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Rod fitment

I'm disassembling a 964 crank. As I withdrew the number five rod from the crank, the sides of the big end seemed too tight. It didn't feel right, at least not like the other 5 rods. Not seized, but like the crank webs were squeezing it a bit, as it came out. My question is what can cause this? I tried inserting the rod back into position from several different approaches. It felt the same. I have a good machinist out here, although not Porsche specific, and I'd like to be able to supply him with as much information as I can. If there is a crank problem, I would like to try and solve it locally, if possible. For the experts in the crowd, does this description ring any bells? Just for reference, all of the crank bearings, and rod bearings all looked good, as do the journals. No visibly unusual pattern of wear. Thanks in advance.
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911 #303129
81' SC project
77' 930
Old 04-04-2008, 06:37 AM
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Hmm, why would there be reduced side clearance on #5? Oil starvation in the middle of the crank causing the bearing to distort?

What is the side clearance measured with a feeler gauge on #5? What is it for the other throws?

I'll stop now and let the real experts answer.
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Old 04-04-2008, 06:53 AM
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Thanks for the input John. It's times like this when I wish I had your skills at measuring and processing the information. I will try and measure the parts today. Yesterday was tear-down day, and I finished the crank right before it was time to go get the kids. I'll gather more pertinent information, "measurements", and report back.
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911 #303129
81' SC project
77' 930
Old 04-04-2008, 08:12 AM
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I guess the next step would be to simply measure the big end and the crank dimensions to see if anything is amiss. Best case I guess would be that the big end is a little "too big"...sorry, couldn't resist.
Why the tear down in the first place...any history that could have caused this issue?
Good Luck,
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Old 04-04-2008, 09:28 AM
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Why the tear down in the first place...any history that could have caused this issue?

No, unfortunately the car came to me with no history. It's a totaled RSA that I bought for my 3.6 conversion. Like most DIY'ers, I enjoy building, so a tear-down was in order. Leak down was poor. Tapping on the exhaust rockers with a plastic mallet improved the score, but not enough to keep me from going inside. No time for any work on it today though. Hopefully I'll get some measurements on it tomorrow.
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911 #303129
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Old 04-04-2008, 05:01 PM
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Just to up-date the thread. I've been waiting for 3.6 specifications. I have the tech specifications book from Bentley, but have not found the con-rod dimension for the width of the big end. I was going to go with the spec for the 3.0, until I saw the differences between the size of the big end bore on the 3.6 vs the 3.0 rod. I've ordered some better reference data. Seems there are many standard dimensions that are shared between the two motors, but not all.

I took the con rods and crank to my machinist. He is advising that I wait to have the rods re-built until I have the new ARP rod bolts. His thinking is that since the ARP bolt is able to withstand a couple of torque experiences, it would be best to re-size the rods using them. For the machinists in the crowd, does this sound familiar? I had thought using the old rod bolts would suffice for re-sizing.
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911 #303129
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Old 04-06-2008, 01:55 PM
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I don't see any difference whether you torque it with the old bolt or new, torque is torque, how does the rod know what bolt you are using?

Now if there is an alignment issue with the new bolts then thats a different matter.
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Old 04-06-2008, 02:29 PM
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I don't see any difference whether you torque it with the old bolt or new, torque is torque, how does the rod know what bolt you are using?

This is pretty much the way I was looking at it, but he mentioned that it's not about just the torque, but about the way the new bolt may sit in the cap. The ARP bolt may sit in the cap differently than the stock bolt, and as long as the ARP can be re-stretched, why not use it. Heck, I don't know. Sometimes I think I spend way too much time thinking about it.
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911 #303129
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Old 04-06-2008, 05:01 PM
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ARP rod bolts are suppose to be stretched .0115 to .012 (inch) and it takes about 50-60 lbs of torque to attain that stretch. If you recon the rods with the stock rod bolts the ARP bolts will distort the big end badly. ARP state a torque value of 45lbs after several torque cycles on the bolt but my experience has been that it takes 50 to 60 pounds to attain .0115 to .012 stretch. If the machine shop does not use a stretch gauge and uses the 45lbs torque number you will only get .006 to .007 of stretch and you do not benefit from the stronger clamping forces the bolt is capable of. I know this because I was assured by several machine shops over the years that they had a stretch tool only to find that the bolts were not stretched enough when I checked, and that the big end would be distorted when the bolt was stretched to spec. The ARP bolts have to be stretched to spec, then the rod reconditioned or there will be problems. ARP states that all bolts should be measured before installation and documented. If any are elongated more than .001 after use they should be discarded. Make sure you measure them at a consistent temperature, it makes a difference!!

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Old 04-06-2008, 09:21 PM
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AWS,

Thank you for the reply. My machinist will certainly use a stretch guage. He mentioned the different loads causing the big-end to change shape, and since the ARP bolt can survive the work-load, we're using it.
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911 #303129
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77' 930
Old 04-07-2008, 06:17 AM
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Arp . .

Bored? You'll think about stroking when you see these rods.
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Old 04-07-2008, 06:26 AM
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Thank you John. I had forgotten that post. I don't know how, other than to plead "mild senility". I actually have a folder with many of your posts collected. I could lie and say it's to keep the tech data at hand, but in being honest, it's for those beautiful photos. My machinist, while not a Porsche expert by any means has, I think, pretty good skills. As AWS had posted earlier, and now your post, once again reminds us; the torque or stretch of the ARP fastener can be, or probably is, different enough that it makes sense to use it when re-building the rods. Also, by using the stretch guage, the torque can be repeated more precisely. I'm by no means in the same league with your ability to measure and understand this stuff. I have to defer to my machinist. If I can get any more usable data I will post it here in hopes of helping someone else, down the line.
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911 #303129
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Old 04-07-2008, 08:06 AM
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Jon, I'm no engineer, I just read a lot. Anyway, I am slightly skeptical of the suggestion that ARP bolts preload the cap to the rod differently-- preload IS preload, at least for the purposes of the machining operation of making the big end round again. I think that only if the shape of the bolt shank or the head were different than the OE VERBUS bolts would it make a difference in the way an ARP fastener clamped the cap.

The way different bolts respond under load MAY be a factor during operation, but during resizing? Is the resizing machine really putting that much load on the cap that the characteristics of the bolt are called into play? Somebody needs to show me some data before I believe this.

On the contrary, I think that the reason ARP bolts should be used is that NEW factory fasteners are torque-to-yield, e.g. when clamp load is applied they permanently elongate, such that a second elongation may move them WAY beyond the yield point into "dangerous" territory; whereas the ARP bolts have a yield point that is WAY up there and aren't designed as "stretch to yield" fasteners. This means, practially speaking, that as long as they are properly stretched when the caps are put on for resizing, you can use them again.

The second point is that the higher strength provides protection against the occasional over-rev. This is not super likely to occur in our street engines as it's rarely necessary to go beyond 7000 rpm when parallel parking. It does provide ham-fist insurance, though, making it less likely that one will have to confess to one's mechanic the inability to consistenly operate a manual gearbox.

Get yourself an ARP stretch gauge, not the cheapo Summit one, and have at it! You can easily flip the gauge when you are done.
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Old 04-07-2008, 09:15 AM
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Get yourself an ARP stretch gauge, not the cheapo Summit one, and have at it! You can easily flip the gauge when you are done.
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I have to confess, the only reason I went with the ARP fasteners was to buy and use the tool. I've watched several guys use it, on this forum, and it looked like a good idea. Plus, I needed a reason to buy another nice tool.
I was a bit perplexed by the reasoning behind what I posted above, but since "I" personally don't know enough about this, I thought I would post the info and let folks bat it around.
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911 #303129
81' SC project
77' 930
Old 04-07-2008, 12:13 PM
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Old 04-07-2008, 12:13 PM
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