According to the experts it's imperative to replace the rod bolts when rebuilding. I've read the reason is because when they are torqued originally they stretch? Are they manufactured specifically to do that? If so, why wouldn't that make them weak? If reused why is that any different from reusing other bolts or parts under high torque? What's the science and logic behind this?

Rod bolt are true "stretch bolts" and are properly torqued by measuring the stretch (not that anyone would do that normally).

To me, the backyard barbaric shade tree mechanic, reuse of the rod bolts for a street engine is OK. A race engine is a different case.

Hell, we used hi-strength aftermarket rod bolts on the race cars and reused them many times. Never lost a rod bolt. Had beam and bearing failures, but not the bolts.

Others opinions may vary.

At least replace the rod nuts, I wouldn't scrimp on that. I wouldn't scrimp on the bolts either, but I'm one of those "as long as" guys.

ARP is up in ventura. I went up there and bought one their stretch gauges and picked up a catalog, which I think is downloadable that answers your questions. My recollection is that when a bolt is torqued the tension initially ramps up rapidly, but then that rate fades as the bolt stretches. When the proper tension is attained, the bolt stretches, but is not deformed, and static tension is much higherr than cyclic variation the bolt would see when the engine is running. If the reverse were true, the cyclic loading would fatigue and break the bolt.

If you buy a set of their bolts, you'll be impressed with the quality - this ain't flaps hardware. No connection.

IN stock standard form the rod bolts..are internal shuffle pins.....AND stretch BOLTS . .try taking them out of the bottom cap. they are really in there. they just dont; fall out. Look closely at the rod bolt at the portion that is in the top (cap) of the rod. you will see some very minor (polishing) rub marks, indentations,...you know that part of the rod bolt is against (something) which is the inside of the top cap. the rod bolt is a bolt...and a internal shuffle pin . Bet you have never even seen a "914 porsche rod bolt failure....goes for 911 porsches also. (I am talking street cars) NOT RACE CARS.....ALWAYS replace the rod NUTS...rod bolts in a low rpm 914...stock...probably not REQUIRED. but like everything...everything new in a fresh engine will give you piece of mind that a failure is impossable.(usually).

So someone tell me about this stretch gauge. How is it used? To check old bolts? Or is it an alternate way to measure the tightening. Just curious.

Rod Bolt stretch Guage....use....The guage is placed in such a way so as to measure the total stretch of the bolt at ANY TORQUE....USE the guage to measure the O.A.L. overall length of the rod bolt (WITH OUT THE NUT IN THE WAY BEFORE TORQUING)...this gives you the BASE LINE LENGTH of the specific rod bolt.(JUST for that specific rod bolt)....so you have got to take notes like real ENGINE builders do. ..so you get the O.A.L. of all (8) bolts individually. Install the rod bolts into the bottom cap...insuring that they are absolutely flush, seated....install top rod cap..tighten the new NUTS by hand easy, ..back and fourth, numerious times. YOU can feel the resistance , by hand , until the NUTS are seated on the upper rod cap., and the rod seperations..bottom and top cap is flush and secure...BUT NOT TORQUED. ...the ,,guage.. is made insuch a way that you can measure through the top rod cap NUT opening to the rod bolt itself , the bottom is the bottom of the rod bolt....Then ....with the specifications/ recommended for torque and rod stretch ..proceed to Torque the rod nuts/ bolts.(back and forth as you should do)...toward the rod nut torque specified....now What I specifically do is to keep shy of the FINAL torque recommendation by at least ..(2) two ft. lbs....I measure the rod bolt stretch..and compair it to the torque recommendation. if the bolt is stretched too much at this point.. that bolt must be replaced..even if its new, it has a defect somewhere. AND, I use a brass hammer, at this ..NON FINAL ..torqued stage. gently but firmly tap on the bottom of the rod bolts...to ensure they are completely seated. then walk off and give the assembly a opportunity to normalize itself. ( usually overnight). The FINAL 2 lbs of torque are applied and a final confermation check of the ROD bolt stretch is done. Each measurement is recorded., as you can reuse these bolts again if they maintain their diemensions... I made my own stretch guage,years and years ago..I use a Inch pound torque wrench.. to help ensure accuracy.( not a regular ft. # torque wrench)...(the CARRELLO rod bolt lubricants should be used for assembly...even if you use the A,R.P. bolts and nuts.

Thx for taking the time to describe how this is used. I understand now that somebody "splained it to me , properly!"

the beauty of using a stretch gauge is that you don't need a torque wrench. Geting accurate and reproducible settings, whether using the torque wrench or torque angle methods, is limited by among other things, your ability to carry out the tightening in one smooth motion. Having to stop, especially near the end point, is going affect the effective torque, regardless of your wrench setting.

Here's the gauge in use. Lube threads and nut face with special lube, mount and zero the gauge, then tighten until 0.012" stretch (3.2 carrera).

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All fanciness aside, I would NEVER assemble ANY 911 engine of any vintage using previously installed rod bolts and nuts.

The Cap'n

I once asked Jake Raby if he thought stock used rod bolts were up to a mild 2056. The answer was a qualified yes IF you know their history. Who knows what they have been through in the past 30 yrs. I bit the bullit and bought ARP's. And no, Jake doesn't sell them.

I have..... and it was in a street car. A dead stock 964 Cab being driven on the highway at the time of failure.

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This is a picture of a 3.6L 964 motor. A broken rod bolt caused the #3 rod big end to separate. This caused lots of shrapnel that destroyed the top of the case, embedded pieces of the rod in the case. The flailing rod remains hit the oil pump and jammed it, and that caused the intermediate shaft to shear off. It also put some very large cracks in the crankshaft, and destroyed 4 pistons and three cylinders.

The top of the case in that picture came off like a banana peel. I could see the rods. I have already stripped it, but I can get a picture of the remains of the case if you like.

Rod bolts are cheap compared with the cost of fixing this type of damage.......