Molnar rods

Discussion in 'Street/strip 400/430/455' started by john.schaefer77, May 17, 2018.

  1. john.schaefer77

    john.schaefer77 Well-Known Member

    I have a set of stock replacement Molnar rods and the paper work has torque specs of 30 ft lbs and then 60 degrees. I torqued one cap and went to go the 60 degrees and it seemed really really tight and felt like it could break the bolt. I know stock rods with the ARP is 50, but those are bolts with nuts. These rods have the ARP bolts.
    Has anyone used these and torqued them and been successful? I emailed the company for advice. Thanks.
     
  2. Bluzilla

    Bluzilla a.k.a. "THE DOCTOR"

    I would ask Jim Weise as he is a Molnar Dealer.

    Larry
     
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  3. john.schaefer77

    john.schaefer77 Well-Known Member

    My thoughts exactly. I called a little too late. I'l try tomorrow. Thanks.
     
  4. 300sbb_overkill

    300sbb_overkill Well-Known Member

    Keep us posted because the old v8 stuff isn't suppose to be torque to yield one time use bolts!

    Usually they give either 2 different torque values, one with ARP grease and a different one for oil OR a stretch dimension to shoot for, never heard of 30 lbs then 60 degrees for a rod bolt?
     
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  5. TexasT

    TexasT Texas, where are you from

    I'm not sure how many things have the angle spec but the 3.1L gm engine we had apart last year had specs like that. I had to pick up an angle thing to measure with.
     
  6. Bens99gtp

    Bens99gtp Well-Known Member

    I will check my molar papers, but mine came with molars special made arp2000 bolts and mine showed to torque them by bolt stretch. This is not an easy thing to measure as it's hard to get in there and takes a special tool.

    I think it did have the angle method listed, I also think if I recall it ended up being about 75 ft lbs on mine.

    The big thing will be to torque them the same way as whoever did the final sizing, more or less and they won't be round. Now mine came in from molar and didn't need final sizing, they were all right on, as molar said they would be, but still had it checked to be sure
     
  7. john.schaefer77

    john.schaefer77 Well-Known Member

    Yes the instructions call for 30 ft/lbs and 60 degrees or .006-.0064 bolt stretch. Just feeling uncomfortable with how tight it was getting.
     
  8. 300sbb_overkill

    300sbb_overkill Well-Known Member

    This is untrue, they can be sized @ 45 ft ibs and if it takes 75 ft ibs to get the right amount of stretch for the final installation, that won't distort the bore.(not with a high quality rod anyway).

    The important thing is to get the bolt stretched the right amount as per the manufacturers recommendation to keep the bores round when the engine is running. If the proper amount of stretch isn't achieved the bolt can stretch when it is running and cause a bearing failure. I'm sure there is a better write up than what I wrote probably on ARP's website if you want more than what I wrote.

    Here's what ARP has in there website;

    "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."

    Here is the link for the entire write up;

    http://www.arp-bolts.com/p/technical.php#p7TPMc1_2

    The first time I installed a rod using a stretch gauge I thought to myself, I'm gonna leave the gauge attached to the bolt and use a 12 point 7/16" wrench, well the head of the 7/16" started bending:mad: before I got any stretch at all! So I had to make sure the stretch gauge repeated when I took it off and back on and it did. So I used my 1/2" drive Snap-On under glass dial indicator torque wrench so I could see what torque value I left off at to slowly sneak up at the proper stretch. That worked out great and no more bent wrenches and the way I still do it.

    I don't think I would trust the 30 torque/60* method, I want to see where the stretch is so I know for certain it is where it is suppose to be. So just get or borrow a stretch gauge and use those numbers to tighten your rod bolts and you'll be good to go.
     
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  9. john.schaefer77

    john.schaefer77 Well-Known Member

    I spoke to JW and Tom Molnar and both of them seemed to support the specs that is on the paperwork with the rods.

    BTW Tom Molnar is a super nice guy and was willing to spend the necessary time with me.

    JW also. I have called him a few t8mes and he is willing to give advice saving me time and money.
     
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  10. 87GN_70GS

    87GN_70GS Well-Known Member

    The instructions that came with my Molnar rods with the ARP2000 bolts also specd a torq plus angle. Using my beam type wrench, they ended up at about 90 ft lbs when the 60 deg was reached.
     
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