Crankshaft stud design

I'm curious about the design of these studs. I got a set with an engine overhaul kit from Norvil and proceded to overtighten them. Removed the studs and took them to work and had the hardness and chemistry checked. Rockwell C 20-22, plain carbon steel. These are definitely not high strength fasteners, yield strength around 85000 psi. On the little wallet card some of the guys carry around it shows the recommended torque on a lubricated 5/16 inch, fine threaded, high strength bolt (130000 psi yield strength) to be 18 ft-lb. This is to achieve a stress in the bolt of 85% of yield.
So how can the correct torque on the not-high-strength crankshaft studs be 25-30 ft-lb? Granted, lots of motors have been built and run successfully using this number but it just doesn't make sense.

Littlefield,

Your 85,000 psi estimate is close to a Grade 5 bolt spec of 92,000 psi. A 5/16" Grade 5 bolt will easily handle 18 lb-ft torque dry. Additionally, an unplated bolt requires more torque for the same preload as a plated one. So if the Norvil fasteners is indeed equivalent to a Grade 5 fastener, it should be loaded to less than yield (80%?) at 18 lbs-ft torque with oil on the threads and faces.

However, material hardness is only a rough estimate of material yield strength. So the above is merely a hypothesis.

I probably wasn't clear in my original post. Based on normal practice I agree that 18 ft-lb is in the right range. The 25-30 ft-lb numbers that are recommended seem very high.

Ooops, my mistake; I stopped reading at 18 lbs-ft.

A grade 8 fastener could go 25 lbs-ft dry and still be loaded to 80% of yield. It might even make 30 lbs-ft without deforming plastically. However, I suspect a grade 5 fastener that is torqued to 30 lbs-ft would be permanently stretched. Interestingly, there are some applications where the fastener is torqued to yield, but I don't know why it's done this way.

One thing is for certain -- torque is a lousy way to preload a fastener. The error is +/-25%, so I guess fasteners are pretty forgiving at seemingly overload conditions!

Well, they do fasten by stretching. Torque is pretty inaccurate but at least it gives a ball park. My old John Deere manual said to tighten the head bolts very firm if I recall correctly.
A lot of vehicles like my Subaru now tighten by rotation on head gaskets and such to ensure stretching properly.
Sometimes there is a reason that folks used to overtorque a part. That reason may or may not be valid today. It may mean that a certain vibration period caused bolts to loosen from the proper torque and folks found out that adding a few pounds over kept it in place. I've seen this fix in Mercedes vibration dampers, VW flywheels, and Subaru vibration damper nuts that come to mind imeadiately.
My temptation would be to use studs from a good firm and install them the reccomended manner. If I was going to race it I might go with the toughest faster I could get from a fastener distributor.

I'm going to see if I can find an alternate source for something good. You can get Grade 8 bolts of a workable length at the hardware store but the bodies are too small to properly locate the pieces. I need to try a bolt in the 2 holes behind the cheek and see if it will even go in. Those 2 may have to be studs.

One thing is for certain -- torque is a lousy way to preload a fastener. The error is +/-25%, so I guess fasteners are pretty forgiving at seemingly overload conditions!

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Very true. The only accurate measurement is tension. Unfortunately most of us don't have the equipment to measure tension.

littlefield said:

I'm going to see if I can find an alternate source for something good.

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hi littlefield,steve maney supplies oversize high tensile fittings designed for racing,but the drawback is your crank will need drilling to accept them

I ended up getting some ARP fasteners for the crankshaft. AG2.700-5G. Good stuff.
The body's the right length but the threads are a little long and need trimming. One end is coarse, the other is fine. I lock tighted the coarse end and did the tightening on the fine end. The body is .310-.3105 so it could be a little bigger but not bad. They have 12 point nuts which are really nice. Much cheaper than the Andover pieces, also. Purchased from CV Products in NC.

I always wonder about "dry" torque values, when you typically use loctite on many of these fasteners.

Personally, I like "hand" pounds instead of foot-pounds.

I let the loc tight dry before tightening fully so the coarse end was locked during the final torquing.

I worked on Porsches for 30 years, I used standard torque settings and a Snap-on beam type torque wrench for many of those years. In the 90's they suggested useing a torque angle gauge. As the air cooled cars gained more displacement, and power, the heat generated went up, and cylinder hold down studs would break, these were long studs, about 9" long used to hold the cylinder and to head down much like a motorcycle engine. I would first titen the head nuts to 15 ft. lbs, let them set a bit then titen the an additional 90 degrees with the torque angle gauge, This seemed to cure most of the stud breakage. I have no idea what material they were made of as they were expensive from Poesche, or the after market. I always used a micrometer to check rod bolt stetch and never used rod bolts twice. Many engines made today have no torque settings just a torque angle settings for heads, main caps, and rods.

The studs in a Norton crankshaft should be replaced if you take the shaft apart as the studs will have been stretched on assembly. There is alot of stress at that joint as the crankshaft does flex. The picture of broken crank studs is not a pretty one.

Ken G.