ARP Rod bolts and resizing

[addictionMS]

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

[ChrisBennet]

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

[addictionMS]

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

[ChrisBennet]

Originally posted by addictionMS
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

[350HP930]

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.

[mpdevelopment]

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.

[addictionMS]

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

[Eagledriver]

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

[Wayne 962]

Originally posted by Eagledriver
My own opinion is that stock rod bolts and clearances are fine for 7500 rpm and under (not on 3.2s).

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

[mpdevelopment]

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.

[addictionMS]

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

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,

[racing97]

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

[mpdevelopment]

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.

[addictionMS]

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

[racing97]

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

[addictionMS]

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

[Eagledriver]

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

[addictionMS]

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

[mpdevelopment]

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.

[Wayne 962]

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