rbahr

I was JUST helping a buddy last weekend on a air cooled motor... '
last resort' was bad phrasing on my part - just meant to say sub-optimal...

Part number is important but here is an example:

http://arpinstructions.com/instructions/204-6301.pdf

Part Number: 204-6301
Application: PORSCHE 996 / 997 / 986 / 987 (M9)
Cap Screw without Washer - Rod Bolt Installation
[1] Please check the part-number(s) for your application against the part-number(s) listed on the instruction sheet.

[2] Make sure there is an adequate chamfer around the bolt hole on the connecting rod cap to clear the radius under the head of the bolt

[3] Use ARP Ultra-Torque Fastener Assembly Lubricant to lubricate the threads of the bolt and the under head of the bolt. Then install the bolts and tighten them hand tight.

[4] ARP recommends using the STRETCH METHOD when tightening rod bolts. Following the instructions for using a stretch gauge, Stretch the bolts to . 0095 - . 0100

[5] If you do not have a stretch gauge, torque the bolts to 45 ft-lbs using ARP Ultra-Torque Fastener Assembly Lubricant

[6] The connecting rods should always be re-sized after new rod bolts are installed.

[7] A log should be kept on the original non-torqued length of each bolt. Bolts that have any permanent deformation or have increased in non-torqued length by more than .001 in. should be replaced.

timtt

Ah, I think this is the confusion: there is no ARP part number for these bolts. You cannot buy them from ARP. They are just the manufacturer. If you call them, they won't have it any catalogs, though one guy did confirm that they're the contract manufacturer when I called them (he had to look it up). The bolt is a Wrightwood Racing part made to spec by ARP and Wrightwood owns the IP, so ARP won't sell it to you directly. Same reason why Pankl won't sell you the OEM bolt directly even thought they make it for Porsche.

Since Wrightwood has no website and are crazy secretive about everything they do, I can't even figure out their part number. Wish I could - sorry!

Regarding the spec, the 45 ft-lbs is what Don from EBS told me and I vaguely remember being told that same number by another company I'd called a while ago as well.

m42racer

Good advice.

The Stretch is different by bolt design I think. At 45 ft/lbs I think you may be well under the correct required value. Stretching I'm told removes all friction errors, so even at your torque figures you may have uneven clamping (because of friction) of the bearing due to the bolts not been evenly stretched.
Hate to suggest but now is the time to double check.

timtt

I guess I'm going by the specs given to me by Don from EBS who sold me the bolts. I just checked my email again. 45 ft-lbs with the supplied assembly lube. I'll check again, though I'm 99% sure that BBI told me the same when I spoke to them a few months ago.

rbahr

Hi Tim,

With no part number it can be difficult. ARP tends to suggest using stretch as a determination of proper tightening. I would suggest verifying with ARP / Wrightwood / Don that this fastener is designed for simply a torque value.

The other methods include torquing to a value and then adding an angular offset, but ARP does not recommend that either...

Ray

Spyerx

BBI builder torques these to stretch specs.

Rob S

timtt,

I suggest that before letting anyone worry you too much about this, maybe you can do a little research to find out what the typical correlation is between torque and stretch for that very combination of rod and rod bolt, using the same lubrication technique. If you bench test identical hardware, you may find that the torque you used corresponds very favorably with the stretch spec. The challenge will be coming up with that hardware. Maybe someone on the forum or in a shop nearby happens to have a GT3 rod that's out and available for a test. (I have one I could lend, but I think it's not identical, as it's from a 2001 GT3 RS race engine.). Then you'd need one rod bolt, which perhaps you could buy. If you ran a test and found that the torque you used reasonably corresponds with the specified stretch, then you'd know you're likely to be okay. If not, then you can decide what to do about it.

I recorded both torque and stretch years ago on a 930 with ARP rod bolts. Somewhere I have the numbers still, but as I recall without looking, to reach the specified stretch required torque that was maybe 10-20% higher than the torque spec. So, I favored the stretch spec and "ignored" the torque spec.

I find it a bit odd to "dual dimension" that spec. That's a no-no in most engineering applications, for this very reason. If it's critical that the stretch spec be used, then that should be the only specification used and a torque spec shouldn't even be given. If either torque or stretch are safe to use, then it would be okay to give both, but instructions should be clear that, even if they don't agree, either will be sufficient.

As a final suggestion, you might like to call ARP and get their top technical guy to talk to you about it.

Rob

Laurence Gibbs

ARP go to great lengths on their website to explain the reasons for using stretch to fasten rod bolts. I wont bother to repeat it. More important for you are two things.
1. You may find it difficult / impossible to get a stretch gauge onto the bolt anyway without tearing the case apart !
2. Porsche specify a torque value for OE bolts and in general it works.

I doubt you'll get any race engine builder recommend Torque over stretch when it comes to rod bolts. If the data exists to use the stretch method.