Should differences in new rod bolts be a concern?
#1110762
11/10/11 12:01 PM
11/10/11 12:01 PM
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Joined: Jan 2003
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Val-haul-ass... eventually
BradH
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Taking time off to work on my car
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This may be "much ado about nuthin'", but I'm still going to ask. I'm replacing the original "Eagle by ARP" 8740 rod bolts in my Eagle (naturally) rods during this rebuild. The replacements are ARPs of the same length and 8740 material, but not the "Eagle by ARP" series, and appear different in a couple of ways: 1. Less total threaded length (same overall length, but threaded section is approx. .050" shorter) 2. Narrower shank by .015" (.360" vs. .375"); also longer due to shorter threaded section noted above 3. The new bolts are also lighter, enough that when I recalc'd the bob weight it worked out close to 20 grams lighter Considering the point of this swap is simply to put some new bolts in previously "touched up" used rods, are the differences between the old and new bolts liable to throw something off dimensionally due to differences in how the two styles of bolts react when torqued? FWIW, the old bolts were installed at 63#s, since I didn't have a stretch gauge at that time. IIRC, the recommended stretch should be around .006", but I don't know how close that comes to matching that particular torque rating. I'm not worried about the bob weight differences, just the potential for the rods not being round when the new bolts are installed. 
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Re: Should differences in new rod bolts be a concern?
[Re: BradH]
#1110763
11/10/11 01:45 PM
11/10/11 01:45 PM
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Joined: Aug 2003
Posts: 43,097
Bend,OR USA
Cab_Burge
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I Win
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Bend,OR USA
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I would have the old bolts check for free length, if oaky then have them magnaflux, if no crack maybe reuse them The other thing is to torque the rods with the old bolts and take them to a shop that can check the bearing bores for roundness, if okay then put the new bolts in and stretch them to proper prelaod and check them again and go from there  Did you by the new bolts from Eagle or from a dealer, speed shop? I wonder if Eagle sells the original style bolts still?
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
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Re: Should differences in new rod bolts be a concern?
[Re: Cab_Burge]
#1110764
11/10/11 02:06 PM
11/10/11 02:06 PM
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Joined: Jan 2003
Posts: 15,439
Val-haul-ass... eventually
BradH
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Taking time off to work on my car
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The original rod bolts have been through at least three previous tear-downs, so I figure it's time to replace them for peace of mind. Unfortunately, I have no pre-usage length measurements to compare against what they are today. The new bolts came from a vendor who I bought some other parts at the same time, as I recall. I figured one 7/16" x 1.800" 8740 ARP cap screw rod bolt is like another until I happened to compare the two side by side. Also, the new bolts are labeled as being for K1 Technologies, so it appears that Eagle and K1 have some different ideas about what their respective bolts should be. I did find a source for the exact replacement "Eagle by ARP" rod bolts, if that's the better choice given what I've noted above.  It's just that I figured I'd already had what I needed until the closer  revealed those differences.
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Re: Should differences in new rod bolts be a concern?
[Re: BradH]
#1110765
11/10/11 02:26 PM
11/10/11 02:26 PM
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Joined: Nov 2003
Posts: 10,384
Upstate NY
Bigcube
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I Live Here
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Upstate NY
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Did you re-size the the rods after you replace the bolts? I think it's a good idea to.
Jim
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Re: Should differences in new rod bolts be a concern?
[Re: Bigcube]
#1110767
11/10/11 02:32 PM
11/10/11 02:32 PM
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Joined: Jan 2003
Posts: 15,439
Val-haul-ass... eventually
BradH
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Taking time off to work on my car
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OP
Taking time off to work on my car
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Quote:
Did you re-size the the rods after you replace the bolts? I think it's a good idea to.
Some of the big ends were touched up after the 2nd rebuild, but the original bolts have been reused the entire time. I figured nothing lasts forever and they should be replaced for peace of mind this time, but wasn't planning to re-do the rods again.
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Re: Should differences in new rod bolts be a concern?
[Re: BradH]
#1110769
11/10/11 03:01 PM
11/10/11 03:01 PM
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Joined: Jul 2004
Posts: 19,317
State of confusion
Thumperdart
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I Live Here
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State of confusion
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Quote:
This may be "much ado about nuthin'", but I'm still going to ask. I'm replacing the original "Eagle by ARP" 8740 rod bolts in my Eagle (naturally) rods during this rebuild. The replacements are ARPs of the same length and 8740 material, but not the "Eagle by ARP" series, and appear different in a couple of ways:
1. Less total threaded length (same overall length, but threaded section is approx. .050" shorter)
2. Narrower shank by .015" (.360" vs. .375"); also longer due to shorter threaded section noted above
3. The new bolts are also lighter, enough that when I recalc'd the bob weight it worked out close to 20 grams lighter
Considering the point of this swap is simply to put some new bolts in previously "touched up" used rods, are the differences between the old and new bolts liable to throw something off dimensionally due to differences in how the two styles of bolts react when torqued?
FWIW, the old bolts were installed at 63#s, since I didn't have a stretch gauge at that time. IIRC, the recommended stretch should be around .006", but I don't know how close that comes to matching that particular torque rating.
I'm not worried about the bob weight differences, just the potential for the rods not being round when the new bolts are installed.
I just went through that............bolts were WAY shorter and didn`t fully engauge the threads in the rod so Pettis sent em back and now I have bolts that are .020 longer but not enuff to be concerned w/balance.
72 Dart 470 n/a BB stroker street car `THUMPER`...Check me out on FB Dominic Thumper for videos and lots of carb pics......760-900-3895.....
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Re: Should differences in new rod bolts be a concern?
[Re: BradH]
#1110770
11/10/11 05:09 PM
11/10/11 05:09 PM
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Joined: Jul 2010
Posts: 2,091
Delray beach, Florida
Performance Only
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Delray beach, Florida
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Brad, i can't imagine anyone would go to the expense to magnaflux 16 rod bolts for cracks, so nix that idea. even if the old bolts were bad, magging them would be a complete waste of time and energy.  here's the facts about rod bolts. over the years, different companies (i.e.) eagle, scat, k-1, oliver, carillo etc, have taken issue with the inconsistency of the bolts ARP has supplied. some of the 8740 bolts were taking over 80 ft lbs to stretch properly, the 2000 bolts in some batches were taking well over 90 ft lbs. that's a real problem since that amount of torque would distort the rods in many cases. these comnpanies have had ARP address this issue by changine mainly the shank diameter, and in some cases the number of threads and overall length. this is to keep the torque values within reason to reach the proper stretch length of the bolts. stretch is what keeps the bolt from backing coming loose. many people have had engine failures because they used a torque wrench instead of a stretch guage while assembly their engines. the bolts never got stretch the proper amount and consequently the bolt will come loose. it happens more than people realize. usually everything ends up so mangled up it's difficult (but not impossible) to tell the root cause of the failure. i would trust that your new bolts will be much more responsive to stretching the proper amount much closer to the nominal torque values you would expect. i deal with literally 1000's of rod bolts each and every year and i can tell you we've had zero failures with the newer redesigned bolts. i may be the lone disenter here, but i say bolt them together, use the stretch guage and move on. .050" difference in length is just a little more than 1 turn on those bolts. 
machine shop owner and engine builder
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Re: Should differences in new rod bolts be a concern?
[Re: maximum entropy]
#1110773
11/10/11 07:30 PM
11/10/11 07:30 PM
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Joined: Nov 2003
Posts: 6,293
Rock Springs
Bob_Coomer
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master
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Rock Springs
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A few thoughts.
The rod bolt shank doesnt locate the cap like the factory rods did, so I dont think the shank diff is a real problem. I assume these are typical H beam rods, with dowels in the rods.
As far as length is concerned I would look how the rod is made, and threaded where the bolt pass's/ends in the rod itself. Sometimes there is some threads cut that really have no value except to allow the threads to continue and end.
Why was the reason you decided to Not get the bolts from the rod manufacturer?
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Re: Should differences in new rod bolts be a concern?
[Re: Bob_Coomer]
#1110777
11/11/11 02:54 PM
11/11/11 02:54 PM
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Joined: Nov 2004
Posts: 2,825
Sk. Canada
RemCharger
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Quote:
Quote:
Do you think the big cap screw is really a better system than the factory bolt and fine thread nut? I mean other than the size and metallurgy aspect? and clearance...
Rem
Not just the bolt itself, but the rod design. look at how the cap is secured, its not a good comparison either. How can you compare a 3/8 bolt to a 7/16 bolt anyways?
All these rods I have seen are 7/16" diameter.
A bit lost in translation... I guess what I should have said was, I'd prefer to see a stud (which is ** essentially ** what the factory style is closer to..)
Not far off the idea of screwing down a main cap versus a stud.
And yes I agree that the rod designs are diff, the bolt sizes are diff, etc... I'm talking more in theory.. 
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