Balancing Connecting Rods

[Hubert_25-25]

I finally received my motor back from the machine shop.  This is a 1925 Buick straight 6 cylinder.  The machinist balanced the crank, pistons, and flywheel.  When he got to the connecting rods, he said he did not want to balance them as there is no place to safely remove any steel.  The greatest difference is 19 grams between the heaviest and lightest rod.  A nut weighs 6 grams.  I agree that this seems like a lot of steel to remove from mainly the big end cap.  These are pictures of the connecting rods and the ends.  

 

The firing order is 142635.  Since 1&6 are on the same throw of the crank, Another option could also be to put the heaviest and lightest connecting rods on these 2 positions.  Do the next heaviest and lightest pair for the next crank throws and so on, and this does look better.  

 

I included a spreadsheet showing at the top, the difference in the connecting rods if I were to grind off each end.  This appears still to be more than I will be able to grind off, but I could make it closer as I am sure I could take a little steel off.  In the lower part of the spreadsheet, I look at what the balance would be if I solely moved connecting rods around.   I can grind out the number and restamp it.  The forum will not let me paste an excel spreadsheet, but PM me if you would like it in that format.     

The machinist suggested just putting it back together, but it sure would be nice to balance these connecting rods.  

So I am concerned about being able to remove much more than 2 grams from the small end of the rods, and 5 or 6 grams is in my estimation max that I can remove on the big ends of the rods.  Renumbering the rods makes a bit of sense because I can pair the lightest rod with the heaviest rod, and put that on one throw of the crankshaft, since there are 3 throws on the crankshaft.  That would be option 1.  At least then my maximum grams out on the other 2 crankshaft throws are 6 to 7 grams instead of being at 19 grams out if I did nothing. 

This is where option 2 says that if I do a little grinding.  A maximum of 6 grams essentially on the rods in the middle of the weight range, I may be able to "in theory" get the weight balanced overall.  It is just different because you start with doing nothing to the the heaviest and the lightest other than pairing them up.  

Just wondering if anyone has done this or their thoughts.     

Thank you ,   Hugh 

Edited October 3, 2018 by Hubert_25-25 (see edit history)

[george gohlike]

Before grinding and restamping, be mindful not to remove too much. Usa file to take the minimum material off that surface because it is a locating surface for that part and if you are not mindful of this, it will be a problem when or if it ever requires babbit repair .One of the first things I look for before I start the rebabbiting is this feature. I will need to compensate my rod machine for each part .(extra time to correct)

oh well if so I have a Jolly Rodger candy (watermelon) wipe the tears away and “damn the torpedos”.

[Leif Holmberg]

Hugh.

Just remeber they worked well when they where new with that differans in weight.

I did weight my rods too years ago with the same differans as yours ,and didn`t do anything just becuse of the grinding problem.

 

Leif in Sweden.

 

[PFitz]

You have another option,..... add weight to the lighter rods.  It looks like there might be enough thread on the big end bolts to add a washer under the nut and still get a cotter pin in.

 

If you pick through a box of the cheap zinc plated hardware store washers you'll see that they are often stamped out of varying thickness of sheet stock. Because of that I buy boxes of them and sort them by thickness to use as shim washers. The fact they also vary in weight might help you. And you can also file the washers to get the weight closer to what you need, thus not having to do anything to the rods.  

 

Paul

Edited September 30, 2018 by PFitz (see edit history)

[stvaughn]

Maybe I’m missing something here but doesn’t balancing a crankshaft require adding bob weights to the crank throws? The weight of the bob weights is a calculation derived from the weight of the rod/piston assembly. Did your machinest allow for the different weights when he balanced the crank? You need to know. If so don’t change anything, just put it together. 

 

Have you talked to your machinest about what you plan to do? If you change something and the engine vibrates, he’s just gonna shake his head and say “to bad, so sad, you should have listened to me”. 

[Hubert_25-25]

Thank you for the comments.  

I did play around some with washers, but I was not making a lot of headway with balancing the big end and the little end of the connecting rod to go along with this.  On the balance scale set up for connecting rods, if you add a washer under the nut, this will lighten the little end weight just slightly because the fulcrum is the center of the big end.   So my lightest rod overall also has the lightest little end on the connecting rod, so a washer is not helping my situation.  A great thought though.    

 

Good question on the bob weights.  I talked to my machinist today and he said that he set up the crankshaft with no bob weights.  Since he found no good place to remove metal from the connecting rods, he felt it was best to just be conservative and leave the rods alone.  We also talked about moving the heaviest and the lightest connecting rods to cylinders 3 and 4 as these crankshaft throws are together.  The other next pair go on 1 and 6, then 2 and 5.  This boils down to Option 1 in my spreadsheet, which says 6 grams heavy on one pair of crank journals and 7 grams heavy on the other.  I am not getting the repeatability on my scale that I would like to be venturing to Option 2 so I do not think that I will be grinding any metal on the rods.  

If I leave the rod numbers as they are, I have one crank throw pair at 15 grams heavy and the other at 19 grams heavy.  By moving the rods, I am about 1/3 of this offset weight.  

All comments are appreciated on this subject.   Thank you,    Hugh

Edited October 3, 2018 by Hubert_25-25 (see edit history)

[automaschinewerks]

on a straight 4/6/8 engine, there is no need for bobweights because the 1-6, 2-5, 3-4 rod journals are equally spaced on the length of the crank from the centerline to the ends, and equally indexed at 120 degrees apart.  on a v8 type engine, there are 4 rod journals equally indexed, but not mating on an equal spacing from the center main on the crank.  that should be as clear as mud

for balancing big ends i ground off at the corners where the rod bolt heads, and nuts seat.  there is material hanging outside the head/nut that is doing anything for strength.  on the small ends i like the idea of washers on the pinch bolt, or on circlip rods i have ground a big chamfer around the pin boss area, and sometimes narrowed the small end width abit, but not too much though!

if grinding on the big ends, don't get the rod hot enough to melt babbit,  (pretty much blue color).  my former boss told me about an employee that did that, and i don't like getting screamed at, so i was careful not to star in that movie!

terry

[automaschinewerks]

i just looked at your figures, from what ive been trained, and read in balancing manuals, you need to lighten each big end to match the lightest big end, then turn rod around and make each small end weight the same as the lightest small end SEPARETLY.  so number 1 rod is lightest on both ends, 750 and 228, so all rod ends much be lightened to match the lightest figure. 

also weigh each piston and face off bottom of skirt to match the lightest piston.  pins should be weighed separately, but always seemed pretty close, if not perfect

[automaschinewerks]

i don't understand the  heaviest and lightest pair on cyls number whatever and next heaviest and light on cyl numbers something else. the idea to balancing is the make the rotating assemblys masses exactly the same.

and yes, i read now that he did balance the pistons

Edited October 1, 2018 by automaschinewerks (see edit history)

[stvaughn]

17 hours ago, automaschinewerks said:

on a straight 4/6/8 engine, there is no need for bobweights because the 1-6, 2-5, 3-4 rod journals are equally spaced on the length of the crank from the centerline to the ends, and equally indexed at 120 degrees apart.  on a v8 type engine, there are 4 rod journals equally indexed, but not mating on an equal spacing from the center main on the crank.

Thanks, learned something new today. 

[Hubert_25-25]

On 10/1/2018 at 11:49 AM, automaschinewerks said:

i don't understand the  heaviest and lightest pair on cyls number whatever and next heaviest and light on cyl numbers something else. the idea to balancing is the make the rotating assemblys masses exactly the same.

and yes, i read now that he did balance the pistons

Thank you for your insight as I am understanding this problem better as well.  The biggest issue is that one rod is much lighter than the other five.  19 grams is too much to remove from these old rods.  If I add 12-13 grams (2 big end washers) to the lightest rod, everything is in the ball park.

1) The conventional approach is to remove weight to get them all to the lightest rod.  I think what I need to do first is add a couple washers under the big end nuts on the lightest rods.  This will also require adding washers under the little end bolt of some to bring these rods back in line.  I was having trouble with this originally but it makes sense now.  Then I only have one rod that I will be grinding a max of 6 grams off of.  I think the rod can handle that but no more.    

2) So the other plan was a concept that will only work with a straight engine like my straight 6 which balances in 3 places 120 degrees apart.  Pair up the heaviest and the lightest connecting rods.  Put them on the same throw.  In fact, If I pair up the heaviest and the lightest, and add 6 grams of washers (2 thin washers) to the lightest rod, then I am within 1-2 gram of the other 2 pairs.  This does require renumbering 3 of the connecting rods.  I listed this as option 3.   Regarding balancing the small ends for reciprocating motion, these also move in pairs and I listed how the pairs look relative to the big end and small ends in the upper right.  They are within 3 grams so I might leave this alone unless someone has a better suggestion.  

Does this second note (shown below as option 3) look reasonable although not conventional?     Thank you    Hugh

 

 

Edited October 3, 2018 by Hubert_25-25 (see edit history)

[automaschinewerks]

your using washers under the nuts is a fine idea, just don't use cheap hardware store type washers, they may deform and lose abit of torque, i also use Loctite on the threads when i assemble and forget about the cotter pins and trying to line up the holes and slots.  get your torque value (3/8 NF = 40 ft/lbs), use Loctite and torque the nuts.  this will avoid trying to over/under tighten to line up holes for cotter pins.

now about your pairing up rods on mating journals. that works for static balance, which is if you are looking at the end of the crank and only seeing a pulley, or a stone being swung around on a string.  by keeping the rods and piston assemblys all the same weight, will keep the crank in dynamic balance, which means in balance along its length.  you go mixing up your rods and having heavier/lighter to one end of the crank just kinda takes away the benefit of dynamic balancing the crank.

so its best to use good hard washers, get the rods even weight, and don't mix up and regrind numbers. it makes things look like a dogs breakfast and I'm kinda fussy that way.

terry

and yes, trying to get 19 grams of a rod that doesn't have much of balance pad on the cap seems like eternity!

[chistech]

My machine shop just did all this on my 31’ Chevy stovebolt. I know he also milled some off the wrist pin ends to lighten the small end of the assembly. My lightest to heaviest was off 17 grams. There was no room for him to grind either. He got all assemblies to within 3 grams of each other. He also told me he used bobs weights on it also. He mixed and matched pistons with rods, used slightly longer pinch bolts where needed, etc. I can’t say exactly how he did everything as I’m not an engine rebuilder. He did say it was very difficult though. My motor runs great.

Edited October 7, 2018 by chistech (see edit history)

[Hubert_25-25]

Here are my final notes on balancing the connecting rods at home.  Thanks to those that gave guidance in the process.     Hugh

 

The machinist balanced the crankshaft, pistons with wrist pins, and the flywheel separately.  The original steel pistons were 40 grams difference between lightest to heaviest.  New aluminum pistons and wrist pins were equally balanced at 542 grams each.  Steel pistons and pins were around 800 grams each, so always nice to lower the rotating mass.

 

When the machinist got to the connecting rods, he said he did not want to balance them as there is no place to safely remove any steel on these old connecting rods.  The greatest difference is 19 grams between the heaviest and lightest rod.  A nut weighs 6 grams.  I agree that this seems like a lot of steel to remove from mainly the big end cap.  The machinist weighed both ends of each connecting rod, so all modifications and checks could be made using a more basic scale at home.

1)     Two connecting rods were light on the little end, so I added a stainless washer under the wrist pin bolt head.  Strange too was that the lightest connecting rod had the heaviest little end, so no washers on this end.  The wrist pin bolts were also replaced with drilled head bolts so that I could safety wire them in place.

2)     The connecting rods were “paired” starting with the heaviest and the lightest as a pair.  This decision alone put the max being out at 6 grams.  I will put these two rods closest to each other which means journals 3 and 4.  One connecting rod was much lighter than the others, so I opted to add washers under the big end nut, otherwise I would be grinding on 5 connecting rods and still not sure if I could safely remove 6 grams.  I used stainless washers as they are harder than galvanized hardware store washers.  I did grind some on the washers.  Then I replaced them with grade 8 hardened washers. 

3)     I purchased a 3kg/.1g digital kitchen scale online.  About $25.  This had good repeatability and measured to the .1gram because it is in the weight range for these connecting rods.     

4)     This set up required me to renumber 3 of the connecting rods.  Not really an issue.  I used an engraver because I did not want to bang on the recently re-babbitted connecting rods.  Probably should have looked at the balance before babbitting and then I could repunch the number.  No real reason not to move them around after everything else was balanced.  All new bearing surfaces and better balancing instruments than these old motors ever saw. 

Not entirely necessary for a low revving engine, but I was able to get the pairs within .1 grams of each other.  The machinist was going to just leave the connecting rods alone.  This would have put me at 19 grams heavy on one journal pair.  The original assembly with steel pistons had one journal pair out by 28 grams.    

Edited October 17, 2018 by Hubert_25-25 (see edit history)

[mikewest]

On ‎9‎/‎30‎/‎2018 at 2:09 PM, PFitz said:

You have another option,..... add weight to the lighter rods.  It looks like there might be enough thread on the big end bolts to add a washer under the nut and still get a cotter pin in.

 

If you pick through a box of the cheap zinc plated hardware store washers you'll see that they are often stamped out of varying thickness of sheet stock. Because of that I buy boxes of them and sort them by thickness to use as shim washers. The fact they also vary in weight might help you. And you can also file the washers to get the weight closer to what you need, thus not having to do anything to the rods.  

 

Paul

Paul are you serious about suggesting "Cheap zinc flat washers under connecting rods?" Would you install Franklin rods like that? I hope not. I think that idea would get you thrown out of most quality shops. There are better solutions.  Just my opinion...

 

[PFitz]

12 minutes ago, mikewest said:

Paul are you serious about suggesting "Cheap zinc flat washers under connecting rods?" Would you install Franklin rods like that? I hope not. I think that idea would get you thrown out of most quality shops. There are better solutions.  Just my opinion...

 

Mike,

Try looking up the "crush specs" of grade 2 hardware store zinc washers and then figure out what happens to them at only 40 lbs. torque before you think someone should be "thrown out" of anywhere. I only suggest it because I know it works from experience. What's your experience using them in that type of application ? 

 

Paul

[mikewest]

3 hours ago, PFitz said:

Mike,

Try looking up the "crush specs" of grade 2 hardware store zinc washers and then figure out what happens to them at only 40 lbs. torque before you think someone should be "thrown out" of anywhere. I only suggest it because I know it works from experience. What's your experience using them in that type of application ? 

 

Paul

Paul,

II have no experience using " cheap hardware washers on rod bolts" and never will. Im glad it works out for you. Your friend , Mike

 

[jp928]

Not very relevant, but I see two different firing sequences quoted above, and neither match the one I have always used - 153624. Whats that about please? I know there are lots of v8 sequences, but 4s and 6s I have only ever seen one order used.

Enquirings minds....

jp 26 Rover 9

Edited October 19, 2018 by jp928 (see edit history)

[Hubert_25-25]

It is the same pairs 1-6, 2-5, 3-4 but perhaps being older the firing order was not a standard.  The first time I looked at this I wrote down 153624 as this is from memory, but then I decided to look in the shop manual to be sure that was how they did it back in 1925.  Sure enough, firing order is 142635 which is just backwards so maybe its a chain driven vs gear driven timing that they decided to use a backwards firing order.  Hugh