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-   -   Whats the deal with connecting rod and bottom numbers? Can I mix them? (https://vansairforce.net/community/showthread.php?t=164743)

filiperosa 10-03-2018 10:28 AM

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

rocketbob 10-03-2018 10:33 AM

No you can't mix them.

lr172 10-03-2018 11:04 AM

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

filiperosa 10-03-2018 12:18 PM

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


j-red 10-03-2018 01:33 PM

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.

DanH 10-03-2018 02:02 PM

Clue: when 1 and 2 are at TDC, 3 and 4 are at BDC.

filiperosa 10-03-2018 02:07 PM

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.


filiperosa 10-03-2018 02:08 PM

Quote:

Originally Posted by DanH (Post 1292973)
Clue: when 1 and 2 are at TDC, 3 and 4 are at BDC.

Got it! number 3 is mounted opposite way. Because should be on BDC and is on TDC. Thanks!

filiperosa 10-03-2018 03:16 PM

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?


brian257 10-03-2018 06:47 PM

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.


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