130" I machined off the cylinder.

Using the degree method to tighten up the nuts is certainly and option, but it would require testing all four studs. Each stud is a different length on this engine. The longer the stud more nut rotation is necessary to achieve the same stress. It is a more accurate method.

Hotrod, I can't rule out the check valve entirely, but it sure looks like it was not the cause of my gasket failure. I'm thinkin maybe the check simply set up the right conditions to push the gasket over the edge. I've been testing the KrankVent, riding around with a vacuum/fuel pressure gage installed on the crankcase. I haven't observed any vacuum/pressure in excess of 2 psi. That's not enough to cause a gasket failure. I have a copper base gasket in there now and its 100% oil-tight.

I have a big problem with the test, the gage was not intended for high vibration application. I broke the gage. I didn't note any major pressure or vacuum spikes so I'm pretty sure vibration did the gage in.

The failure was so weird that it would be difficult to explain. I have it on a series of videos. If any of you are willing to review the vids and comment I would certainly appreciate it. I can post links to the vids so you can see for yourself what took place. I have observed a lot of gages in my career and have never seen one behave like this. :o

Title: Re: Cylinder Head Fastener Torque
Post by Armen on 12/15/19 at 15:27:22

Hey DBM,
Maybe instead of measuring stretch of each stud, just use the assembled motor with the nuts coated in your fav juice, and a torque angle gauge below an indicating torque wrench. Just see how many degrees it takes to reach the stock spec? Then use the degrees after gooping up the nuts with your fav magic juice?

Title: Re: Cylinder Head Fastener Torque
Post by DragBikeMike on 12/16/19 at 18:05:56

That's an interesting suggestion Armen. I will have to ponder it for awhile. The tests I'm doing correlate stud stress-level to friction. I determine the stress level by how much the stud elongates. Regardless of stud length, the friction should remain consistent for the desired stress level. Using your proposed method might be an option. It would be cumbersome when you consider that you want to pull up the fastener in at least four increments.

Degrees of rotation generally determines elongation based on thread pitch. These studs have a pitch of 1.25mm, so for every quarter turn the stud is elongated .31mm (.012"). That would be every quarter turn past the point where the stud first starts to stretch. That's the real tricky part.

There's a head gasket in there, and it's also crushing as the nut is turned. I just received a set of four studs so now I have one of each length. Maybe I'll see if I can figure something out. As I recall, my Evolution Harley had a spec that was something like torque to 7 ft-lbs, then tighten 90 degrees. But all four studs were the same length. I would have to figure out the correct torque necessary to achieve the "just starting to stretch" condition. Then I imagine I would end up with four different rotation values in degrees. The shortest stud requiring the least rotation, and the longest stud requiring the most rotation. Argh!!!! Too complicated. Maybe I'll just stick with torque.