Rod Bolts Revisited

Jeff Higgins · 06-01-2008, 03:35 PM

The ARP rod bolt torque thread caught my eye, as I'm right in the middle of getting my motor back together. I bought a new set of Raceware rod bolts, replacing the stock rod bolts I initially built it with. Since the stock bolts are a one-time use, stretch to yield kind of a bolt, I obviously could not reuse them.

The only reason I replaced the rod bolts was because I wanted to inspect the rod bearings after what appeared to be some severe detonation. There was no reason to split the cases again, which also means there was no way to use the stretch measurement to gauge torque on those rod bolts. So I used the alternative method in Raceware's instructions, where they are torqued and loosened twice before the final torquing. I re-oiled the threads with 20W-50 between each torquing, as specified in the instructions. So, the question of the day is how much confidence can I have in the torque applied to my rod bolts with the alternative method described by Raceware in their own literature?

The rods were just sized prior to the initial build. The machine shop used my new rod bolts to attach the caps, torquing them to the initial 25 ft lbs used prior to the final 90 degree additional tightening from there. In other words, they cannot use the full torque value on them to size the rods, as they would "use up" the one-time stretch of those bolts.

Now it sounds (on the other thread) as though the full torque value of one kind of rod bolt changes the roundness of the big end when compared to the full torque value of another kind of rod bolt. I'm sorry, but I just cannot see how this can be true. Once the cap is in full contact with the rod, I don't care how much more those rod bolts are tightened, they simply cannot distort the bore unless the rod and/or cap material around those bolts is being displaced. I think it would take a great deal more torque than we are applying to any kind of rod bolt to achieve deflection of the mating surfaces between the cap and the rod, or the bores common to those mating surfaces. Could some one clarify this?

Jeff Alton · 06-01-2008, 08:18 PM

I too was skeptical of one of the posts in that thread, and posted such. I have a spare set of 3.2/3.3/964 rods here (and bore gauge) and will check for myself.....

Having said that, how many rod/rod bearing issues of you heard of form folks who use ARP or Raceware and have not had the rods resized using this method?

Cheers

Steve@Rennsport · 06-01-2008, 09:19 PM

Quote:

Originally Posted by Jeff Alton

Having said that, how many rod/rod bearing issues of you heard of form folks who use ARP or Raceware and have not had the rods resized using this method?

Cheers

Plenty!!!!

aws · 06-01-2008, 09:19 PM

I have always had the big end of any rod I was using ARP bolts on reconditioned with the new bolts stretched using a stretch gauge, not a torque value. Over the years I have had several machine shops use the recommended torque value from ARP and when I unseated the big end and reseated it and then tightened to the correct stretch the big end would distort .0008 to over .001. I believe the big end factory spec is no more than .0005 out of round. Even using the stretch method the big tends to move around .0001 to .0002 out of round when unseated and then reseated and stretched. I have always blamed this on the way the cap aligns on the rod bolt shoulder. Carrillos show the least amount of change and I feel the use of the alignment sleeve between the big end and cap is the reason this is so, it's a better (although more expensive) way to maintain alignment. My opinion about reconditioning the big end of a Porsche rod is the same as Henry's---- WHY NOT?-- considering the price of motor and peace of mind it's pretty cheap insurance

aws

Jeff Alton · 06-01-2008, 09:48 PM

I agree the rods need to be resized. However the stanard has always been to have them resized with the old rod bolts. I am curious enough to check with a bore gauge myself.

Steve, how many failures that can be solely attributed to the fastener choice? Where the only point of failure was due to an out of round big end???

Flieger · Flieger's Garage

So, Steve's way (the correct way

) is to use a new set of rod bolts, stretched beyond yeild point, to resize the rod and make it round? Then get another new set of bolts to install the rod onto the crank? If you use 2 different measurements (stretch vs. torque) how close do you think the clamping force and therefore the roundness would be between the stress during resizing and stress during installation?

Steve@Rennsport · 06-01-2008, 10:41 PM

I measure the big ends before the resize process as well as measure the rod lengths (OEM) as they vary as much as .0020. I'll resize them with the old bolts, then install the new ones and torque them to stretch when assembling them on the crank.

If the case will not be split and the rods will be assembled through the spigot holes, I will install the new bolts and measure the torque when the bolts reach the stretch spec. This allows me to properly stretch the bolts on the crankshaft when I cannot use that guage.

Jeff,

I've seen failed OEM and some aftermarket fasteners so I am VERY particular about what I use.

I never counted exactly how many rod bearing and engine failures were due to out-of-round big ends,......I've just seen plenty of these with worn/pinched bearings resulting from rods that were either improperly rebuilt or fastener-related issues. There are a LOT of variables at work here.

Jeff Higgins · 06-02-2008, 07:43 AM

O.k., so bottom line - with rods that were recently resized and run for about 5,000 miles, new rod bearings and new Raceware bolts torqued through the spigot holes with their three step process, am I at risk of a rod bolt or rod bearing failure?

I had the 7,000 rpm chip in my MSD 6AL, and will set the rev limit on the Electromotive to 7,000 as well, if that helps make a determination. There was no evidence of rod bearing damage when I took it apart. I'm kind of wishing I had left that part well enough alone, but I guess that sentiment is the product of not having found any indication of detonation induced damage. If I hadn't taken them apart, of course, I would have never known they were o.k. I would never have had any confidence in them. Funny, now my confidence is waning because I took them apart. Talk about a nasty "catch 22" situation.

aws · 06-02-2008, 07:48 AM

I have never seen a failed ARP Porsche rod bolt. I have seen a a blow up from a race engine that the only thing left of the big end of the rod was the ARP bolts and I have always joked with ARP that I should have magged them and put them back in the new engine they looked so good. This particular blow up was due to styrofoam packing peanuts being in the new oil tank when it was installed, the tank had been washed with soap and water and blown and shook out but when the engine was torn down there was a styrofoam peanut stuck in the suction line. The peanut was soft enough you couldn't hear it in the tank when shaking it and the soap and water didn't dissolve it. A good reason to use lacquer thinner when cleaning oil tanks and then sending them to a ultrasonic cleaner. The point I was trying to make with my recommendation to re-size the rods when using ARP bolts was that some machine shops do not have stretch gauges and if they use the ARP torque value to re-size the rod it will not be round when you stretch it. This has been my experience several times in the last 10 years or so.

aws