I inadvertently over torqued my con rod bolts, looking for advice or suggestions. 1. Rod bolts are 3/8 dia by approximately 2 inches long. 2. I torqued to 46 ft lbs, should’ve been torqued to 35 ft lbs. Took value from wrong page in manual 46 ft lbs is for a different engine OOPs. 3. Clearances etc. checked out ok with rods torqued to 46 ft lbs. My questions are : 1. Did I damage the bolts , can I still use them? 2. If I replace the bolts - ARP bolts are a little over half the price of stock replacement bolts (pricey), my understanding is if ARP bolts are used the rods should be resized, is this correct?. Again with stock bolts torqued to 46 ft lbs the big ends didn’t seem to have changed much if any dimension wise. Thanks Dan
Torque values are calculated as a percentage of the yield strength of the bolt. Yield strength is the stress value above which the bolt permanently deforms. Usually torque values are somewhere in the range of 65-85% of yield. You overtorqued the bolts by 31%. I can't predict whether that resulted in a stress on the bolts above the yield strength or not, but it may or may not be. You can risk it and run them, or you can replace them. Chances are it may be fine, but nobody can predict that with complete certainty.
I would either leave them as is, which will probably be fine, or replace them. Don’t loosen them and retorque as they could be stretched a fraction. Possibly.
Well, a 'standard' torque chart says a grade 8 3/8" NF bolt is good for up to 49 ft-lbs, I can't imagine that those would be less than grade 8, so I'd say re-torque 'em and call it good.
I would not put a lot of worry into it, when was your torque wrench calibrated? Is it dead on and there is a +- on them.
Answering your questions- 1. It would depend on the age of the bolts, and how long they have been run. Although most likely not stretched, the easiest way to confirm would be to pull a couple of caps and mic the shank diameter comparing to specified dimensions. 2. Replacing the bolts (ARP) would require the rod ends to be resized.
Just curious. What year, make and model engine are you working on? Is it a Ford flathead V8? QUESTION? Why must rod ends be resized when using ARP bolts?
ARP bolts are designed to be tightened to a level that results in the proper stretch to achieve more clamping force than OEM bolts. This increased torque can make the big ends of the rod out-of-round, which will reduce oil clearance and possibly be disastrous. Resizing the big ends restores the clearance by making them round and to proper size under the increased torque. HOWEVER...checking the big ends for size and round after installing and properly tightening the ARP bolts may or may not show the need for resizing...but measuring is the only way to know for sure. Terry
chicken: Thanks for explaining ARP bolt use. Instead of using ARP bolts would it be ok for the original "poster" to buy some new OEM connecting rod bolts and use them?
Sure, in fact...the overtorqued bolts may still be fine. If they tightened up nice and solid without a stretchy feeling (technical mechanic talk there ) I'd use them. First thing would be to loosen them and turn them with fingers to make sure they turn freely. If so, that's a good sign. I'd sure like to know what engine this is.....probably not a Ford Flathead since the bolts are part of the rods on those.
Thanks all. The engine is for OT car a 64 Austin Healey Sprite 4 banger. The bolts turn by finger fine I don’t have any specs for shank diameter I loosened and torqued to 35 ft lbs I don’t feel any stretchy springyness (new technical word) I am a cheap skate and really want to get this engine running BUT I think I follow my own advice “spend the money necessary and have patience” I think I’ll purchase the ARP bolts and resize if I have to. The original bolts may be fine and I would’ve used if I didn’t dumb ass the torque. If a con rod bolt were to fail it would probably trash the engine that I have a fair amount of money invested into. Thanks All for helping me sort this out in my pea brain. Dan
Has anybody else ever wondered why rods with nuts and bolts need to be resized, and rods with capscrews just put the new bolts in? Just asking...
Bolts are a tight press fit. New tight bolts can distort the rod via the installation process. I've never measured it so I can't say how significant this is.
Dan, during mock-up and blueprinting an engine, a machine shop may torque and re-torque rods and mains more than once. IE: when you are checking bearing bore vs. shaft dia. to figure oil clearance (rods or mains). You can torque rods up, with bearings, to measure side clearance. They start out a little "football" shaped to account for stretch. There usually is a spec for this. You could check yours vs. the crank pin for oil clearance and side clearance. If they make it, run them. 3/8" is a generous bolt in something the size of an A-series. If you install new bolts and have to have them re-sized, they are going to torque and re-torque at least a couple times to do it. It's fair game. Mike
I contacted ARP after a similar question I asked here awhile back, my specific question (mains) was would the bolts hold properly at factory spec vs ARP spec. “No problem” they said. Which was in line with some of the responses to my thread. But since I also had a set of rod bolts also, I asked about why I’d need to have the rod ends resized. If memory serves, I was told just installing the ARP rod bolts could distort the rods. I didn’t ask specifics, but decided to just use what was in there.
Normally when the rod bolts are tapped in with a mallet , the shank [sometimes knurled or wavy] will swell the rods and increase bearing crush in the centre. You can sometimes install them over a crank journal with some bearing shells in place . Torqued both bolts up , then loosen one side and knock it out! Then knock in a new bolt and torque that up. [then repeat on the other bolt] I've always checked with plasti-gauge after and have gotten away with this method on numerous occasions [usually overhauls] On my Race engine BBC with ARP Wavelocks the bolts swelled the rod quite a bit [I always get the rods resized on and expensive engine]
The engine was bored 0.020 over, decked for piston 0.004 below deck, head was milled , valve job ,new springs, Isky style lifters Cam reground to a 274 duration by Schnieder all new bearings ARP main studs header The engine was not balanced and the crank was not touched the pins and main journals are well within specs for roundness and size ,crank pin oil clearance is at 0.0015 to 0.0017 and the mains are at 0.0015 using plastic gauge the surfaces looked beautiful upon disassembly and the bearings showed very minimum wear - could’ve reused if necessary
I guess that is my real dilemma I don’t know if I if torqued beyond the yield strength I saw that torque rating charts for 3/8 fine thread grade 8 bolts at 49 ft lbs but I believe that was for dry threads. I also believe that SAE fine 3/8 bolts may by stronger than the British Standard Fine 3/8 bolts as the SAE fine bolts have more threads per inch.
Well I tend to be a bit anal about doing things. If it were me I would just replace the bolts seeing they are cap screws and are easy to change out. Sounds as if you spent some money on the engine build to gain some performance so why chance a rod failure that could destroy the engine. The price of new bolts is minimal for the piece of mind while doing some spirted driving. If you did over torque by 31% you could have yielded the bolts and changing the rod bolts is one less thing to worry about.
Hold the phone. Since this is an English engine, are we talking about SAE threads or a British size? If the are British, their 3/8” may not be the same size or torque spec as SAE. Just a thought.
I would seriously consider replacing them. The BSF is Finer thread than SAE fine [UNF?] Usually finer threads have less Torque "resistance" for the same given bolt stretch. You over torqued them with a Torque wrench measurement, not by Rod bolt stretch so you are risking stretching them beyond yield. [If they were UNC it would be a non issue] A good way to explain this, is to get a 3/8" UNC and a 3/8" UNF bolt and torque them both to the same value [eg: 40 Ft/lbs] BUT also measure the degrees of rotation. The finer thread will rotate more for the same stretch [which is OK] BUT there is also less resistance so the finer thread will stretch more until the torque wrench measures this torsional resistance. Being an Austin Healey Sprite, I assume this is an "A" engine!! Enthusiasts tend to rev these engines into the stratosphere. Replace the bolts , then you can sleep better. It was a "Formula 1000" that We designed and built. [we being me and my wife] I even styled the body , designed the suspension, brakes etc. Powered by a snowmobile engine with a CVT They were used for a driving school That is my wife Mimi posing beside it
Cool car Kerrynzl, yes the bolts are British standard fine I couldn’t find a torque chart for the British threads. I’m going to replace with ARP cap screws and check for distortion on the big end, really strange that the ARP bolts are $90 and the OE bolts are $160 + even if I have to resize the rods I’ll be close the cost of OE bolts alone, a no brained even for me. Thanks you everyone for your input I learned a couple things in the process.
Just a technicality, but all bolts stretch when tightened. Unless they are torque to yield, that stretch is in the elastic range. No permanent stretch, the bolt returns back to original length when loosened.
Just a thought, how accurate is your torque wrench? If it's been used/abused for a while, and not been checked, the tension might be within the specs..
Not absolutely certain on my torque wrench accuracy been a couple years since it was calibrated although I do check against a beam type torque wrench ,they read within a lb or two of each other could be they’re both off. All bolts stretch but when yield is reached they don’t return to their original length My problem is I don’t know the original length or original diameter so can’t use those dimensions as a gage and for sure I was at or maybe above max torque, just too close for me to sleep well at night ha ha.
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