Connecting Rod Balance

[TimberHole]

I am rebuilding a 3 cylinder Yanmar and will be replacing one connecting rod. I purchased a used rod which I believe was cast 10 or 15 years later than the originals. This rod is heavier, I'm thinking 14 grams give or take but I need to re-weigh all the components. These rods do not have balance pads for grinding. Even if I buy a new Rod it isn't necessarily going to weigh the same.

How important is Rod balance in these relatively slow turning diesels? How close is close enough?

 

[pmsmechanic]

Does John Deere parts list different connecting rods? I'm assuming it's a Deere.

 

[bigrich954rr]

Weigh both ends is one end heavier then the other or is the weight equal across?

Are you having the assembly balanced?

 

[Pete O]

Not important at all.

 

[rScotty]

I am rebuilding a 3 cylinder Yanmar and will be replacing one connecting rod. I purchased a used rod which I believe was cast 10 or 15 years later than the originals. This rod is heavier, I'm thinking 14 grams give or take but I need to re-weigh all the components. These rods do not have balance pads for grinding. Even if I buy a new Rod it isn't necessarily going to weigh the same.

How important is Rod balance in these relatively slow turning diesels? How close is close enough?

How much does the rods weigh? Can you tell where the extra weight is?

There are several ways to balance crank/rod/piston assemblies. One way is what you are doing - which is to assume the crank is balanced and that only the rods are different. That may or may not be true, but it's a pretty good bet that it is. And anyway, that assumption is the only choice you have if you don't want to take the crank out of the block, or if the old rod can't be weighed for some reason.

I have a pretty good Yanmar 3 cylinder rebuild manual and there's nothing in there about balance. If you want to grind a little on the big end - mostly around the big end cap - then I can't see that hurting anything. Hopefully that's where the extra weigh is. But without taking the whole crank out and doing a dynamic balance on all the parts I think you don't have much choice but to bolt it together as is. That's a stout engine & turning slowly. I think you are overthinking the importance of doing that much balance on this engine. My bet is that you never notice whatever imbalance. How far do you take this? Do you balance your injector spray?
best of luck,
rScotty

 

[Langanobob]

Not a Yanmar expert but 14 grams isn’t all that much for a relatively slow speed engine and I don’t think your engine will know the difference. Especially since the extra weight is probably spread over the length of the rod and not all concentrated at the big end. What happened to the original rod? Is it too far gone to have it machined?

 

[TimberHole]

Thanks for the great responses.

This is a Yanmar 3T90J out of a John Deere 950. I did pull the crank to have it turned. This engine ate a rod bearing and buggered a journal. I don't think the original rod can be repaired and additionally the rod shows signs of heat. I'm not comfortable using that rod.

The rod part number is CH10683. Beyond the part number each rod has a casting number on the rod. I believe only rods from the same casting number were installed on each engine. Was this to introduce some very basic, crude balance to the assembly? I'm not sure.

Here is a pic of an original rod. I believe the casting number 78-5 indicates this rod was manufactured in 1978 and was the 5th batch.



This is the best image I have with me of the replacement rod. It came out of a 950 and is dimensionally correct. The casting number is different. I'll have to look at it tonight but it has a boss with 94 stamped on it. I'm assuming this is a much later rod.



As far as overthinking this..........I've been accused of that before but I don't think that is the case here. Understand that I'm not trying to build a "balanced" engine, just wanting to make sure I'm not using components that are far beyond a reasonable out of balance condition. I will try to reweigh the rods tonight and report back the results.

Thanks again for the help.

 

[LouNY]

It is most likely not critical, but if you have an accurate enough scale to weigh the big and small ends of the other two and and have enough material to bring the new rod to the same weight at both ends as the average of the others it shouldn't hurt and I would be tempted to do the same.

 

[Langanobob]

Don't think you are overthinking this. It's a lot better to get everything right the first time. Most guys probably wouldn't even have weighed the replacement rod. You're being smart and conscientious, not overthinking.

 

[TimberHole]

Weighed the rods tonight. The 78-5 rods are original and the 47-2 is from the salvage yard.

Total Crank end Piston end

No. 1 (78-5) 2# 137.5g 1# 207g 445g

No. 2 (47-2) 2# 154.5g 1# 215.4g 455g

No. 3 (78-5) 2# 131.8g 1# 205.5g 439.4g


Total difference 22.7 grams
Crank end difference 9.9 grams
Piston end difference 15.6 grams


I am surprised the larger difference was at the small end of the rod. Should be able to help that by weighing the pistons. If I do that I think it looks ok. What do you guys think?