ARP Rod bolts and resizing

[Wayne 962]

Originally posted by Eagledriver
These things are in the spec book from Porsche. They don't have to be found out by word of mouth.

All of the spec books have been out of print for many years now. That is why I consolidated and duplicated all of these specs in the engine rebuild book.

-Wayne

[addictionMS]

Thanks for the pointer Wayne, I was just talking to Eagledriver on the phone and mentioned I did not have the little book, but I do have yours, so in effect I have the little book. GReat Stuff, I will have to look this stuff up so I get more used to checking in the back of the book

Cheers

Jim

[350HP930]

One of the things I like the most about Wayne's book is its abundance of specs.

[cstreit]

Er...


Question here: ARP Rod bolts are torqued (or stretched) with around 45 ft lbs... They're saying this is so much more than stock it deforms the rods? What's stock?

Besides, how does putting clap pressure on two flat surfaces create distortion?

[350HP930]

Rod bolts with much greater clamping forces can cause bore distortion by imparting a greater squish to the portion of the rod that is being clamped by the bolt.

Before I resized my rods I measured them and they were close enough to spec to use again without resizing. Since it was possible for me to DIY this job I took about a half a thousandth from each rod mating surface and got it dead nuts on spec for the sake of my borderline OCD status.

If my abused turbo rods were that good after a ton of miles I can see how it might be overkill to rezise a set of late model porsche rods without measuring them to see if they need it first.

[mpdevelopment]

Stock rod bolts are stamped 12.9 on the head. 12 is the ultimate tensile strength in hundreds of MPa, .9 multiplied times the ultimate is the yield strength in hundreds of MPa. So the stock bolt has an ultimate strength of 1200MPa and a yield strength of approx. 1100MPa. This works out to approx 177000psi and 160000psi in english units. ARP 2000 material is stated to have an ultimate strength of 220000psi, no yield strength is listed but typicaly .9 X ultimate would be close giving 198000psi. The factory stresses the stock bolts beyond the yield point (stretched or plastic deformation) this puts the installed stress at some point over 160000psi I dont know the exact number but I will estimate at 168000psi. ARP bolts are not plastically deformed when installed so the installed stress is less than 198000psi I will estimate 185000psi. This increase in stress results in an increase in clamping load of 10 percent if the smallest bolt cross sections are the same. This is not a very large increase. The greatest advantage of ARP bolts in my estimation is an increase in fatigue resistance of the material and possibly better quality control.

[911pcars]

Don't forget to balance the entire crank assembly (crank, rods and rod bearings, piston, piston pins, rings and clips, rod bearings, flywheel, crank pulley and woodruff key).

Sherwood

[Wayne 962]

Originally posted by 350HP930
One of the things I like the most about Wayne's book is its abundance of specs.

I had to fight the bean counters to get that stuff in there. Worth the price of the book alone, in my opinion...

-Wayne

[mpdevelopment]

I measured the diameter of a stock 2.7 bolt at 0.297" .The clamping of one stock bolt stressed to 168ksi is 11638 pounds. The ARP at 185ksi works out to 12802 pounds. Since the loads induced by rpm go up as the square of rpm the ARP bolt gives a 4.8percent rpm margin of safety.

[addictionMS]

can you fill in the jump from the pounds to percent, there is something I can learn here, I can't see the connection from A to B


here is what I tried

ans= sqrt((12802-11638)/11638

got 3.1625

no idea what I was doing, but thought the sqrt because loads go up with the sq of rpm,

uneducated guess, but wrong

[snowman]

Woooha. Let me take some of the mystery out to rod resizing, and you can make your own decision on what is required.

Connecting rods, when brand new are perfactly round, thats where the bearing fits. After significant wear, they MAY become slightly egg shaped due to the stretching effect and compressing effect that happens every time the crank turns around. Porsche rods are so well made to begin with that they do not tend to become egg shaped at all or if so , so little as to be totally insignificant. I would say that many if not most Porsche rods do not need resizing to begin with. What is resizing? It is restoring the shape of the rod to perfactly round, no more , no less. Rod bolts will not change this.

How is a rod resized? Usually the bottom half is ground down, at the mating surface, making it slightly shorter, it is then remated with the top half and a hone is run thru the center in an attempt to make the hole round again. Thats it One can juggle taking metal off the top or bottom half or both to make minor compensatins for rod lengths. The width is unchanged, because there is no way to make the witdth smaller. This is not a problem as the width isn't changed to begin with due the direction of the forces on the rod.

Thats all there is to it folks.

For referance look at any stock chevy or ford rod, then look at any Porsche rod, you will instantly see why chevy and ford rods NEED resizing and Porsche rods do not.

As to balance, the crank is totally independant of the rods and pistons. If the rods are resized, then they must be rebalanced. The Pistons are usually independant of the rods unless someone has compensated for rod imbalance using piston imbalance. If you understand what I just said you know what to do, if not get all the info on balancing you can find and learn. In any case an opposed 4 or 6 cylinder engine is not like any V type engine where the balance is dependant upon the piston and rod weights in conjunction with the crank.

[addictionMS]

agreed on that is how to resize, the info I was getting on the board for others to see was when you change to ARB style bolts you need to resize as well. So say the experts. It still is not entirely clear to me how a greater clamping strength by the rod bolts deforms the rod/crank bearing surface, but they say it does, and have experienced rods that were fine with stock bolts, not be fine with ARP, so the rods get resized with the new bolts.

possibly the greater force squeezes the rod half moons more so the flare a little more and thus get out of round and tighter on the crank shortening the distance when measured along the axis of the rod.

Jim

[snowman]

As to the bolts, I strongly suspect it is the shape, size, and exact placement of the bolt the may change the "sizing " of a rod, not the clamping force. I understand you get the same effects using new stock rod bolts.

I would like to add that this effect is usually so small as to be totally insignificant, and in fact may be one of those "anal" type of things people do.

[addictionMS]

well, I am dealing with some of the best engine builders around so they are about as anal as you can get, if it aint perfect it aint good enough, a blessing and a curse.

The tough part is separating the nice to have from the must have, I only have the bucks for the must have and a few nice to haves. Oh but which ones, which ones indeed....

Jim

[snowman]

Problem is, if an anal guy is fixing your car, you got to go with anal. Human nature And I for one can "unfrotunately" completely agree with their position. If I am one of the best and I fix an engine, it ain't gona fail, period. Good philosophy, but it does cost.

How to get around it? You don't. You shouldn't even try. UNLESS you got the guts (and know how) to do it yourself.

[lonehill]

Jim,

Just tell Ted you are going to use the ARP bolts and he should know what to do, not that its going to help you lap times at Thunder Hill anyhow . When I put in my ARP bolts in my car, the con-rods were simply re-ground and then reinstalled with stock bearings and the ARP bolts. I forgot how many races we did that season but it was a few and never had a problem. I think you'll be just fine. So what day are we going to put the uber-motor back together?

[350HP930]

Originally posted by snowman
For referance look at any stock chevy or ford rod, then look at any Porsche rod, you will instantly see why chevy and ford rods NEED resizing and Porsche rods do not.

I made a post on this very subject a while ago . . .

To give you an idea how they compare against other types of rods, here is a pic of mine beside a ford small block rod, a chevy small block rod and a rod from an 87 grand national 3.8L turbo motor.

[/B][/QUOTE]

[ChrisBennet]

This rod deformantion effect should be easily measurable with a bore gage. I don't have any ARP or Raceware rod bolts that aren't holding something together or I'd do this experiment myself.
-Chris

[mpdevelopment]

Take the sqrt (12802/11638)

[addictionMS]

picked up the rods after resizing, look good.

I had another good suggestion, test fit the pins of the pistons into the rods before I close the case up, just in case.

Jim