Does someone know why the oem con rod nuts have so much play even from the factory?

ahh911 · 02:24 PM

Hi Guys,

Does anyone know why the connecting rod bolt nuts have so much play, even the ones from the factory that were removed? I can wobble the nuts back and forth much more than a usual nut/bolt combination that you'd find on a modern car. My new nuts and bolts wobble even a little more. I'd be interested to know why the factory/designers went this way. 81 sc, but it'd be the same for any 10mm Porsche rod bolt/nut combination of the time I'd imagine.

Phil

al lkosmal · 08-24-2020, 08:17 AM

Phil,
I'm not sure I am understanding your question. It's not been my experience that there is any play in the connecting rods/nuts.....can you provide more information/pix. Are you sure you have the correct bolts for your 3.0?

regards.
al

Bigtoe32067 · Bigtoe32067's Garage

Yea it sounds like the wrong ones to me too.
Tony

ahh911 · 05:39 AM

Sorry guys, to clarify, when I take a bolt in hand and just screw on the nut (not mounted, no torque) then just wobble the nut on the bolt I was just checking to see how loose the nut would feel. I went to a fastener store and tried the same thing, they do it as well, the originals would wobble too, but just a little less. I was just surprised that the fit wasn't a little closer. I guess the threads on the new oem bolts are just made to a slightly different tolerance to the originals, but there is not much difference between them, they are both sloppy, I thought maybe Porsche had a reason for doing so. The new bolts will torque up with new rod bolts well past the yield point, I tried and permanently elongated the rod bolt in a test fixture intentionally.

Dpmulvan · 08-25-2020, 05:55 AM

Hopefully the bolt you stretched was an old one.

dannobee · 08-25-2020, 06:58 AM

So you're saying the class of fit (tolerance) is different between the two bolts? Or that your old bolts are stretched? I don't know what class fit Porsche specifies for their rod bolts, but I do know this much.

Even though a bolt designation is, say, M10 x 1.5, it doesn't tell the whole story. There's actually a thread class fit, too. "Medium" class fit bolts can omit the class fit in the description.

Where an SAE bolt might be 3/8"-24 2A, an ISO 965-1 metric bolt would be M10 x 1.5 6g. If you wanted a tighter class of fit, the SAE bolt would be 3/8"-24 3A and the metric bolt would be M10 x 1.5 4h6h. Furthermore, the series of bolt can be either an M or MJ. MJ bolts have a slightly increased root radius, giving the bolt more strength.

ahh911 · 08-25-2020, 03:07 PM

Extra rod bolt and nut, the nut actually flares out to the diameter of the socket.

Dannnobee, new bolts, old bolts, new nuts, old nuts doesn't matter, it seems like there is a lot of wobble on nuts when you thread them on the bolts. I was wondering if Porsche had a reason for not making the specs a little tighter, they fit as hardware store quality. Maybe Porsche decided to have standard to loose fitment to minimize the impact of grit or particles when assembling at the factory using the torque method? Maybe Porsche decided that the fit doesn't really matter, maybe they thought under such high torque the bolt and nut threads need room to expand into each other, I don't know. Just wondering. If you ever have a rod bolt and nut from Porsche for the 10mmx1.25 style, give it a try, you'll see them wobble about.

Phil

stownsen914 · 08-25-2020, 03:25 PM

Different situation obviously, but years ago I bought ARP rod bolts for 2.0L rods, and found that some of the nuts wobbled more than I wanted. In my case, buying a few additional nuts solve the issue.

ahh911 · 03:54 PM

Could you believe I still haven't torqued down the 911 sc rod bolts? Been working with people and on my own test torquing with extra bolts that I have to figure out if Bentley was right about the 15ftlb+90 or go with the 41ftlb as Wayne says to do for older than 84 cars and Porsche spec'd up to the published 911 sc spec book in 1987. Looks like 15ftlb+90 takes you right up to the yield point of the bolts, but it comes back to original size. Very very tiring. I have several views of competent pelican forum member current shop owners/individuals/past shop owners. It's interesting but difficult to get consensus. I called Porsche today to see if there is any update on this area.
Phil

Jeff Alton · 08-25-2020, 08:57 PM

When you called "Porsche" who did you call?

ahh911 · 07:46 PM

Porsche Langley service department, a lady passed my message to the manager who was in charge of the rebuild, he was on holiday but replied nonetheless to her with the 20Nm on all the bolts with 00 sufix in the part number, then all direct to 55Nm. I asked about angle, she said no mention of it. She mentioned a name, I didn't write it down. I can find the name of the gentleman if you'd like me to, or I can call back. I saw you're in Langley as well, this dealership made a pretty nice 7 part video on an SC build last year and that's why I specifically called them, maybe a short drive and hop away for you

PM me if you want me to find out, but seeing as he's on vacation it makes no sense in calling this week at least. I'm not saying torque or angle torque, I just wanted to get to the bottom of this and it lies at Porsche.

Jeff Alton · 09-04-2020, 08:25 PM

Ok,

Let me do some checking. We are 1 block from them... Rob, the shop foreman there, is a great guy, very knowledgeable. We did the final CIS tuning on that car for them. They did a very nice job on the build.

ahh911 · 09-06-2020, 01:34 PM

Hi Jeff, thank you very much for your offer.

I spoke with Rob for sometime, very knowledgeable and helpful, indeed 50-55Nm oil on threads, 81SC.
I've done half of them today, the other tomorrow.

Phil