The 'Maverick' Classic Porsche Forum and Classic Porsche Club for Porsche 356, Aircooled Porsche 911, Porsche 912, Porsche 914 & Porsche Replicas. 'We join for the cars but stay for the people'
Con rods on 911s are now well beyond their initial design life and whilst failure is rare it does seem worthwhile to spend a little time reconditioning them to improve confidence in their condition.
We start by checking that the rods are not bent or twisted and then re-size the big end.
Small ends are replaced using Colsibro Bushes which provide an improved performance compared to the standard bush.
The rods are then balanced end to end and finally shot-peened to ensure that any accumulated fatigue damage is removed from the surface. The rods are peened using a #230 Austenitic Stainless Steel Shot to an Almen A Level of 6-10 and a 200% coverage. The shot is screened continually to ensure and broken shot particles are removed which guarantees the integrity of this process.
The rod bolts are also checked to ensure that they still behave elastically and the nuts are replaced using a high strength 12 point nut.
Changing bolts is an interesting idea and seems to be an accepted practice but the current cost of geuine Porsche Rod Bolts seem to vary between £10 to £25 per bolt depending on engine type it can be quite costly.
The early version of the Workshop Manual (Volume 1) describes rod bolts as 'malleable' which is probabaly a poor translation suggesting that the bolt may yield when fully torqued.
This is now taken to mean that 911 rod bolts are 'stretch' bolts but the concept of Torque to Yield Bolts wasn't developed until the Eighties so I doubt this was the reason.
The design theory of a rod bolt is an interesting concept and is one of the main reasons why rod bolt failure is relativley rare.
The preload applied to a rod bolt should always exceed the alternating loads that the con rod sees in service as this prevents the bolt from being subjected to the alternating stresses that could cause metal fatigue. By acheiving a sufficiently high preload we do not need to design for fatigue and a simple 'strength of materials' approach is sufficient.
If the preload used is insufficient to acehive this objective then bolts would experience fatigue loading and failure would be much more commonplace.
If the bolt is tightened beyond its yield point but to a stress which is lower than its Ultimate Tensile Strength it will perform in a satisfactory manner but it would be wise to replace a bolt that has been subjected to this type of deformation.
In general most bolts are designed to operate in their Elastic Regime and typically the maximum recommended torque is equivalent to a preload of 80% of the bolt material's tensile strength.
ARP Rod Bolts are always used 'elastically' and the recommended tightening regime for rod bolts is to use a Stretch Gauge rather than applied torque as this is a much better indication of preload.
The simple way to test that a bolt is still 'elastic' is to measure the length of the bolt in a unloaded conditon and record the value. The next step is to fully tighten the bolt either to torque or to stretch and then uload the bolt and ensure it returns to the original length.
When we re-size the big end we tighten the bolt to its maximum preload prior to checking/correcting so measuring bolt length before and after we carry out this process is neither complex or time consuming.
If the bolt doesn't return to its original length after tightening it should be replaced.
Later versions of the Workshop manual do not descibe rod bolts as being malleable and only suggest replacing the nut.
The 9mm bolts used in the later 3.2 Rods are a different proposition and I do believe these bolts commonly yield but we are not refurbishing this type of rod as they do seem to have some issues.
It is intersting to calculate the preload in a 911 rod belt when stretched to the limit suggested by ARP and to compare this with the preload developed by a bolt tightened by a Torque + Angle method which Porsche used in later versions of the Workshop Manual.
The ARP Bolt would have a preload that is around 15% lower than the bolt using the Porsche method and would therfore be closer to the point at which fatigue loading could occur, I asm sure that this would not be an issue but the observation is interesting.
We re-size using the Torque + Angle tightehning method hence producing the greatest potential preload and if the bolt is still elastic after this loading history I am confident that it is fit for purpose.
The metallurgy which supports this conclusion is quite straightforward and very well documented.
I am sure many people will still change rod bolts but this will not invalidate the refurbishemnt process providing the same tightening methodolgy is appled and we have an instruction sheet we will supply with a set of rods.
The Nuts we are supplying are new and have not been used during the evaluation process as we use the original nuts which are then discarded.
Sorry to be long winded but it was a serious question that needs a considered response.
