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Old 04-23-2013, 02:34 AM
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Rod bolt strength,180,000 psi stock 3/8 inch bolts

Do you really need ARP bolts installed in a stock rod (3/8 inch bolts) if the engine is not exceeding 5500 rpm. Isn't it the high rpm that produces the torsional stresses that distort the big end bearing seat,regardless of horsepower production (perhaps crankshaft rotational acceleration can be a factor as well) Anyone got knowledge on this ,I can't find exact information on it but suspect the horse numbers are not failure critical as are the high rpm stresses placed on the rod bolts (centrifical force multiplied geometrically as rpms are increased past a critical point. Can this not be determined by comparing the clamping pressure forces of the tightened stock bolts to the centrifical forces acting on the rod big end at a given rpm. Isn't this how the factory figuares out whether to put in boran bolts or to increase the bolt diameter to 7/16. There must be a formula the factory uses or is this a trade secret.

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Old 04-23-2013, 04:06 AM
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Probably not........Many stock engines go hundreds of thousands of miles without failure. However, I dont think the bolt strength is determined just by RPM. Look at it this way....Most top fuel engines dont turn much more than 5500...Do they use stock fasteners?
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Old 04-23-2013, 05:55 AM
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Remember that top fuel engines run aluminum rods because the explosion in the combustion chamber is so intense that the whole rod flexs from the percussion of the fuel (basically TNT chemically) so I think the physics involved is quite different as is the acceleration of the spin of the crankshaft with a load on it. I believe the stock rod bolts have a pretty high tensile rating out of the factory somewhere between 160,000 - 180,000 psi which is not weak.
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Old 04-23-2013, 06:20 AM
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These engines don't need ARP quality rod bolts for low RPM, low stress usage. A standard Gr 8 rod bolt meets or exceeds 150,000 psig, BUT - are they the same quality as those ARPs' and the reason that my car's engine has them.
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Old 04-23-2013, 07:53 AM
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thot top fuel buzzed em up in the 8000 range.....just sayin
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Old 04-23-2013, 08:01 AM
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Quote:
Originally Posted by yankee phil View Post
Do you really need ARP bolts installed in a stock rod (3/8 inch bolts) if the engine is not exceeding 5500 rpm. Isn't it the high rpm that produces the torsional stresses that distort the big end bearing seat,regardless of horsepower production (perhaps crankshaft rotational acceleration can be a factor as well) Anyone got knowledge on this ,I can't find exact information on it but suspect the horse numbers are not failure critical as are the high rpm stresses placed on the rod bolts (centrifical force multiplied geometrically as rpms are increased past a critical point. Can this not be determined by comparing the clamping pressure forces of the tightened stock bolts to the centrifical forces acting on the rod big end at a given rpm. Isn't this how the factory figuares out whether to put in boran bolts or to increase the bolt diameter to 7/16. There must be a formula the factory uses or is this a trade secret.
for the little the arp bolts cost.. I'd add them.. remember the weight of the rod and piston is try'n to rip those bolts apart..
if you are having the rod big end resized, you might as will put in new bolts..
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Old 04-23-2013, 10:38 AM
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Originally Posted by yankee phil View Post
Do you really need ARP bolts installed in a stock rod (3/8 inch bolts) if the engine is not exceeding 5500 rpm. Isn't it the high rpm that produces the torsional stresses that distort the big end bearing seat,regardless of horsepower production (perhaps crankshaft rotational acceleration can be a factor as well) Anyone got knowledge on this ,I can't find exact information on it but suspect the horse numbers are not failure critical as are the high rpm stresses placed on the rod bolts (centrifical force multiplied geometrically as rpms are increased past a critical point. Can this not be determined by comparing the clamping pressure forces of the tightened stock bolts to the centrifical forces acting on the rod big end at a given rpm. Isn't this how the factory figuares out whether to put in boran bolts or to increase the bolt diameter to 7/16. There must be a formula the factory uses or is this a trade secret.
The factories use the Elfini finite element stress/strain model inside the engineering design system called CATIA. A manual lo
ok that gives an idea of what this is doing can be found in simplified version at this link <<< Tension and Compression in Connecting Rods >>>.

Using a high strength bolt in an old used rod is mostly a waste of money. The factory bolts are suitable for a grocery getter and a bit extra. The extra comes from over design for load limit strength that is used to get life cycle fatigue resistance. So the part is designed to operate for so many cycles before failure. The design also includes a fudge factor for production variation in part and material quality. So the first problem with a rebuild is knowing how far into its design fatigue life is the part? There is no way of knowing this, so the best choice for anything other than you grandmother's grocery getter is to use a new rod, not suped up old ones even if recondititoned, which is often done incorrecctly anyway. For a moderate pwerformance build you can find new rods made from 4130 or 5140 steel for only a little more than a reconditioned rod. While 4130/5140 generally isn't considered racer stuff like 4340, none-the-less 4130/5140 is a quantum leap in material quality over the factory's typical low carbon, non-alloy steel of 10xx something.

Funny thing about bolt sizes, the manufacturers and sellers of these things discovered that between a lighter 3/8ths bolt made from super materials and a 7/16ths bolt made from a lesser material but both having the same ultimate strength, that the average purchaser of bolts goes with the larger rather than the weight savings of the equally strong but smaller diameter bolt. (A human nature thing) For the professional competition engine builder; weight hanging on the crankshaft is of extreme importance so the smaller bolt made from unobtainium would be the choice over the larger diameter bolt having the same strength but made from a lower grade material.

Bogie
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Old 04-23-2013, 12:59 PM
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what about the oem powder rods, they sell them new mighty cheap..
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Old 04-25-2013, 05:50 AM
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Originally Posted by oldbogie View Post
The factories use the Elfini finite element stress/strain model inside the engineering design system called CATIA. A manual lo
ok that gives an idea of what this is doing can be found in simplified version at this link <<< Tension and Compression in Connecting Rods >>>.

Using a high strength bolt in an old used rod is mostly a waste of money. The factory bolts are suitable for a grocery getter and a bit extra. The extra comes from over design for load limit strength that is used to get life cycle fatigue resistance. So the part is designed to operate for so many cycles before failure. The design also includes a fudge factor for production variation in part and material quality. So the first problem with a rebuild is knowing how far into its design fatigue life is the part? There is no way of knowing this, so the best choice for anything other than you grandmother's grocery getter is to use a new rod, not suped up old ones even if recondititoned, which is often done incorrecctly anyway. For a moderate pwerformance build you can find new rods made from 4130 or 5140 steel for only a little more than a reconditioned rod. While 4130/5140 generally isn't considered racer stuff like 4340, none-the-less 4130/5140 is a quantum leap in material quality over the factory's typical low carbon, non-alloy steel of 10xx something.

Funny thing about bolt sizes, the manufacturers and sellers of these things discovered that between a lighter 3/8ths bolt made from super materials and a 7/16ths bolt made from a lesser material but both having the same ultimate strength, that the average purchaser of bolts goes with the larger rather than the weight savings of the equally strong but smaller diameter bolt. (A human nature thing) For the professional competition engine builder; weight hanging on the crankshaft is of extreme importance so the smaller bolt made from unobtainium would be the choice over the larger diameter bolt having the same strength but made from a lower grade material.

Bogie
The thing is the rods are the heavy truck type and they're low mile. Engine was pulled due to an electrical problem that apparently wasn't diagnosed as the problem but made the engine skip and studder. Although the oil was left in passed several schedualed oil changes, guides and valves are near perfect as are the cylinderwalls. I'm not building a dragster and will not rev passed my own redline of 5500 . This said I believe the motor will produce near 500 hp with the parts I'm going to install so most everything has got to be reasonably strong when new. The rods are perfectly sized and have not been apart till I split them. All rod bolts are equal length(none stretched). The block is a high performance/ pass(on front of block) and 4 bolt. If I wasn't so strapped for dough I'd buy the eagle rods and install the new pistons(10 to 1) but I gotta try to use what ever I got that'll make it. I'll tell you, I had a mild 305 in the car that had stock rods and I pulled 6 grand with it on a regular basis,the rods bent a little but the stock rod bolts held the big end quite well,only evidence of the high rpm bending was the uneven bearing wear on the upper bearing shell. I don't recommend it but stock bolts,if they're not stretched, are pretty strong,perhaps enough even for a mild upgrade like mine. But this is a big block with considerably heavier pieces swinging around the crank. I hate to remachine a big end when I got a perfect factory machined big end right now to work with. I'm also such a miser that I hate not using near perfect parts.
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Old 04-25-2013, 09:40 AM
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Quote:
Originally Posted by yankee phil View Post
...All rod bolts are equal length(none stretched).
The original bolts were torqued, they are stretched. Not that you couldn't reuse them, that's your call.
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Old 04-25-2013, 01:25 PM
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The original bolts were torqued, they are stretched. Not that you couldn't reuse them, that's your call.
The bolts have an elasticity and return to their untorqued length when loosened,although some GM machinery here in europe, opel for instance, use head bolts that can only be stretched once as they loose their elasticity with the first stretch. Some other german brands also require new bolts after one torque stretching, but most quality built engines, Mercedes, BMW, use bolts that can be checked for elasticity and stretch when they will be reused. Over torqueing of course will kill the bolt but a factory torqued bolt should be able to be retorqued (correctly) no problem.
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Old 05-06-2013, 03:07 AM
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Originally Posted by oldbogie View Post
The factories use the Elfini finite element stress/strain model inside the engineering design system called CATIA. A manual lo
ok that gives an idea of what this is doing can be found in simplified version at this link <<< Tension and Compression in Connecting Rods >>>.

Using a high strength bolt in an old used rod is mostly a waste of money. The factory bolts are suitable for a grocery getter and a bit extra. The extra comes from over design for load limit strength that is used to get life cycle fatigue resistance. So the part is designed to operate for so many cycles before failure. The design also includes a fudge factor for production variation in part and material quality. So the first problem with a rebuild is knowing how far into its design fatigue life is the part? There is no way of knowing this, so the best choice for anything other than you grandmother's grocery getter is to use a new rod, not suped up old ones even if recondititoned, which is often done incorrecctly anyway. For a moderate pwerformance build you can find new rods made from 4130 or 5140 steel for only a little more than a reconditioned rod. While 4130/5140 generally isn't considered racer stuff like 4340, none-the-less 4130/5140 is a quantum leap in material quality over the factory's typical low carbon, non-alloy steel of 10xx something.

Funny thing about bolt sizes, the manufacturers and sellers of these things discovered that between a lighter 3/8ths bolt made from super materials and a 7/16ths bolt made from a lesser material but both having the same ultimate strength, that the average purchaser of bolts goes with the larger rather than the weight savings of the equally strong but smaller diameter bolt. (A human nature thing) For the professional competition engine builder; weight hanging on the crankshaft is of extreme importance so the smaller bolt made from unobtainium would be the choice over the larger diameter bolt having the same strength but made from a lower grade material.

Bogie
Your right,the cost of resizeing a rod over here and ARP bolts installed along with the small end honed for a free floating wrist pin is about 20 euro's cheaper than a new eagle I-beam rod. I gotta go with the new rods . Thanks for the info and the link to the technical site on clamping pressures vs internal stress. I was always amazed at how the aluminum rods must compress to withstand the pressures of supercharging yet the bolts don't shear,makes me thing the actual load of the power stroke is carried mostly by the upper bearing surface of the rod and the upper rod body although I haven't studied fuel motors when they're torn down and only partially broken/bent where you can really see the internal forces and their effect on the parts before they actually break and explode the motor.
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Old 05-06-2013, 06:24 PM
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Your right,the cost of resizeing a rod over here and ARP bolts installed along with the small end honed for a free floating wrist pin is about 20 euro's cheaper than a new eagle I-beam rod. I gotta go with the new rods . Thanks for the info and the link to the technical site on clamping pressures vs internal stress. I was always amazed at how the aluminum rods must compress to withstand the pressures of supercharging yet the bolts don't shear,makes me thing the actual load of the power stroke is carried mostly by the upper bearing surface of the rod and the upper rod body although I haven't studied fuel motors when they're torn down and only partially broken/bent where you can really see the internal forces and their effect on the parts before they actually break and explode the motor.
I think that new rods is a good choice. There's just too much unknown about old rods, as you've seen Eagle and Scat have many replacement choices. The GM PM rod is overall pretty good but it seems to me that the Vortec truck has seen more rod failures with these than the older forging. But that's a feeling not a measurement.

Bogie
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Old 05-06-2013, 09:11 PM
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thot top fuel buzzed em up in the 8000 range.....just sayin
My buddy sits his clutch weight to turn 8100 at the hit on his T/f dragster and he turns 400 to 500 less than everyone else.
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Old 05-06-2013, 09:34 PM
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Please help me to understand. Am I missing the point? I thought rod bolt failure happened in tension. This only happens on the intake stroke (it seems). So, the acceleration at about 30 - 45 degrees ATDC rotation, plus the delta-p on the piston area would seem to be it. This leads me to believe that rod weight, piston weight (both linear functions) and RPM (an exponential function) are the primary drivers of bolt failure. I repeat, am I missing something? (Help me Bogie)

Thanks for any input.

PatM
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