Tech Tips

Diagnosing Connecting Rod Problems

Written by  February 29, 2008

Winter has been so weird this year and you just can’t help wondering when will be that magical day to get your bike back on the road for another fabulous riding season. However, have you addressed the items that plagued you last fall? Were those items really bad, like a knock in the bottom of the motor? Well, if you had an engine that was knocking the last time you remember it, here is the problem with this tech tip. This is for the very serious weekend warriors, and if you even consider tackling this repair and fixing your engine you have many parts and pieces that will need to be addressed. Here we try to cover the high end items; however, I have received e-mails from the readers and they would like to know more about the crankshaft and connecting rods that are currently installed in your engine.

Always in the past I have stressed the need to purchase the correct service manual because it covers so many different items and the engine itself. One of the easiest ways to identify a major connecting rod bearing problem is to really pay attention to the used engine oil after it is drained from the oil pan. The next thing is to inspect the oil filter and even cut it in half if it is a spin-on canister type because this will truly show you the metal pieces in the bottom of the engine that have made it into the oil filter system. If you find any kind of copper chunks or little pieces in the oil or filter you will need to make a decision whether to stop or have the shop fix this. I have been sitting on an engine with this problem and it just so happens that the # 2 connecting rod bearing failed and it produced the exact problems we are explaining in this article. At the initial teardown of this engine the crankshaft did suffer damage and was needed to be repaired by Falicon Crankshafts of Florida. They do a super job and you can have them do the hard work and have the measuring part of this procedure done for you! So if you have decided to inspect your connecting rods and crankshaft, be prepared to spend many hours in this project. You will need to have several special tools and friends to help you get the engine out of the frame and back in when complete!

Since this tip is so labor intensive I just want to cover the basic info on the plain bearing type crankshafts. These types of systems have also been referred to as “Babbitt” style as well, and these are in a great percentage of the sport bike and cruiser type power plants.

Rod bearing failure is a problem where the bearing prematurely wears out around the crankshaft journal, and you will most assuredly know the sound because it is a deep knock in the lower part of the engine. What needs to happen in the repair of this situation is the engine must be removed from the motorcycle and basically needs to be disassembled and cleaned for inspection of the worn parts. Since this is the main rotating assembly the engine will just keep rotating until the connecting rod actually breaks and possibly even comes through the engine case. If your engine has not had this scenario as of yet then you may get lucky and only have to have the crank fixed and the replacement of rod bearings and gaskets to put the engine back together. Now I know that a great percentage of you do not have the means or tools to even consider this, and that’s ok! If that’s the case, please feel free to take your motorcycle to a quality repair shop and get that engine fixed before the spring thaw hits our doorstep. Most shops have a flat rate guide to these repairs and can give you a pretty accurate price to this engine repair.

Crankshafts that are equipped with two piece style bearings have color codes on the bearings, and this remains true to the metric world. The bearing color tables have measurements of thousands of an inch when it comes to these bearing sizes, and this is explained in detail in your service manual for your exact engine. Now don’t be nervous when it comes to these codes of different thickness because the crankshaft usually has stamped on one of the webs the actual codes, like AABBA or whatever the case may be. There is a trick to this, and when you see how easy this is to understand you will be 10 steps ahead of the other people that just don’t try. If your crankshaft has received damage, then it will truly have to be sent out for repair. Here is a tip: ALWAYS ship your parts ground because FedEx and UPS will get these parts to the destination in just a couple of days. So, if you have a crankshaft that needs to be reworked, ask them what size the fixed journal is after repair and this will aid in the ordering of the bearing selection. For those of you that just prefer to have a “new” crankshaft and rods for the motor rebuild, this will still have to be measured with plastic gauge and micrometers. However, the new crank will have no wear and the codes will be easily identified on the crank web, and this will give you the crank journal codes for the cases as well and make the repair a lot faster and back to service condition.

After you have determined what color of bearings you will need, I highly recommend that you check everything one more time because the phrase “measure twice cut once” stands true in this case, and it just makes good sense when you are repairing an engine down to this level. Selecting the appropriate bearing colors will be the biggest challenge that you will face, and when in doubt, call your local shop and see if they can assist if you find yourself lost. This is a problem that many engines have had, and rest assured this repair has to be done upon an overhaul situation. That’s why this procedure costs so much.

So the moral of this tip is when it comes to rod bearing failure, catching this right away can also save you dollars because when a connecting rod breaks the engine cases most assuredly will be damaged and those items are “not” cheap. So change and check your engine oil for any copper looking pieces, and if the engine is running than now is the time to have the oil pan removed for further inspection and repair if need be!

B-safe out there!

Dave Miller