A good question.

[Neil Harvey]

Had a telephone call this morning from an owner rebuilding his own engine. He asked what I think was a great question. I thought I would share. This is his first Porsche 911 engine to be rebuilt but has rebuilt some V8 engines in the past.

His question was, do I need to send the Rod Bearings with the rods for resizing and balancing? The main concern was the balancing part. In the past, with the V8 engines he sent the bearing shells when the Rods were balanced. The Bearing shells were required not to balance the Rods but to balance check the crankshaft.

The Porsche 911 Crankshafts have opposed throws so one side balances out the other. Don’t confuse this with running balance. This applies only to dynamic balancing (checking rotating balance) when there are no combustion events happening. Crankshafts that do not have opposing throws require “bob weights” or simulated Rod weights added when checking the balance. In the calculation of these attached weights the Bearing shell weights and even some oil weight is included. This weight is a percentage, often what is called over balanced upwards of 50% or under balanced as low as 30%.

We do not need to add the Bearing shell weights as no added weight is required to be added to the 911 Crank. Balancing Rods is a check to make sure all Rods have the same weight overall and the same end for end weights. If you want to add the Bearing shells into this, you can. It will make no difference unless there is some small amount difference in the shells.

Where the big end weight with the shells is important is to find the max loading on the rod bolts. The engines RPM, big and small end weights etc are all factors in this calculation. Unless you are designing a custom rod with custom bolts this calculation is not necessary for the typical DIY assembler. You may be supplied with an RPM limit, usually well above the engines’ ability to make any usable performance. This should not be an excuse not to assemble the rod bolts correctly with the suggested stretch or torque limit, provided.

[PeteKz]

Neil, you have a lot more experience rebuilding Porsche engines than I do, but I concur with your analysis. Because the engine has opposed pistons, you do not need to calculate balance factors, as with a V engine. Thus the weights of the rod shells do not matter, as long as they are equal.

As for sending the shells for doing resizing of the rods, that makes sense in theory, but have you noticed any difference in practice? The shells should be manufactured to tight enough tolerances so that the rods should all be resized the same. However, several people here have found significant variances in the shells, causing too large or small oil clearance. First, I would measure the resized rods with the shells installed, and see what I've got.