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#21
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I started using a stretch gauge about 25 years ago. Some of the bolts can be REALLY tricky to get to, with only about 1/4-turn possible without repositioning the wrench. And unless you are a gorilla, you are NOT going to get the proper stretch with a normal boxed-end wrench! I've got some extra-long Snap-On box-end wrenches I use, and still have to wrap rags around the end to keep from making my hands sore.
I've refined my technique over the years. I now measure a rod bolt, remove the gauge and torque it to (for example) 50 - then remeasure the rod length. Then add 5 and repeat until I get the desired stretch. Once I've determined what the magic torque number is, I repeat for another rod bolt - but I again measure the rod before and after to verify. I check every bolt that way, and tweak the magic number as needed as I go along.
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#22
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There’s allot of good videos on this whole subject from professional engine builders. I particularly like this one seems to be the simplest way using a good stretch gauge
https://youtu.be/sxW7TJTWoOY
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#23
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Another one using torque angle torque wrench
https://youtu.be/im1YgvBvV_8 And another one using regular all torque wench and stretch gauge. https://youtu.be/kA1elSAnE7g
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Last edited by Gach; 04-04-2023 at 04:48 PM. |
#24
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#25
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But I'm still thinking inertial loading will cause cyclic stress on the bolt, even if it is clamping and the joint design prevents separation of the cap and rod. I'll have to think about this more but will try not to whirl totally out of control! I have used Miner's rule in the past to effectively "sum" cumulative fatigue damage from complex loading like vibration stress, but I need to think more about the "smaller cyclic stress" on top of a preload.. And all this makes me wonder why a stock rod bolt would ever fail. Is it all about the rpm and resulting inertial loads overpowering the clamping force? All interesting so thanks again. Mike |
#26
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Was just watching a video about this very thing from a former writer on high performance pontiac magazine. he is testing rodbolt torque vs strech and checking the results with a micrometer . To see if they need to be resized (on a ford rod) But he still has the gto he was buiding during the HPP days.
https://www.youtube.com/watch?v=7Lv0jAD9PtE
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Murphy's law - "Anything that can go wrong will go wrong". -469 ,Butler 310+ Eheads, Hurricane intake portmatched by butler, Butler roller 230/236 @0.50 112 LSA, Johnson lifters, pypes 2 1/2" exhaust, 3.42 yukon duragrip lsd, holley sniper efi, hyperfuel efi tank +++ Last edited by djustice; 04-04-2023 at 06:51 PM. Reason: Can not spell. |
#27
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Molnar gives you a rod bolt stretch chart on the back of their instructions to keep track of it. Post #3 was very telling on just using a torque wrench. + - 25% is terrible accuracy. Proves Tom is right on this.
Another thing about Molars ARP 2000 bolts is that they are propitiatory to Molnar. No one else can use them. There is actually a slight thread interference fit when hand tight. The threads do not perfectly engage. But when the bolt is stretched to the proper length either by using a stretch gauge or by the torque + angle method the threads line up perfectly, full engagement. Tom thought of everything it seems. Nothing left to chance. This is the 31$ tool i used for the torque + angle method. I have tightened for more bolts than the average guy in my life, and I am telling you the 30lbs + 60 deg was a lot more than the 75lbs torque many 7/16 rod bolts use. https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1 My buddy who has every engine building tool in the book used the stretch method and the torque it took to get it was 92lbs ! He was the one who suggested I use the torque + angle method. This thread is about a Chevy guy who used Molnar rods and crank on his 1002.7 HP 582ci pump gas BBC build . Dyno sheet is on page 3. Screenshot would not enlarge. https://www.chevelles.com/threads/58...#post-10912784 Last edited by Dragncar; 04-04-2023 at 08:03 PM. |
#28
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This all makes me wonder my BME aluminum rods required no stretch gauge to torque them. He give me a specific torque number. They Have 30 dyno pulls about 20 quarter mile passes and a ton of street miles
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#29
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Micrometer-style torque wrenches should be turned to the lowest torque setting on the tool, not to "0". For example, a 50--250 ft/lb torque wrench would be turned down to 50 ft/lbs when you're done using it, not turned down to "zero". "Split-beam" torque wrenches are also "clickers" but they don't need to be turned down at all. They can be left with the torque set to any value. Quote:
However, don't coat the underside of the washer that goes against the casting/forging. Coat only the side of the washer that bears against the fastener. Coating the casting- or forging-side of the washer can lead to over-tightening the fastener. |
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#30
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I think if you are using the stretch only method it would not matter if you had grease on both sides of the washer. Stretch is stretch, In torquing the bolt to 30lbs then 60 deg it could effect it a little bit.
Torque wrench only, even more so. It is kind of tough to get the grease only on one side, kind of messy. Question, coarse threads vs fine threads like main studs vs stock bolts. Is 100lbs of fine thread torque really more than 100lbs on coarse threads. If so, how much ? I was going through that with my played caps. Trying to replicate what Whitmore did when he line bored it, but he passed away so I just had to go with stock numbers on fine threads. Machinist side it was dead straight and round when he checked the mains. He was impressed. |
#31
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Ok. I learned in school to turn it "all the way down". Maybe it was supposed to be lowest setting. That was a really long time ago. Lol! It makes more sense to me to take all of the load off of the spring. Thanks for the info.
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#32
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As far as Molnar rods having some kind of interference fit between bolt and threaded hole? It's possible Molnar has some sort of special machining process that produces a slightly higher engagement % than the standard 75%-up to 85% on a precision fastener set. Or he has ARP spec. a slightly larger fastener blank to produce the same higher %. Actual interference would create friction which is the enemy of the joint and a wild card when tightening. Next time i talk to him at PRI, I will ask. When I run the bolts into his rods, I don't feel any resistance until they seat. |
#33
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Now if you apply 200 lbs of external loading to the cap, the bolt load does in fact see that extra 200 lbs. But....the cap parting line load is reduced by 200 lbs. So now you have 800 lbs clamp load at the parting line plus 200 lbs external load and the fastener gets the sum of the two (800+200) and still is at 1000 lbs. If the external load is right at 1000 lbs, the cap parting line has zero load and the bolts sees 1000 lbs external load plus zero load from the parting line = 1000 lbs. If external load is 1500 lbs, the bolt gets 1500 lbs of load from that and since the cap parting line is open and waving in the wind, it doesn't contribute any load to the fastener; so the fastener gets 1500 lbs + zero lbs = 1500 lbs. So bolt load doesn't exceed preload until you totally unload the parting line joint. As a side, we've done detailed FEA analysis of bolted joints and the actual fastener load ends up about 2-3% higher than the above discussion. Eric
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#34
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Excellent information...thanks!
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My money talks to me-it usually says goodbye! |
#35
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Eric, thank you! I appreciate you taking the time to educate. Your explanation is perfect and I now "get it". This should be one of those interview questions like thermal expansion of a donut, as it's not intuitive, at least for me... and I worked with MechEs for decades mitigating fatigue risk!
After my last post, I tried to educate myself and found a couple useful sites online. Wiki has this cartoon illustrating exactly what you just clarified: ARP has more insight (link): "Pre-Load Finally, although not a design parameter, the subject of bolt installation preload must be addressed. It is a fundamental engineering concept that the force in a bolt in an ideal preloaded joint will remain equal to the preload until the externally applied force exceeds the preload. Then the force in the bolt will be equal to the external force. This means that fluctuating external forces will not cause fluctuating forces in a preloaded bolt as long as the preload exceeds the external force. The result is that fatigue failure will not occur. In a non-ideal joint, such as in a connecting rod, the bolt will feel fluctuating stresses due to fluctuating rod distortions. These are additive to the preload, so that fatigue could result. In connecting rods, precise preloads are required because if they are too low, the external forces (the reciprocating weights) will exceed the preloads, thus causing fatigue. If they are too high, they provide a high mean stress that combines with the fluctuating stresses due to rod distortion. Again, fatigue is promoted. The objective, then, is to preload a bolt so that it just exceeds the external load, and no higher. To sum up: both insufficient preloads and excessive preloads can lead to fatigue failures." Great education, thanks all. Mike |
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#36
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Stan
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#37
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Stan.I don’t really know but this video just makes one question it. Would it be something that would make me want to check every rod bolt length. I’ve never used rods that needed rod bolts stretch type of set up. Mgarbilk and Eric did an excellent job on helping us get more educated.
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Last edited by Gach; 04-05-2023 at 03:05 PM. |
#38
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What they are, asymmetrical threads. Or threads like a big C clamp would have. Flat on the top. If you put a Molnar 7/16" rod bolt in a Scat/Eagle rod there would be interference. They won't fit. Standard "V" shaped threads have more load on the bottom/base of the thread and less on the tops. The uneven loading is where problems/cranks start. The Molnar thread results in a even load across the threads when tight. |
#39
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The deciding factor for me would be listening someone with real experience and knowledge vs someone with very little experience . Plus what investment and what procedure I would be comfortable using.
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Last edited by Gach; 04-05-2023 at 03:30 PM. |
#40
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My last post comes straight from the # 2 guy at Molnar, Ed. He is the one who sent me the 3 D cutout pics of the 3 rods.
Now seeing if we can get some 3 D printed cutouts of the PA and PA+ rods. They have a 3 D printer used for prototypes of rods. |
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