Connecting rod bolt tightening specifications

[jbranch]

The shop manual I am using for my Boxster 2.5L rebuild gives two different tightening specifications for the connecting rod bolts. The pre-torque is the same for both, but for the connecting rod they designate as "7R" the final torque is 90 deg. For the rod they designate as "8R" the final torque is 110 deg. Does anyone know what these designations are and how to determine them?

[jbranch]

I learn that the R number is stamped on the rod so I check the rods. Shure enough, there it is. Only one small problem. My shop manual gives specifications for the 7R rod (90 deg final turn) and the 8R rod (110 deg final turn). My rod says 6R (or course). This implies to me that I may not want to do 90 degs on the final turn. If anyone just has suggestions as to where I might start looking for this info, I would really appreciate them.

[J-Dub]

Glad to hear you are taking on the rebuild your self however I have no knowledge of what rod bolt these rods use or what torque specifications to use. I just tried googling and got nowhere, likely just like you did.

Between friends, are we going to notice the difference between 90 and 110 degrees? As you are torquing them try to feel the bolt stretch. You can tell the initial stretch and then give it a bit more. See where that lands you on a few of them, might gain you some confidence.

[jbranch]

This stuff wouldn’t be fun if it were easy. In my first post I indicated what the shop manual said about the 7R and 8R requirements. That was in the Specifications part of the manual. However, in the procedure for installing the rods, the manual goes a different direction. It says that this procedure applies to all 986 engines, and all previous information is no longer valid. It then says to apply a final torque of 110 deg. Perhaps Porsche made the change to address the rod bolt problems they were experiencing, and the person who made the changes only made them to the procedure and missed the specification section. Or maybe my shop manual wasn’t even written by Porsche. We’ll never know. I will probably torque them to 110 deg. Before I do that, I am going to measure the new Porsche bolts I have ordered and compare them to the bolts I just removed. These bolts have only been torqued and stretched once and have never seen service. If they show significant permanent stretch, I will stay at 90 deg torque. If they show no permanent stretch, I will torque the new bolts to 110 deg. I suppose all of this will seem silly to an experienced engine re-builder, but right now it makes sense to me. I do agree with your suggestion that it probably doesn’t make any difference.

Another confusing statement in the shop manual procedure is:

CAUTION!
Connecting-rod bolts must not be re-used after unscrewing because they cause damage to the engine.
- New connecting-rod bolts must not be screwed on more than 4 times during mounting.

I don’t know what all of this means, but it is really confusing for an amateur like me. Does the second line mean torqued and stretched, or just torqued? And what is “damage to the engine”? Oh well, I have new bolts coming anyway. My mission in life is to make the parts suppliers happy.

Thanks for your input and advice.

[J-Dub]

They are saying that the rod bolts should not be stretched more than 4 times. After that many stretches the bolt manufacturer cannot guarantee that the stretch amount for a given torque value will be the same. By the book for a given amount of bolts stretch the faster should provide a certain amount of clamping force. For this it helps to not think of the bolt as a solid object however it is really a spring. As you know as metal heats it expands and as it cools it shrinks. The goal is to have no chance that as the rod shrinks there is ever a point where all of the stretch is taken out of the faster. Hard to imagine that but rod bolt designers have lots of time to think about this.

At some point this is a topic for F1 engineers to worry about and the average guy in the garage, you and I, can simply bolt the engine back together taking some considerations for clearances and fit and let the hard work to the designers of the parts. If it works for the other 100,000 2.5 liter engines they made that year it will work for you as well.

[jbranch]

That's good and helpful info and it makes sense to me. After I made my last post I re-visited the procedure. It was a little clearer then. They were talking about doing an initial torque to 15 ft-lb no more than 4 times, but that did not include the final torque to 110 deg. It's now clear to me that they only intend for you to do that final torquing once.

I'd like to get back to Yuma the middle of next month but I have to get this car running before I do that. Shouldn't take long once I get the case halves back together.