Connecting Rod Orientation

Can anyone explain to me why lycoming connecting rods are supposed to be installed so the rod identification number is facing downwards towards the sump at all times? The rods are symmetrical so why does it matter? I?m assembling my engine and realized that two of them were backwards when I was dropping the crank into the case. I?m guessing I can?t reuse the connecting rod bolts and nuts and need to buy new ones since they have already been torqued once?

I can’t answer why the need to face down, but since that is what’s specified I would want to fix those that are not. And yes, you will have to replace those very expensive bolts and nuts since they are one time only stretch bolts.

Because that's the orientation when they were line bored. Yes, theoretically, they should work either way, but believe me, I have seen problems when this orientation was not followed.

Mel said:

Because that's the orientation when they were line bored. Yes, theoretically, they should work either way, but believe me, I have seen problems when this orientation was not followed.

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Thanks. That makes sense.

Oil squirt

I don?t know about lycoming engines, but I?ve seen some engines where the rods have a little peephole at the parting line, and installing correctly, it?ll squirt the cam.

Mel said:

Because that's the orientation when they were line bored. Yes, theoretically, they should work either way, but believe me, I have seen problems when this orientation was not followed.

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Mel, way more important here is the fact that your next post will be number..... 10,500 You are entitled to a new keyboard .....

gasman said:

Mel, way more important here is the fact that your next post will be number..... 10,500 You are entitled to a new keyboard .....

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Thanks for pointing out! I hadn't noticed. OOPS, Now I've done it! Please contact me at BR-549 for delivery instructions.

Taltruda said:

I don?t know about lycoming engines, but I?ve seen some engines where the rods have a little peephole at the parting line, and installing correctly, it?ll squirt the cam.

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That is what I always assumed was the reason.

Taltruda said:

I don’t know about lycoming engines, but I’ve seen some engines where the rods have a little peephole at the parting line, and installing correctly, it’ll squirt the cam.

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Update: I just looked at many photos of Lyc connecting rods and am reminded there are no holes, they and machined all over and some are polished (almost) all over. I will +1 Mel's comment. They had to put the numbers somewhere.

Mel said:

Because that's the orientation when they were line bored. Yes, theoretically, they should work either way, but believe me, I have seen problems when this orientation was not followed.

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from my understanding it is because of the orientation when weighed. The connecting rod is laid on it's side (Holes horizontal) and secured by each end, then weighed at the other end (they weigh big end, small end & total weight). Material is removed from strategic locations until the weights are in spec (many shops will endeavor to get all weights into a tighter spec, like 1 gram). While it looks symmetrical, it is not necessarily so from a weight stand point. If you flip it over, the weights will not necessarily be the same.

Many auto engines also have the big end slightly offset from the rod centerline. Not sure if that is the case for Lyc rods. It is not usually obvious visually. Even if the are centered, there would be some machining tolerance for the hole location that could affect the small end weight, based upon orientation.

Larry