IO360 connecting rod #'s up or down does it matter

[sansoneservices]

Installing all 4 connecting rods and removed them with the part numbers facing up. Service manual says numbers down. I carefully examined the rods and can't see any difference top or bottom. Anybody know why the manual states part number down?

 

[lr172]

Usually makers specify this because there is an offset in the big end bore (i.e. bore is not in the center of the rod). Sometimes it is the way they balance the rods. On shared journal cranks this is to match up chamfers. Have no details on lycoming engineering but strongly believe they do not put things like this in the manual just to make it longer or harder for the mechanics to follow it. Also, they often stamp the cyl number on the rod. Be sure whether they want the cyl number or the part number up. Been too many years since i last read it.

No real surprise they were installed upside down. I have been universally unimpressed with most aviation mechanics I have met. They are so intelligent, there is no need to read a manual or follow best practices.

 

[IO390]

The manual says numbers facing the sump, but doesn't specify if that's with all cylinders at TDC or BDC. Also, when the assembly is installed in the engine, two cylinders will be TDC where the other two are BDC. So on my engine we assembled it in this way - #1 and #2 at TDC and #3 and #4 at BDC all with the numbers facing the sump.

It could be that yours were installed as I mentioned above, and now you've placed all cylinders at either TDC or BDC two of them will now be facing the other way, but that isn't how they would be installed in the engine.

 

[sansoneservices]

Thanks for the input fellas. I looked through the engine logs and 20yrs ago the engine was tore down due to a prop strike. Interesting that the engine ran fine for that long with the rods upside down. I left voicemail with Lycoming tech support but no response after 24hrs. If there's any offset in the rods comparing them flipped on top of each other didn't reveal any. If there's no response from Lycoming today I'll pull the jugs and flip the rods regardless

 

[lr172]

The manual says numbers facing the sump, but doesn't specify if that's with all cylinders at TDC or BDC. Also, when the assembly is installed in the engine, two cylinders will be TDC where the other two are BDC. So on my engine we assembled it in this way - #1 and #2 at TDC and #3 and #4 at BDC all with the numbers facing the sump.

It could be that yours were installed as I mentioned above, and now you've placed all cylinders at either TDC or BDC two of them will now be facing the other way, but that isn't how they would be installed in the engine.

The down side of the conn rod stays down throughout the 360* rotation. It doesn't matter where the crank is at when you install them.

 

[lr172]

Thanks for the input fellas. I looked through the engine logs and 20yrs ago the engine was tore down due to a prop strike. Interesting that the engine ran fine for that long with the rods upside down. I left voicemail with Lycoming tech support but no response after 24hrs. If there's any offset in the rods comparing them flipped on top of each other didn't reveal any. If there's no response from Lycoming today I'll pull the jugs and flip the rods regardless

Unfortunately they don't tell us why they want it that way, so we don't really know how critical it is that we follow it. I speculate that Lyc dynamically balances the rotating assembly and therefore rod orientation must be maintained to insure that the balance remains within the limits. A quick look at lyc docs state that several components are dynamically balanced.

 

[IO390]

The down side of the conn rod stays down throughout the 360* rotation. It doesn't matter where the crank is at. The manual says "down." That means with the engine in its normal orientation, numbers point toward the ground.

What I’m getting at is whether you are looking at the numbers with the rods like this




Or like this




The manual does not specify, however I assumed it was the second option, so when installed in the engine the numbers face the same way.

However I’ve seen videos of people installing the rods oriented as per the first photo, with them all at BDC. Maybe this is obvious to engine builders and I’m just over thinking it.

 

[lr172]

What I’m getting at is whether you are looking at the numbers with the rods like this


View attachment 85841

Or like this


View attachment 85842

The manual does not specify, however I assumed it was the second option, so when installed in the engine the numbers face the same way.

However I’ve seen videos of people installing the rods oriented as per the first photo, with them all at BDC. Maybe this is obvious to engine builders and I’m just over thinking it.

Generally when an engine maker gives info like this, they are referencing the engine is in its normal running orientation. In your pics, you flipped the piston positions on 1&2 and that is why the numbers were on one pic and not the other. The top pic has the 1&2 rods pointing in the wrong direction.

 

[sansoneservices]

Lycoming finally called me back and said the connecting rods are SYMMETRICAL, no offset between the crank end and piston pin. They DO want the rod part numbers all lined up on the same side, facing the oil sump or facing the camshaft. That is for BALANCE purposes. The rod caps are indexed to only assemble in the proper orientation. CRITICAL importance is to be certain the rod caps are installed on the rod they were manufactured with. They are machined as a single mated component... Thanks for the replies here and hopefully this info will help a builder assembling his Lycoming 320 or 360

 

[Aden Rich]

There should be a cylinder number on the con rod top and cap, that will mirror each other when you mate them together and those two numbers point down at the sump. Not casting numbers or part numbers, the actual rod number. Usually if they are older rods, folks at A.S.S will stamp them after they have been shaved and rebored. Those two pieces now cannot be interchanged with other rod pieces.

 

[sansoneservices]

Yes the cylinder numbers were stamped on the sump side of the rods and caps. The part numbers were on the camshaft side of the rods. That's what prompted me to call Lycoming. The technician said face the markings on the top or bottom, doesn't matter, just place them all in the same direction for balance.