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Whats the deal with connecting rod and bottom numbers? Can I mix them?

filiperosa

Well Known Member
Hello,

I was measuring my connecting rods and I got:
#1 top part: 686,5 grams, bottom: 165 grams, total: 851,5 grams
#2 top part: 691 grams, bottom: 163 grams, total: 854 grams
#3 top part: 692 grams, bottom: 168,5 grams, total: 860,5 grams
#4 top part: 684,5 grams, bottom: 167 grams, total: 851,5 grams

As it is possible to see there is 9 grams difference between connecting rods and also 9 between opposite connecting rods. My idea would be:
#1 top 2 plus bottom 1 total 856 grams
#2 top 3 plus bottom 2 total 855 grams
#3 top 1 plus bottom 4 total 853.5 grams
#4 top 4 plus bottom 3 total 853 grams

Like this, I would have maximum 3 grams between connecting rods and 1 gram between opposite rods. However, the overhaul manual says to assembly the rods with the corresponding numbers with the corresponding bottoms.

Can someone clear me on this? what is the best thing to make? Leave it as before or should I mix them as I was thinking?

Thanks a lot,
Filipe
 
As Bob mentioned, they must stay together. Also, gross rod weights are not the key balancing factor. In balancing rods, you are getting all the big end weights the same and all the small end weights the same. A special method and tool are used to weigh them at the big end and the small end. Not saying that yours are well balanced or not, but gross weight comparisons doesn't tell you if they are or aren't.

Please do not try to balance your rods without proper guidance and tooling.

Larry
 
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Thank you Bob and Larry,
I will place them in the same positions. However, I arrived another problem. I saw that serials of the rods much match and face forward the sump. I installed rod 3 and was installed rod 1 now. Serial down and the dot/notch up. However, this ended up putting on number 3 the dot facing the rear of the engine and the numbers can be read from the flange and the number 1 putting the dot facing the flange and the numbers can be read from the rear. Can you help me find what is wrong? Thanks, Filipe

Sem_T_tulo.png
 
One of them is backwards. Don't know which because I can't see the whole crank, but they should both look the same when sitting on the same side of the crankshaft.
 
If one I turn opposite I will have the numbers up and the dots/notche to the sump.

This is the picture of all the crankshaft.

image.png
 
Redid n3, now n1 and n3 have the dots/notches to one side and n2 and n4 to the opposite side. I think this is normal, right?

image.png
 
No! You can't mix them up! That big super precise hole that the crank bearings go in was finish bored and honed with the matching cap on it. Start mixing up the caps and you will get holes that are offset from the top and bottom half, out of round, etc. Sure recipe for trashing your crankshaft or maybe the whole engine.
 
The questions you are asking and your assembly mistakes would seem to indicate that you may not have the appropriate expertise to assemble your own engine. You would be much safer getting a qualified person to do it for you. If the engine parts are already overhauled then getting it reassembled should not be a costly exercise. An aircraft engine is a life support system. This is not an area where you want to put your life, and your passengers lives, at risk just to save a modest amount of money.
 
The questions you are asking and your assembly mistakes would seem to indicate that you may not have the appropriate expertise to assemble your own engine. You would be much safer getting a qualified person to do it for you. If the engine parts are already overhauled then getting it reassembled should not be a costly exercise. An aircraft engine is a life support system. This is not an area where you want to put your life, and your passengers lives, at risk just to save a modest amount of money.

Hello Bob,
I had rebuild one O200 on the past. My question was the flexible I had about mix them up since I knew I shouldn't do it according to the manual but not why or if it would not be a problem. Now I understood I am not able to make it and the reasons for this is why I appreciate the help on the forum. I did not make the mistake of mix the parts, just wanted to know if that was possible and the reason.

About the rod mounted with the number opposite, I realize the direction and this was the reason for my question. I am following all assembly according to with the manual step by step with the help and supervision of a certified mechanic, however, he know how to do the things but not the reason. My main aim to post on the forum was to understand not to do according to or against the manual but the why/reason of it.

Thanks!
 
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If it were a car...

Hello Bob,
I had rebuild one O200 on the past. My question was the flexible I had about mix them up since I knew I shouldn't do it according to the manual but not why or if it would not be a problem. Now I understood I am not able to make it and the reasons for this is why I appreciate the help on the forum. I did not make the mistake of mix the parts, just wanted to know if that was possible and the reason.

About the rod mounted with the number opposite, I realize the direction and this was the reason for my question. I am following all assembly according to with the manual step by step with the help and supervision of a certified mechanic, however, he know how to do the things but not the reason. My main aim to post on the forum was to understand not to do according to or against the manual but the why/reason of it.

Thanks!

If you were building a car engine your thoughts about balancing would be correct, but as mentioned in the previous replies, there ARE differences in aircraft engines.

Daddyman
 
I am not an engine expert, so no advice there. Just wanted to say don't let any negative feedback deter you form asking questions!

Tim
 
No, please no

Please don?t mix up or invert the rod caps on anything, aircraft or auto. Balancing of rods is not performed in any field that I?m aware of by mixing and matching. The rod caps are typically milled, bolted on, torqued and then line bored/honed to size making each assembly unique. In the automotive field once this occurs then minor sanding or grinding of certain areas of the rod is performed in order to balance. I?m sure others know much more than I about this however it is very alarming thinking someone would attempt to switch rod caps as it is certainly a recipe for catastrophic failure.
 
I am not an engine expert, so no advice there. Just wanted to say don't let any negative feedback deter you form asking questions!

Tim

Thanks Tim,
That was the primary objective of my question. An idea came to my head and I decided to ask about that idea. From the beginning, I knew what was on the manual but I think is always a good idea to discuss things and why something don´t work or it might work.


Please don’t mix up or invert the rod caps on anything, aircraft or auto. Balancing of rods is not performed in any field that I’m aware of by mixing and matching. The rod caps are typically milled, bolted on, torqued and then line bored/honed to size making each assembly unique. In the automotive field once this occurs then minor sanding or grinding of certain areas of the rod is performed in order to balance. I’m sure others know much more than I about this however it is very alarming thinking someone would attempt to switch rod caps as it is certainly a recipe for catastrophic failure.

Thank you all. I am not mixing up anything and not doing any balance. I saw the manual and something come to my head and I decided to come to the forum to ask the possibility. I understood that is not possible but more important than that. I understood why and good explanations and that was what I was looking. Thank you all for it.

Filipe
 

Now that we've thoroughly covered "don't swap the rod caps", so let's go back to the last photo. Per the manual, the numbers should face down, toward the sump. Number 1 is the right front cylinder. See the error?
 
If you were building a car engine your thoughts about balancing would be correct, but as mentioned in the previous replies, there ARE differences in aircraft engines.

Daddyman

This is incorrect. The rods in auto and aircraft engines are fundamentally the same design and balanced the same way. The OP's approach would be just as wrong on an auto engine.

Larry
 
Now that we've thoroughly covered "don't swap the rod caps", so let's go back to the last photo. Per the manual, the numbers should face down, toward the sump. Number 1 is the right front cylinder. See the error?


Hello Dan.
As per the photo all the numbers are facing down. My red circles are the dots/markers/notches, do not know the name in English since it is not my main language. But the numbers are down and the dots up. Do not see the mistake, can you point me?
Filipe
 
Hello Dan.
As per the photo all the numbers are facing down. My red circles are the dots/markers/notches, do not know the name in English since it is not my main language. But the numbers are down and the dots up. Do not see the mistake, can you point me?
Filipe

Ok then, you're good. I was mistaken; thought you had numbers up.
 
Ok then, you're good. I was mistaken; thought you had numbers up.

Thanks Dan Maybe I expressed myself wrong. Happy all is good. Measured journal diameter, rod with bearing Id.gap and side clearance all inside the values. Torque all 100 by 100 steps to a total of 480.
Filipe
 
Hello Bob,
I had rebuild one O200 on the past.

Dear Filipe, I did a Lycoming disassembly/reassembly course under professional supervision with Mattituck in 2006. I learnt a lot of things from that and in particular the following:

1. There are LOTS of little tricks involved in assembling a Lycoming. Failing to understand what all those little tricks are could be catastrophic.

2. Assembling an engine is like flying a plane....you need to do it regularly to be competent.

Despite having done the course I would not personally reassemble my own engine. I now know enough to understand the pitfalls awaiting the novice. And besides, getting a pro to reassemble a 4 cylinder Lycoming is just not that expensive. It?s 2 day?s labour.

As Clint Eastwood famously said: ?A man?s got to know his limitations?. ;)
 
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