Connecting Rod Bolts

[OIL BURN]

On A 6.2, Is Is Neccessary To Replace The Rod Bolts And Nuts After Four /five Cycles Of Disassemble&assemble. Past Rebuilds And Plastigauging Crank Changes/bearing Replacements Were The Reasons. Do These Bolts Fatigue From Retorquering And Stretch,can Red Locktite Be Used To Positively Secure The Nuts? Where Can Replacements Be Bought? Does G.m. Or Arp Stock These? Have Searched And Cannot Find Any Information. Any Help Or Advice Would Be Appreciated. Thanks , Oil Burn, New Member.

[john8662]

During a rebuild, where the rod is resized they probably should be replaced, but it's not a requirement.

I'm sure the bolts stretch, although I cannot think of anywhere to find a specification of what the stretch is and what is allowable on these rod bolts.

You'll want to only use engine oil for the torque down, to get the proper torque, so I don't know what affect you'd have on the torque with the loctite (you'd get less actual torque, not sure how much less).

ARP doesn't have a listing for the 6.2 or 6.5, GM has the bolts, as does Pioneer. Just doing a few sets the bolts aren't cheap.

John

[Burbinator]

Mmm...after all the engines I've built and rebuilt, one of the things I'd never do is re-use a set of con rod bolts. You have to consider that not only does a fastener stretch when it is tightened, it also undergoes torsion, which simply put is twisting. And because of torsion, you have what is known as shearing.

To put that into simple terms, when you tighten a bolt it stretches and twists like a piece of licorice and then it relaxes, which is why you should always torque a fastener in increments up to the specified amount. As it relaxes, the applied amount of torque lessens--the fastener becomes lose and you have to tighten it again. Plus, in addition to these factors the torque and shear effects produce heat, which also causes the fastener to spring back or slip in the threads. This is why you see the bent metal lock tabs on exhaust manifold bolts, and even on con rods in older heavy duty motors (gas and diesel). Now imagine the 20+ years of heating, cooling, re-heating, torsion and shear effects those con rod bolts have gone through. Suddenly, re-using any fastener in a critical area becomes an obvious no-no, don't it? :D

This stretching and relaxing of metal is what is known as 'work hardening'. As an extreme example, if you take an old metal coat hanger and bend one spot back and forth, the bend will heat, deform, become brittle, and break. That's what is happening when you re-use a con rod bolt, and that's usually the reason why con rod bolts break; they fail due to shearing stress.

Now you need to consider that a 6.2 N/A diesel has a healthy compression ratio. And you also need to consider the fact that ignition of the fuel/air mixture during the combustion cycle is going to translate that power conducted through linear motion into rotary motion and power through the wrist pin and con rods, which are attached to the crank. Those rods, which are held to the crank by the rod caps and specifically, the con rod bolts, are undergoing an incredible amount of g's during rotation, which translates into stretching and bending of the rod bolts as the crankshaft makes a revolution.

So really, when you look at the factors or torsion, shearing stress, stretch, and work hardening, you should never reuse con rod bolts during an engine rebuild. Ever.

On the same note, the most accurate method by which to ensure that the con rod bolts are torqued correctly is by measuring bolt stretch as you torque the fastener--which requires the cap and tip to be machined flat for precision measurement with what's called a 'stretch gauge'.

IIRC, the con rod torque spec for the 6.2 is 48 lbs ft, which isn't too far away from a 305/350 SBC (45 lbs ft). The SBC rod bolt stretch using fastener lube (some folks use clean 30 wt oil, others use Marvel Mystery Oil, I like ARP Ultra Torque when using moly fasteners) is about .006". So in theory only, that's about the stretch you'll want when you tighten the rod bolts on your diesel. However, keep in mind that's a theoretical estimate and not a fact; I wouldn't use that because the material the bolt is made from, the diameter, and pitch all play a role in defining the final torque spec. I'm just throwing that out there so you will get an idea of what's going on.

Anyway, I'm pretty sure ARP makes con rod bolts for the 6.2/6.5 and you can grab their latest catalog in pdf format here (http://video.arp-bolts.com/catalog/ARPCatalog.pdf). There's a lot of really good info in the catalog that goes into more detail about what I explained, so if you want go and grab it. Plus, you can get in touch with ARP and get the rod bolts you need for your rebuild--if they don't have it, they can make it.

If you don't want to go that far and spend money on something like a set of moly con rod bolts, you can always motor by the local Chevy parts department and have them look up the parts and numbers for you. Keep in mind that the stockers might be as expensive as a set of moly rod bolts or more because you're getting it from the dealer. But if you get in good with the local machine shop where the block, heads, rods and crank are being worked you might be able to get a discount. While you're at the dealer (or the shop) ask what the stretch should be on the stock bolts--either the dealer or the machine shop should be able to tell you. Just keep in mind what I mentioned about having to machine the rod bolt flat on both ends to use the stretch gauge correctly.

When I build my spare 6.2 I have sitting in the corner of my shop, you can bet that's what I'm gonna do about the rod bolts, and I'll be studding both the bottom and heads, and bracing the bottom end as well. There's nothing worse than investing in a quality engine rebuild only to have it crap a rod or two on you because the rod bolts busted the first time you put a decent load on the motor. And by the way, the studs for the mains and heads aren't necessarily a race only mod that is a waste of money on a street motor. Studding those areas reduces crank flex on the bottom end while reducing the load on the main bearing saddles and webs (that's a problem with the older 6.2's), which lets the bottom end live a long, long time under adverse conditions. Up top, the studs distribute the toque load more evenly and uniformly, which lays the groundwork for uniform pressure throughout the cylinders on each bank--not to mention you really gotta do something wrong to pop a head gasket. And with the compression ratio a 6.2 has, that's not a bad way to go. Granted, most would call this overkill, but when I build one I want to give it every chance I can to let it last forever.