re-useing most low performance OEM connecting rods



re-useing most low performance OEM connecting rods

Postby grumpyvette » September 17th, 2008, 5:56 pm

should you re-use those stock rods,...

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I get asked all the time,
"should you re-use those stock rods, when I rebuild my 350 or when I build my 383 stroker"
most sbc gen I stock rods are designed to be cheap, and dependable in engines spinning under 6000rpm that make under 400hp,
this is one area where Im simply amused at the lack of thought shown in sellecting components, by some guys.
most chevy small block rods are VASTLY inferior in strength to many of the mid range and better aftermarket rods available.
a 7/16" cap screw type ARP rod bolt is EASILY 200%-300% stronger than a stock 3/8" factory rod bolt and frankly, the cost & TIME to correctly modify and prep stock rods is a total waste, its almost always cheaper to buy decent aftermarket rods.

example

http://www.sdparts.com/product/12495...ctingRods.aspx
$265 for a set of stock rods and then you should still have ARP bolts installed, pollish, ballance and sized your looking at easily $500-$600 or more for a set ready to run

compared to something like this below its a joke

http://store.summitracing.com/partde...1&autoview=sku

http://store.summitracing.com/partde...6&autoview=sku


keep in mind theres far stronger rods available if you have some extra cash, and that connecting rods and thier rod bolts are under a huge amount of stress at high rpms....one rod bolt stretching at high rpm will usually result in engine failure and its comon for only the intake, valve covers, distrib, and water pump and a few other parts to be salvagable if that were to happen at high rpms...stretch a rod bolt and the piston contacts the head, or bends a valve, the rod bends, the heads destroyed, the block can be history and it can go down hill rapidly from there as fragments work thier way around thru other of the moving parts as the engine locks up
IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!
grumpyvette

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Re: re-useing most low performance OEM connecting rods

Postby grumpyvette » September 17th, 2008, 6:09 pm

rod bolt stress vs rpm



many guys don,t realize that the rod bolt material and cross sectional area are critical to durrability , especially in a high rpm range combo,while the rods themselfs ocassionally fail, its much more likely that the rod bolts lost thier clamping strength, stretched a bit first and that was a major contributing factor in the bearing failure or the rod failure process.



interesting info from ARP
ImageImageOther Stresses

It must be realized that the direct reciprocating load is not the only source of stresses in bolts. A secondary effect arises because of the flexibility of the journal end of the connecting rod. The reciprocating load causes bending deformation of the bolted joint (yes, even steel deforms under load). This deformation causes bending stresses in the bolt as well as in the rod itself. These bending stresses fluctuate from zero to their maximum level during each revolution of the crankshaft.

Fastener Load

The first step in the process of designing a connecting rod bolt is to determine the load that it must carry. This is accomplished by calculating the dynamic force caused by the oscillating piston and connecting rod. This force is determined from the classical concept that force equals mass times acceleration. The mass includes the mass of the piston plus a portion of the mass of the rod. This mass undergoes oscillating motion as the crankshaft rotates. The resulting acceleration, which is at its maximum value when the piston is at top dead center and bottom dead center, is proportional to the stroke and the square of the engine speed. The oscillating force is sometimes called the reciprocating weight. Its numerical value is proportional to:
It is seen that the design load, the reciprocating weight, depends on the square of the RPM speed. This means that if the speed is doubled, for example, the design load is increased by a factor of 4. This relationship is shown graphically below for one particular rod and piston


http://www.arp-bolts.com/Tech/TechWhy.html
This chart confuses me and (for me) indicates the smaller the bolt cross section, the higher the tensile strength?).


I did a quick DOUBLE TAKE on that graph the first time also....look closer at the edges of the graph, its points out the STRONGER the material USED the SMALLER the dia. necessary for a given tensile strength, your limited in clearance on rod bolt max size so the material needs to have higher yeild strength, and potential durrability, to increase the rod bolt strength

FROM ARP

"Metallurgy for the Non-Engineer

By Russell Sherman, PE

1. What is grain size and how important is it?

Metals freeze from the liquid state during melting from many origins (called allotropic) and each one of these origins grows until it bumps into another during freezing. Each of these is a grain and in castings, they are fairly large. Grains can be refined (made smaller); therefore, many more of them can occupy the same space, by first cold working and then by recrystallizing at high temperature. Alloy steels, like chrome moly, do not need any cold work; to do this – reheat treatment will refine the grain size. But austenitic steels and aluminum require cold work first. Grain size is very important for mechanical properties. High temperature creep properties are enhanced by large grains but good toughness and fatigue require fine grain size-the finer the better. (High temp creep occurs at elevated temperature and depending on material and load could be as much as .001 per inch/per hour.) All ARP bolts and studs are fine grain – usually ASTM 8 or finer. With 10 being the finest.

2. How do you get toughness vs. brittleness?

With steels, as the strength goes up, the toughness decreases. At too high a strength, the metal tends to be brittle. And threads accentuate the brittleness. A tool steel which can be heat-treated to 350,000 psi, would be a disaster as a bolt because of the threads."

http://www.arp-bolts.com/Tech/TechMetals.html
are all rod stretch gauges created equal "

obviously no more than all girls are equally good looking
but most of the gauges are functional, some just have more features or more precise calibrations, some are adjustable in length ,over a wider range, some have digital read outs, ETC.
Image

http://store.summitracing.com/partdetai ... toview=sku

http://www.carcraft.com/techarticles/11 ... index.html

http://www.chevyhiperformance.com/techa ... index.html
IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!
grumpyvette

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