arp rod bolts?

[carrerarsr65]

just had ollies rebush my sc con rods, i would like to use arp rod bolts/nut kit on em' ddo i y?eed to have my rods modified in any way? or can i just order a set from arp? thanks guys al from mass

[brighton911]

You should (read must) get the big end resized after installing the ARP bolts. The act of pressing the bolts in can change the shape, ie no longer round. I will be installing mine on the rebuild this winter.

[Walt Fricke]

Huh? I've never heard of this. ARP doesn't suggest it. Typically guys use old stock bolts for clamping for resizing, then put in their new bolts. I don't recall that the ARP bolts were any more of a press fit than stock or Raceware - a few taps with a brass hammer at most, but tightening the nut draws them in without that.

What am I missing here with respect to Porsche rods? Or Pauter, for that matter. Pauter says nothing about this either unless I missed something.

[Walt Fricke]

Oh - ARP does make bolts specifically for Porsche con rods.

[Ronnie's.930]

brighton911, I'm curious as to where you got this information as I have never heard/read of this before and have never done any resizing due to a rod bolt type?

[Mark Henry]

A 3.0?
I've been told unless you are racing a 3.0 doesn't really need the ARP rod bolts, stock will do.
But on a 3.2 the ARP's are a wise upgrade. The 3.2 has a smaller rod journal.

[Lapkritis]

There is a line of thought out there that the increased strength of the ARP fastener would deform the large end when torqued. This has fallen out of favor in the last decade... many now replace the stock rod bolts with ARP without even removing the assembly from the engine. Practical experience says there is no need to resize when proper care is taken for assembly. This resizing train of thought likely began as just another way for a pro builder/machinist to extract more money from your wallet. Expect those types to "sell" you a service that is unnecessary.

[brighton911]

Re-sizing recommendation was from the ARP installation sheet for part number 204-6005 3.2 rod bolts. My guess is ARP is being on the safe side in case their bolts deform the rod slightly. I haven't got my rods out yet, but I am interested in how much of a press fit the ARP bolts are. If they are relatively a light press fit, then I probably won't be getting them sized. I would be interested in the experiences of other ARP users. Thanks

[Lapkritis]

Never thought of that... CYA policy for ARP itself.

[Gordo2]

I will start by saying I recently had my rods reconditioned - largely due to the fact that I am using ARP rod bolts.

I think most of the pro's here on the board recommend reconditioning with installation of ARP bolts .

My machinist (who's not even a Porsche guy) made no question about it - ARP install, he highly recommended resizing the big end (and while you are at it, recondition and balance...).

I haven't found anything that suggested the rod resize is due to the pressed fit of the bolt in the cap. Appears the recommendation is based on the increased clamping force that the ARP's provide and the distortion it can cause. This is assuming you are tightening the ARPs to stretch length vice standard torque #.

Above is purely theoretical - repeating what I've found through my own searches.

Good luck.

Gordo

[Lapkritis]

Did you ask the machinist if the spec was correct before machining? Chances are it was fine without any work. ARP likely put the resize instruction in there to avoid any and all blame in a failure scenario. Builders love to blame materials. ARP likely put the instruction in to call the bluff of the crappy builder.

http://forums.vwvortex.com/showthread.php?5777647-Tim-Mullen-9-57-157mph-the-Fastest-MK3-VR6-street-car-ever-lol/page2

[johnsjmc]

Resizing is recommended by my machinist when replacing rod bolts in any rod . He has said you could install the bolts and torque them and use a dial bore guage to measure them for round . If you want to save buck or two but he insists they need to be at least checked for round after the bolt change.
Stock 3.0 rods and stock bolts are quite strong , stronger than 3.2,3.6 and 3.3 turbo rods

[Gordo2]

Originally Posted by Lapkritis

Did you ask the machinist if the spec was correct before machining? Chances are it was fine without any work.
VWVortex.com - Tim Mullen 9.57#157mph, the Fastest MK3 VR6 street car ever lol

I asked about measuring the big ends prior to re-boring it but my machinist said reboring was pretty standard when going to ARP rod bolts.

This actually gave me more confidence in going with this local machinist - his recommendation mirrored what I found searching Pelican and other sites.

Was it no-kidding needed? Possibly not, but I planned to get my rods reconditioned and balanced anyway, and the total cost too include re-boring the big ends was about $190.

How much of that was reboring vs. balancing, I don't have a clue. Meanwhile, I'm confident my rods were done right.

If I had a good internal bore micrometer, I might have installed the ARP bolts, measured and made the decision. Meanwhile that's one tool I just couldn't justify - as such I went with the best recommendations I could find.

Engine rebuild - so many decisions...

Gordo

[AlfonsoR]

Yeah, the 3.2 uses stretch to yield bolts from the factory. So as the name implies, it is a one time use fastener. ARP upgrades the material so that at designed clamping load, the material does not exceed the tensile strength of the material. However, the clamp load, as designed by ARP, is greater with than that of the OEM stretch to yield fastener. This additional clamping load induces additional stresses into the rod which change the shape of the rod bore.

If you don't resize the rods, it should lead to reduced bearing life. How much of a reduction? I don't think anyone can tell you that.

I too have seen that it is recommended by several pro builders here and have not seen where a pro builder recommends against it. It makes sense to me that you would want to have two round bores on your connecting rods for optimal bearing life.

Lapkritis: I didn't read the complete thread that you posted a link too, but if the clamping load is the same ARP vs OEM, then the stresses remain the same and therefore resizing is not necessary. I don't know if that might have been the case for deciding to not resize the rods?

[Lapkritis]

In the majority of cases, the rod and cap simply don't deform like you're being told by the person who wants your money. The cap and shoulder of the big end are quite rigid... as is the case with many engines such as the VR6 example that I've posted above. Have your rods checked with a stock torque to yield fastener. Then have it checked with the ARP properly installed. Your machinist will blush and tell you better to give him your money anyway for a "resizing" that you could do at home yourself with a clean paper towel.

[Walt Fricke]

While I am not as suspicious of the motives of machinists as Andrew is, I share his wondering just how that large bearing surface between the rod and cap is going to be distorted by increasing the compressive load on it in the range we are discussing.

The ARP and other super bolts are pretty stout, but we aren't stretching them to the extent that they are likely to alter the much larger cross sectional area (or expand the sectional volume, which would be even more) which is put under a compressive load by stretching the fastener. Come to think of it, isn't the compressive strength of steel substantially higher in PSI than the tension strength?

I can see some possibility that if you are pressing a larger fastener into a smaller hole, there could be sideways displacement, which could affect roundness of the bore. But you don't need a press to install rod bolts.

Pauter rods, if I remember mine, don't have locating bushings. They use the precise cross section of the bolt in that area to do that locating, which is why it is a tight fit on both cap and rod.

But the Porsche rods use a steel bushing for location, don't they, so unless the ARP bolts have to be squeezed through that bushing I don't see how they are going to exert a sideways force.

A guy might make a quick check of this as follows. With the rod and cap together and the bolt snugged (say, to stock torque), measure the distance between the fastener end surfaces of the rod. Then torque (using stretch) the bolt to spec, and remeasure. Is it going to be smaller?

Or, if you have a bore gauge, just do before and after checks of the bore. Is it distorted post fastener tensioning?

[johnsjmc]

I asked my machinist . He said even changing the lube on the bolts will cause some distortion without touching the bolts and at the same torque. maybe 1/2 a thou.. Change bolts from one requiring .063 stretch to .068 stretch can induce up to maybe .0018 out of round.
His recommendation change bolts and resize Use same lube and torque as when resized.
Mine at very least measure with a bore guage.

[AlfonsoR]

The metal most certainly does move. The question may be, "is it significant?". I don't know, I haven't measured it. Maybe Laptkritis has measured and found no significant change for him to make the statements above.

As I understand this, you are supposed to torque the bolts to proper spec, then machine the bore for a "perfectly" round bore. When you release the tension on the bolts is when you should see distortion in the bore.

[Lapkritis]

Originally Posted by AlfonsoR

The metal most certainly does move. The question may be, "is it significant?". I don't know, I haven't measured it. Maybe Laptkritis has measured and found no significant change for him to make the statements above.

As I understand this, you are supposed to torque the bolts to proper spec, then machine the bore for a "perfectly" round bore. When you release the tension on the bolts is when you should see distortion in the bore.

Lots of should and supposed in here. The experience is not magic, luck and certainly not unique if you've been around engine building for a long enough time.

The question is not distortion found in an un-torqued cap. The question is what difference is there between a properly torqued ARP fastener and a torque to yield fastener. How much more force is applied by a properly torqued ARP fastener than a torque to yield fastener? If installed properly, is the ARP applying significantly more force or does the ARP fastener simply have the capacity to resist failure more than a stock fastener which is torqued to yield? Hmmm... what about all the reading on the internet from other people who read something on the internet and the machinist who claims resizing fee must be paid? Unquestionably the ARP material has higher yield ratings than the OEM fastener but ... you're hopefully not applying the ratings to their maximum for ARP material during assembly. This riddle was figured quite some time ago by the community building significantly higher horespower German VW/Audi engines. Polish the crank, plastigage the bearing fit, ensure it spins freely, lube liberally before first start and if you're using a properly installed ARP fastener then expect a long service life. Confidence is now so high in that large community that, as in the example I provided above, they're dropping the oil pan and swapping fasteners without removing the rod caps even. Then they are making close to 900hp on a 174hp tall block. The absence of failure is wide spread and longevity is proven. I have my own example that has been over a decade and across the continental USA multiple times. Through the Mojave, up through the Rockies across the great plains. Raced at Napierville in Quebec, Speedworld outside Phoenix and California speedway in Fontana over 8yrs apart without ever dropping the oil pan or riding on a tow truck. I lost a fuel injector in 2012 and opened it to replace a piston. Bearings looked like new.

I suppose I don't expect anyone here to really change their own minds. Self inflicted p-tax if I've ever seen one.

Feel free to ignore if you're building a Formula 1 engine and have infinite resources. I wouldn't question the logic in that type of application.

[911pcars]

Here's an installation sheet from ARP.



Sherwood