Rod and main bolts-replace?

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Captainkirk

Captainkirk

Old School Mopar Warrior
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There are two schools of thought out there. One says ALWAYS replace your main and rod bolts with new during a teardown. Seems obvious. The other, less-often heard school says if they look OK, they're already pre-stretched and have proven their integrity by not failing. Most bolt failures occur in fresh engines with new (read; defective) bolts. Any thoughts on this from the racing crowd? (who tear motors down quite frequently)
 
If I was using said engine in a drag race forum, then I would have measured the rod bolt on first assembly and on disassembly. There is a given amount of acceptable stetch by manufacturers before replacement becomes necessary. If you don't know your measurements then I suppose one would have to weigh the factors like, bolt size, piston/rod weights, RPM used and number of passes, and then your best educated guess. I know of regular racers that run a good two years on the same rod bolts with no problems. Ask racers. Just remember that the rods should be resized with new rod bolts.
Replacing main bolts. I'd be using studs. And I would think they'd last longer. Same thing for the main bores. They'd have to be checked for proper alignment after replacing studs.
 
Heres a thought that'll help square things away for you.

How serious are you about your engine and it's life?
(Stock or otherwise)

(Thinking outloud again here,; Price of rebuild vs price of new hardware.)
 
"The other, less-often heard school says if they look OK, they're already pre-stretched and have proven their integrity by not failing."

I would say this is misleading. A bolt that is over-stretched, or whose material is not adequate for the intended use is not visibly bad..yet both will fail if asked to perform beyond their design. A torque wrench used on a bolt that has been over stretched will still reach it's spec in many cases. But it does not have the ability to maintain clamping force.

" Most bolt failures occur in fresh engines with new (read; defective) bolts."
It is extremely rare to have a fastener from a reputable company be defective. It is MUCH more common to have the bolt nicked, or not installed properly, or get torqued improperly causing premature failure. I would venture to say it's usually assembler error that leads to failures attributed to bolts. Not the bolts themselves.

I am one of those who replaces all rod bolts during every rebuild. I also install main studs or ARP main bolts in every engine like Len does. All mine get line honed after too. A freshening is not a rebuild. I do reuse them when I know the history, and they come apart looking decent.
 
I'm inclined to agree with you all on this one; in fact, I've already ordered new rod bolts. I'm not one to take catastrophic failure chances when I can avoid them.
Wasn't too sure about the main bolts; but if I follow your line of thinking it would probably be money well-spent. Thanks!
 
I'd do the rod bolts as a minimum. If the motor sees street usage the mains they should be fine. If you are concerned get them zyglowed or magnifluxed.
 
FWIW, my first job out of college was developing torquing specs for the assembly of jet engines. This assignment taught me a lot about threaded fasteners. Here are a few things to keep in mind;

1. All fasterners have some amount of elasticity and is refered to as modulus of elasticity. As you tighten a bolt it stretches. If you do not exceed the yield strength of the bolt it will return to it's orignal length when the torque is released.

2. A properly designed and tightened threaded fastener will apply more clamping force than there is force to seperate the joint (combustion pressure, thermal expansion, dynamic forces such as the piston stopping and changing direction at BDC and TDC). What this means is the bolt does not stretch during normal use. And this is also why an over heated (abnormal conditions) engine quite often blows the head gasket.

3. There are two types of fastners used in automotive engines. Torque to yield and non torque to yield. Torque to yield has come into favor because using a torque wrench to tighten a bolt is only slightly more accurate than doing it by feel. The point at which the metal yields in a fastener is very consistent and once that happens you can no longer increase the clamping force. This makes for very even and consistent clamping force across a bolted surface such as a head. This is much more important in a clamped joint made from dissimilar materials such as aluminum heads and iron blocks.

3. A torque to yield bolt will always have a shank that is slightly smaller than the root diameter of the threaded section so the yield will occur in the shank and not in the threads. This also contributes to consistency in the clamping force between fasteners.

4. Torque to yield bolts are a one time use item. The reason being is that each time they are tigthened the bolt stretches and takes a permanent set which also results in a reduction is diameter of the bolt. Even though the material on subsequent tighteneing cycles has the same strength in pounds per square inches there is less square inches so you have less pounds of clamping force.

5. A non torque to yield bolt operates below the yield point so it never stretches and takes a permanent set so it can always provide the same clamping force. As stated in #2 a properly tightened joint there is no motion so there will not be any fatiguing of the fastener over time. So in theory there is no reason to change a bolt that has not been over tightened, over heated or otherwise subjected to anything that can compromise the characteristics of the bolt. The question is has anything happened to the fastener in it's life time?

6. Another consideration; High quality fasteners from outfits like ARP will roll form the threads verse cutting them on a less costly fastener. Disregarding the superior material these fasteners are made from the rolled thread has less friction associated with it so you will receive higher and more consistent clamping force with the same torque. If you use the lubricant they provide you get even higher and more consistent clamping force which now takes advantage of the superior material. Down side of this is that the clamping forces increase to a point that it starts to distort the parts being fastened. That is why mains and rods need to be resized (or at least checked) when stepping up to these types of fasteners.
 
66dartgt said:
I'd do the rod bolts as a minimum. If the motor sees street usage the mains they should be fine. If you are concerned get them zyglowed or magnifluxed.
Click to expand...
This is what I do. I allways replace Rod bolts, and just check the main bolts for strech. I always use ARP. Just a side note to the 4 cylender guys, that SB engines use the same rod bolt as 2.2 and 2.5's.
Just my opinion, and so far no problems. (fingers crossed)
 
Damn, I thought I knew everything. :lol:

I learned something new about TTY bolts today, I guess you can teach an old dog new tricks.
 
dgc333 said:
If you do not exceed the yield strength of the bolt it will return to it's orignal length when the torque is released.

A properly designed and tightened threaded fastener will apply more clamping force than there is force to seperate the joint (combustion pressure, thermal expansion, dynamic forces such as the piston stopping and changing direction at BDC and TDC).


The question is has anything happened to the fastener in it's life time?
Click to expand...


So working backwards...If you dont know the history, use the right bolt for the job, and dont overtighten it. :)

Thanks Dave!
 
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