Tech Tips: Rod Failures

This tech tip was published in the Veterans BMW Club newsletter Vol. 6 No. 1

I was approached by Richard about writing an article about some crankshaft connecting rods that failed. We thought the information would be helpful to others to keep them from loss from defective rods. Not all replacement rods are bad, but some have apparently slipped through quality control.

Photo 1

Photo 1

Photo 2

Photo 2

I saw my first crank with connecting rod problems early last year. The rods had broken off about midway on the rod on both sides. The customer stated that it was his first trip on the newly-built R69S motor; he was shifting from first to second gear when it made a bad sound and stopped. The customer stated that the pistons were easily removed from the cylinders from the top, so the pistons did not seize in the bore, and the motor had run a bit before the first time out on the road. I looked the crank over and checked it on the truing stand. It was out-of-round but not by a lot. I then pressed it apart and examined the rods and they were both copper-colored. I had seen this before in a replacement rod kit I bought previously, where I used the rod in a stationary display for a customer. The pins had severely scored the crank webs - they literally peeled metal from the interior of the pin bore. The replacement pins were then compared to original pins I had, and were found to be 2 hundredths of a mm larger (Photos 1 and 2). Note: Pins have an interference fit that is specified by BMW. Oversized pins don't allow for the specified interference fit. If they're going to be used, adjustments have to be made to maintain the interference fit. This R69S crank was no longer rebuildable and another crank was found to replace it.

Photo 3

Photo 3

The next crank with problematic rods that came my way had run for about 900 miles, as reported by the customer. The crank had developed a "rod knock" and was sent to me for examination. The crank had some out-of- roundness on the rear journal and the chrome plating that was used to build up the journal was starting to peel off. This was also noted on the front journal (Photo 3). This crank also had copper-colored rods on both journals. The crank was pressed apart, revealing the same damage to the crank webs from the pins as I had seen in the earlier R69S crankshaft. The bearing surface on the big end had broken down at the top of the rod on both rods. There was no discoloration so I do not think it was caused by lack of oil. The crank web was not rebuildable and was replaced.

Photo 4

Photo 4

The third problematic crankshaft to come my way also had a spontaneous failure. Both rods failed at the upper midpoint of the rod (photo 4). This motor had been run a short time up and down the street, and when the customer tried to shift to third, the motor made sounds you don't want to hear and stopped. When the motor was taken apart, it was discovered that part of the broken rod went through the side of the block, and was stopped only by the frame. The remaining part left on the crankshaft had sliced through the oil galley and pinned the lifter in the bore. The over-sized pins had also destroyed the crank webs. The block was repaired, and the customer found a used crank that I rebuilt.

With these failures, and ones I've seen since, I started to look for reasons for the failure. The first and easiest way was to conduct a Rockwell hardness test (HRC). An HRC of 10 is mild steel. Bearing surfaces will test to HRC 60 or better. I tested the failed rods and compared them with known factory rods starting from the bottom of the big end of the rod. Five factory rods (two R69S rods, two R60/2 rods, and one R50/2) were each tested at the same place. The findings were fairly close on the big end - the HRC test results were HRC 59 to 61. There was more variation in the lower mid section - HRC 28 to 35. The upper mid section and the wrist pin area both varied from HRC 20 to 25.

Photo 5

Photo 5

Two failed copper-colored-rods were tested - one that broke mid-section and one that failed at the big end. The rod that broke mid-section had the following test results: the big end HRC at 55 to 58, lower mid section HRC 25, and just above the break point the HRC was 10. The rod that failed at the big end tested out with HRC at 58 at the bottom, to 52 at the top of the big end causing the metal to spall. (Photo 5 -- note spalling). The mid-section tested at HRC 20 to 25 and at the wrist pin site HRC 20.

 

Conclusions:

The results suggest there are some problems with manufacturing quality control in heat treating and tempering of new rods. I realize Rockwell testing is not conclusive, but doing an HRC test of new rods prior to installation could help avoid disastrous results from a defective rod. Testing can be done at most machine shops, and is non-destructive, if done properly. (On a side note, I've seen and used "I-beam" rods from China that are also copper-colored. I HRC tested them and found them to be similar to the BMW factory rods they were replacing.)

Connecting rods need to be very hard at the bearing surface area. They need to be a bit softer in the mid-section to avoid being too brittle. Some rod makers make an insert of hard material (which has to be heat-treatable to an HRC of 60) for the bearing surface area, due to the difficulty of tempering the rod material.

Photo 6

Photo 6

Note: Photo 6 shows two bearing carriers. The left (silver-colored) is a factory carrier. The right is an after-market carrier that has no provision for replacing the rollers. If this one wears out, the crank rebuilder has to find a new cage and rollers to replace the worn one.

 

I hope this is beneficial to the readership.