Question for machinist's/engineers

Discussion in 'Small Block Tech' started by Mark Demko, Jan 14, 2018.

  1. philbquick

    philbquick Founders Club Member

    I know my 300/225s have nuts and bolts and my GN and TTA have bolts but I didn't know why until now, thanks for that info.

    Two reasons studs are better than bolts:

    1) Studs guarantee a class A thread fit. If the threads in a block are tapped when the taps are new, the thread fit would be different then if a block was tapped right before the taps were at the end of life and replaced. You could end up with a class A thread when new and a class B thread with a tap that's worn out and ready to be replaced.

    2) Measuring the torque of a bolt is not accurate but it's better than nothing. The best way to measure a bolts (or studs) clamping ability is to measure how much the bolt stretches. Instead of measuring torque put a dial indicator on the top of the stud and measure the stretch. This can't be done with a bolt since it's being screwed in and turning. This is easy to do with a stud since it's stationary. All studs have stretch specifications, this is the way racing and aircraft engines are built.
     
  2. sriley531

    sriley531 Excommunicado

    Another reason studs/nuts are stronger is because whenever force is applied on a surface where a 90 corner is present, you'll find stress risers at that corner (ie where the bolt shaft meets the base of the bolt head). With a stud/nut setup, the force is better distributed. I use a software at work called Ansys to do finite element analysis' on machinery components/assemblies to find stress risers/shear strength/total deformation/modulus of elasticity/etc to predict failure points before we ever build and test the equipment, and I'm always watching for these "corners" as it's often times the weak point in the system. If I had a little more time, I'd even be glad to run an FEA on these items, but this is my last week on the job so I don't think I'll have enough free time to accurately model and analyze everything.
     
  3. Bens99gtp

    Bens99gtp Well-Known Member

    I think most of the major aftermarket rods use cap screws, not just or our motors but across the board on a lot of motors, so there must be a reason as to why
     
  4. sean Buick 76

    sean Buick 76 Buick Nut

    I have one here where the old style rod broke and it took out the block, hole in the side. I know Buick had more than one rod break on the dyno before they switched to the new style. Like many things a lot of it comes to luck and not all parts are created equal even though they are supposed to be the same... look at the fellow who broke a Buick 350 crankshaft in half at about 420 HP and 6500 rpm while other stock cranks handle 600+ hp like Bobb mackleys 370 stroker. A lot of it is luck of the draw...
     
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  5. 8ad-f85

    8ad-f85 Well-Known Member

    As mentioned, aftermarket rods having capscrews are used in platforms in which the head part of the bolt (in a stud/nut type) would be very close to the cam. If you recess the bolt head deeper, you are weakening the rod too much. Strength is sufficient in the capscrew, leaving more beef for the rod.
     
    Last edited: Jan 15, 2018
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  6. Bens99gtp

    Bens99gtp Well-Known Member

    But there is even ma job differences even between the same bolts. A lot of rods used ARP2000 bolts, but some such as Molnar, use a custom made arp2000 series bolt, they told me it's special made to their wants cause something about the regular 2000 series bolt had weakness in the design causing it to be more likely to failure at a cetain spot along the Holts length.......so even though they use a 2000 series as other rods.....but even them are not on an even field
     
  7. 8ad-f85

    8ad-f85 Well-Known Member

    True and definitely a long ways past the OP's question wondering why a supposedly lesser method of clamping a rod was a 'better way'.
    (It wasn't specified for just Buicks or even OEM rods)
    Not discouraging further rod bolt theory discussion, only pointing out that in some cases the bolt isn't the weak link...like when you cut too much of the rod away.
    "We" made billet rods for OEM sponsored race efforts in several different styles.
    Their needs changed based on many things.
     
  8. Bens99gtp

    Bens99gtp Well-Known Member

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  9. 8ad-f85

    8ad-f85 Well-Known Member

    Didn't mean to sound like I was busting your chops! :) :)
    As I drink my morning cofvefe, calling the kettle black, guiltlessly spinning threads into tangents of their own.
    There's really good info in this thread from all of us opinioners.
     
  10. sean Buick 76

    sean Buick 76 Buick Nut

    Well said. Unfortunately there were more than a few rod failures in the early rods and when does let go there usually isn’t much to salvage. This one I have here the crank, block, piston, and rod all scrap. Once things start hitting it goes bad quick.
     
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  11. jay3000

    jay3000 RIP 1-16-21

    As was mentioned, the stud / nut portion of the rod is not the issue. The latter style rods are much beefier in many places. The stud nut / does seem to make for better fit however.
     
  12. sean Buick 76

    sean Buick 76 Buick Nut

    The engine that Jay built for was a simple comb0. 74 rods, ARP bolts, TRW high compression pistons, and ported heads. I ended up selling it with my old car and it was about 600 Hp with boost and it was reliable, no issues with the rods..
     
  13. Jim Blackwood

    Jim Blackwood Well-Known Member

    Nobody mentioned that studs often have fine threads for the nut where the bolts are coarse thread. This has to have an effect on clamping forces. I don't recall ever seeing a sheared off bolt head on internal fasteners, usually there is enough of a radius to take care of that. On the rods, capscrews generally allow enough room for ribs beside them, at least on the aftermarket rods, and the cam interference problem was up on the shoulder of the rod where the head of the older style "stud" seats in the rod. It's not really a stud though, is it. More of a bolt and nut. So it seems rod fastener selection is down to rod strength rather than fastener superiority. In the block, longer thread engagement can only help. That ratio between diameter and thread engagement varies with the material of each threaded part, so with high tensile studs going into cast iron a 2:1 ratio or better does make sense. Plus it spreads the stress out over a larger area which is always a good idea if only to help control warpage.

    Jim
     
  14. sean Buick 76

    sean Buick 76 Buick Nut

    Agreed, my Hershe rods have the locating dowels to ensure the rod caps are aligned properly. I think this also strengthens the rod because once they are tightened together it takes a fair amount of force to separate them even once the bolt is removed. I bet this takes a lot of the stress off the rod bolt.
     

    Attached Files:

  15. Fox's Den

    Fox's Den 355Xrs

    Everything in my motor used ARP fasteners top to bottom.
     

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