ARP Rod bolts and resizing
 

I thought I would post this info since I had not found a reference to it on my searches.

I am replacing my rod bolts with ARP rod bolts. Most everyone I talked to, Jerry Woods, IPB, and German Precision, all confimr that when you switch to ARP or Raceware rod bolts you need to have the rob bearing surface resized. It seems that the new bolts take more torque and deform the rod ever so slightly, that you run the risk of having uneven pressure on the bearing.


Estimated cost $270

I hope I put enough key words in there for the next person who tries searching http://www.pelicanparts.com/support/smileys/chopper.gif

Are you saying that rods need to be sent to the machinist with the ARP/Raceware rod bolts instead of old (stock) rod bolts?
-Chris

that is what I was told, you take the rods with the ARP bolts to the machinist, and have him (or you I guess) install the rod bolts to the correct stretch, then re-machine the bearing surface to round.

Since ARP bolts can be re-used, this is not a problem, but they do put more force on the rod so it deforms and "can" cause a rod to seize on the crank.

Jim

Thanks! Great info.
-Chris

Having your rods cut and resized is a good thing to do to old rods anyways whether or not you change the bolts.

You can see a few pics in the thread linked in my sig from when I changed my bolts and resized my rods.

If you are building a high rpm motor try to keep your rod bearing to crank clearences at .0025"-.003" this may seem loose to you but it will help the bearings survive.

again some definition clarification, what do you consider "high rpm" I am building a stock 3.0, late pistons intakes ported to 39mm, with ARP bolts and JWE springs so I figure 7500 max.

Jim

Danger Will Robinson,

We don't know much about mpdevelopments background (no offense). He maybe correct on this but I've not heard this before. When I checked the clearances on my rod bearings after 90k miles and 3 race seasons they showed about .0015 clearance. Clearly the further you get from stock on these engines the more chances you are taking. My own opinion is that stock rod bolts and clearances are fine for 7500 rpm and under (not on 3.2s). This is what I have on my car. The problem would be if the engine is over-reved. (money shift). In that case a prudent man would replace the rod bolts (Russ taught me that the hard way).

-Andy

7500 is too high for stock bolts. Try more like 6000-6500 to play it safe.

As to the issue of resizing your rods - you should have this done during a rebuild. Typically you send your old rod bolts with the rods so the machine shop can use them to tighten the rods together when they are resized. If you are upgrading to ARP bolts, then indeed yes, you should send the bolts along with your rods, as they will every so slightly change the geometry of the rods.

If you're replacing the rod bolts with a set from ARP and you're not machining the rods, then I wouldn't worry about it - especially when you're talking about a stock engine. Jerry Woods is a great engine builder, but everything he touches is basically for extreme racing. For stock 3.2s, I have not heard of any problems with replacing the stock bolts with the ARP ones without machining the rods.

-Wayne

Sorry I should have said racing high rpm motor, high rpm (8000+) for sustained periods. Always check size of con rod big ends when rebuilding, if ok with new bolts torqued no need to resize. Stock clearences work well for engines limited to stock rev limits. Stock rod bolts are good for 7500rpm in 2-3liter engines 3.2-3.6 are marginal (smaller dia. greater loads). I would use the aftermarket ARP bolts.

I am working on the delicate balance of what needs to be done and what is nice to have done. I don't want to spend $10,000 on a $3,000 motor :D

But I want to have a solid bottom end, so I opted for the ARP bolts, and found out that "requires" the rods be resized, if I had stuck with stock rod bolts, the resize is "advised"

subtle difference, but an important one, with some sound explaination behind it. JWE told me they have seen rods that are very stiff after tightening the rod on to the crank when switching from stock to ARP, so stiff the rod would not fall under its own weight. So I think it is a "required" step, atleast in my mind.

As for clearances, I will go with stock because I am not planning on 8000+ rpm,

You can't go wrong with a clearance of .001 ths. per inch of journal dia.
+ .0005 for rpm application. As you know it is the tension on the exhaust stroke at high revs that is the hardest on the rod and the brg. It is for this reason that a medium or high eccentricty (wider in and around the parting lines of the rod and brg) are imperative for engine life.

Best regards

Yes eccentricty in the bearing or extra clearence is required at high rpm in order to compensate for distortion of the con rod big end due to extreem inertia loads.

So if I read this right I should put on the rod bolts and have them checked first, I may not need the machining, but I may, possible place to save some $$ if they are ok, and I should look for a clearance of 0.001 with possible +0.0005 cause it is a rcae motor.

Jim

Whoa
That is .001 per inch of journal dia. If the Journal is 2.047 for example that would be 0.00247 + .0005 which is .00297 and of course rounding off is okay don't make it to big of a job. I assume you will be using this for a track car and even though you don't intend high rpm it is sometimes unavoidable.
Best regards

thanks, I am going to see the machinist this am, he would have looked at me funny, he does work for many of the shops around here so he would have known (I hope) that I was out to lunch.

IF possible, coud someone outline how you take this measurement.

thanks

Jim

These things are in the spec book from Porsche. They don't have to be found out by word of mouth. Bearing clearance for 72-73 Porsche 911(including 2.7RS) are as follows:

Mains 1-7 .010mm-.072mm (.0004-.0028 inches)
Rods .030mm-.088mm (.0012-.0036 inches)

As I said my rod bearing clearance was .0015" on a well used engine.
These specs are for new motors from the factory. If you go outside them you are taking a risk. It looks like the word of mouth numbers are within the spec but a little toward the loose end.

-Andy

so I dropped off the rods, should have them back on Wednesday, learned more about resizing, when you resize you must also have the bronze bushings replaced so the distance from crank to rod is once again made equal on every rod. Because there is a chance the resizing of the crank bearing surface is not perfect, it is possible the rod lengths will be off ever so slightly. replacing the bronze bushings and re-sizing them is done after the crank hole is done, that way you can be sure the holes are the equal distance on every rod, because this is done in a jig where the hole for the piston's pin is made relative to the crank hole. The resizing of the crank hole is done independent of the piston hole.

I also got to see and compare a GT3 rod made out of titanium to mine, much nicer, looks like the hole sizes and distances are the same, but the base where the rod connects to the crank is thicker. So someone with too much money could put GT3 rods in their 3.0 with a little machining.

jim

Yes they are in spec and if you turn 8000+ rpm for any extended amount of time the "extra" clearence will help the bearings live. One reason the titanium rods are thicker around the big end is titanium has half the modulus of elasticity of steel that means an identical cross section is half as stiff. The stiffness is determined by the moment of inertia of the cross section and the modulus of elasticity of the material. Since the moment of inertia is dependent on the fourth power of the dimensions a small increase in dimensions will restore the stiffness lost. Since titanium is approx 1/2 the mass off steel one can add additional material if there is room and this will increase the stiffness while still having a weight less than steel.
Titanium has other problems when used in this application but for a racing engine that is torn down and inspected frequently it has its advantages.

One thing to note, if you run larger clearances, you need a heavier-duty oil pump to keep up with the expected drop in pressure.

-Wayne

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

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

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

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?

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.

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.

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

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

-Wayne

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.

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

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.

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

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.

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

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.

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 :D. 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? http://www.pelicanparts.com/support/smileys/pint1.gif

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.

http://forums.pelicanparts.com/uploa...1097943911.jpg [/B][/QUOTE]

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

Take the sqrt (12802/11638)

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