When replacing stock rod bolts with the ARP rod bolts, can ANYONE explain to me why the "Big End" of the Rod MUST be "machined"? I've gotten the same answer from 3 or 4 different engine builders, but no one can EXPLAIN "why" it is NECESSARY!o No: Is the bolt a different size? Does the ARP bolt fit differently than a stock rod bolt? What does it actually affect, other than just a torque adjustment? What part of the Rod "HAS" to be MACHINED for the BOLT to fit correctly? It just doesn't make any sense to me or other "engine builders" that I've discussed it with. What information are we "missing" about ARP bolts? o No:
Basically, the rod bolts are pressed into the rods, they do not just fit loosely, It is not a huge press, but a press none the less. The ARP's tend to have a tighter press than the stock bolts, so when you change the bolts, you can change the roundness of the rod by just a little bit, we are not talking a huge amount here, but round is round and even fraction of a thousandth is out of spec. The rod bolts also locate the cap to the rod, so any chnge in bolt here can also slightly change the cap alignment . One other note, when you use ARP bolts, you need to chamfer the area under the head of the bolt as ARP uses a radius under the head whereas the stock bolts are pretty much a 90 degree configuration. Jim Burek
So, will that require "oversize" bearings, also, then? Or is it possible to use the "standard" bearings (that I've already bought!)?o No: The reason that I ask is that I have NOT bought the ARP bolts, YET, if I can buy "stock" replacement rod bolts without having to do ANY "modifications", if possible. I'm only building a "street" engine, so I don't feel I really NEED "ARP" everything. I'm trying to do a "budget" rebuild (not "cheap", but "practical"!:Smarty: ), and I don't want to spend twice as much if I don't have to!:TU:
As was said, the bolts themselves are what locates the cap on the main section of the rod. Change the bolts, you may change the position of the cap,and that's not happy when you're a rod bearing with thousanths of an inch clearance to begin with. Re-sizing rods uses the same process as align-boring/honing. Mill the cap to make the hole smaller, then hone out to exact blueprint size. The OUTSIDE of the bearing is standard size. Overall center to center length changes a minute amount, though, just like align boring will change the distance from cam to crank centerlines a tiny amount. By the way, it does not matter WHAT new bolts you shove into a rod...you need to resize the rod afterward. This is more complex on modern car engines that use "cracked cap" technology. This is a process that cuts manufacturing cost of the rod by a huge amount. With a "modern" cracked cap rods, the rod is forged in one piece, bolt holes drilled, then the cap part is BROKEN off. When re-assembled, the broken cap exactly fits the broken main section, and so ensures a perfect fit. Since you can't shave the cap to make it smaller, you can't re-size the rod to "new" specs. On these rods, you hone the bearing hole diameter oversize and then buy oversize OD bearings. See how complex this gets? You need bearings in special OD sizes to fit the re-machined rods, and special ID sizes to fit the re-machined crankshaft journal. This is progress? By the way, the first time I came across "modern state-of-the-art" "cracked cap" technology, it was in a 1959 chain saw engine. I thought it was a stupid thing then, too.
Go ahead and spend the insurance $ on the ARP's, Scott. Money WELL-SPENT! I threw a rod at about 7200 rpm's. The ONLY pieces of metal that were still GOOD-AS-NEW anywhere NEAR THAT LOCATION were the two ARP rod bolts from that rod!:eek2: :jd: I also KNOW that they had NOTHING to do with the demise of the rod! The throttle stuck wide open, and detonation TORE the piston to pieces, leaving the rod NO CHOICE!:ball: :ball: Like everyone already said, ANY new bolts will REQUIRE the rods to be re-sized! Don't take ANY short-cuts!:Smarty: My .02
