Connecting Rod Balancing

[Matt Wilson]

Good afternoon all,

I've got a question that has lingered with me for many years.  Not sure why I didn't ask someone knowledgeable before, but here goes.

When I had the connecting rods in my 230 balanced many years, as part of the rebuild, the machinist said the rod weights were so far off from each other that he had to take off a LOT of meat from some of them, and boy, he sure did.  He completely removed the ribs from the caps on a couple of those rods.  This alarmed me, as it seems that those ribs are there for strength.  The 230 rods are different from rods I see on other engines, which usually have pads on the caps, and those pads don't contribute to the structural strength of the cap, and metal can be removed from those.  I ended up getting another set of rods and taking them to a different machinist, who removed only a little material from the ribs.  I never had an issue in running this second set of rods in my engine, but I only put less than 10,000 miles on the engine before removing it for other problems.

So, my question is:  Is it a big deal to thin down those ribs in order to balance the rods?  Is that the purpose of those ribs?  Or will this be weakening the caps and asking for a trashed engine when a cap fails and a rod and piston fly out of the block?

Also, are the bigger engines - 251 and 265 - also set up this way, meaning with ribs?  Or do they have actual pads?

 

Thanks,

Matt

[DrDoctor]

  Back in the day when I was much thinner, and younger, I drove a slingshot dragster, only because I was thin enough to fit into it. We were using a Chrysler 392 hemi with a GMC 6-71 blower, Enderle injectors, along with all of the related sophisticated goodies on the inside, and the outside. Anyway, I remember going along with the car’s owner to take a new reciprocating assembly to the machine shop for balancing. The rods were Keith Black, and had pads on both ends, so each rod could be balanced end-to-end, as well as to one another (the lightest one), before they were spun-balanced (after the crankshaft had been balanced by itself . . .). I recall the car’s owner, and the machinist, commenting about the rod’s rather large ribs. The machinist stated that he couldn’t/wouldn’t touch them, as they were an integral part of the rod’s strength, and he refused to take any responsibility for compromising their integrity in the name of lightening them even more. Besides, with the pressure being packed into the cylinders with the blower, a weak rod could yield catastrophic results, and since I was the one sitting behind that monster motor, I wasn’t too keen on having such “a catastrophic result/event” occur in front of me, with me travelling in its direction. So, given that background, I’d say that messing with the ribs, which are for strength, would be compromising the rod. However, the pads are typically oversized specifically for machining to achieve balance. I guess the key here would be to not go overboard with removing material from the pad – only remove what’s necessary to get all of the rods in the set to match the lightest one. Thx.

[Andydodge]

Agree with the Dr............I've only had a couple of engines balanced however in both cases the metal was removed from the pad on the cap.......not from the ribs.....I would have gone somewhere else if the ribs were to be thinned...........my 2 Oz cents worth......andyd 

[Matt Wilson]

Thanks for the responses, guys.  I agree that thinning the ribs seems to be asking for trouble.  Andydodge, just to be clear, there were no pads on the caps on my 230.  There were only ribs.  I suppose one way to remove weight without thinning the ribs would be to remove a bit of material from the cap surfaces that mate with the main body of the rod, as well as those opposing mating surfaces of the rod.  Of course that then requires the hole to be opened up back to the correct size to fit the bearing inserts and be round again.  Seems like this could be a tedious and expensive process, but maybe the only way to do it properly.

[Andydodge]

Am certainly not an expert here but have seen somewhere where they remove some material from the edges or beam of the rod and also on the edges where the caps fit up to the big end.............I'd be finding a local engine shop or maybe speak to an aftermarket rod manufacturer..........would be interested in seeing what answers you come up with.........andyd   

[DrDoctor]

   My direct experience with balancing an engine’s internal reciprocating assembly is with the Elephant motor I described above. I’ve purchased an engine or two that had already been internally balanced, but wasn’t involved in the process. Andydodge is right – the rod end caps may have some extra “meat” on them (perhaps around the bolt-hole passages) somewhere that can be sacrificed without compromising the integrity of the rod itself. But, I’d go with his recommendation and check with some reputable machine shops, and/or manufactures of engine internal parts. Regards . . .  Thx.

[Matt Wilson]

Ok, I'll see what I can dig up on this.  I figured as much as people talk about it being a good idea to balance these engines, there would be a number of people on this forum who would be very familiar with that.

[Tom Skinner]

I believe I read in "The Restoration Bible" by Matt Joseph, Page 100, and I quote:

"Balancing is very optional in Old Engines" "The simple fact is the Radial Thrusts generated by small imbalances in most engines are minor if those engines are low speed long stroke mills. Most older Engines fit squarely into those categories." in other words your wasting your money. He also goes on to say removing to much  material from Connecting Rods Ribs or Pistons can actually cause catastrophic failure. Also mentioned on page 99 - "Balancing Long Stroke Old Engines is a Fruitless gesture". Please don't blame the messenger here, but I would tend to agree with Matt, because I have rebuilt these engines before and one can instantly see that tolerances left much to be desired on old flatheads back in the day. They tolerate about as much slop as one could imagine and still run good. They ain't racing engines.

Tom Skinner

Edited December 20, 2016 by Tom Skinner
spelling

[pflaming]

I had my rods and matching pistons balanced by removing material from the inside of the piston. I possibly threw away some money but the machinist was about as old as am and he had balanced thusly for most of his career.  I asked him what the benefit balancing gave. His response was, not that much on your engine but your pistons are not close so balancing will smooth out the engine and take some pressure off the bearings. As to performance, negligible in your case. THe $$ was reasonable so I had him proceed . FWIW. 

[Matt Wilson]

Perhaps it's not necessary to balance the rods, but for as long as I've owned this truck with the 230 engine in it (26 years), I've always heard of people recommending to do it.  Maybe those folks were talking about race engines, but I had made it clear that mine would never be used in such a manner.  Or maybe those folks were just blindly passing along info they had heard, but I doubt it as some of those folks were well-experienced with Chrysler/Dodge flatheads.  In any case, as pflaming said, if it's not that expensive, then it's probably not a bad idea to do it.  As I recall, it didn't cost much when I rebuilt my 230, many years ago.  An additional qualifier I would add is that if it doesn't do any harm, then it's probably not a bad idea.  Hence my original question about whether it's ok to remove material from the ribs of the caps.

It's interesting - I found some photos of 251 engine rods and 218 engine rods and they both seemed to show some kind of raised pads in between the ribs.  That would be a good spot for removing material, I would think.  But what's odd is that the rod caps out of my 230 have absolutely no raised/sacrificial pads between the ribs.  Nothing at all.  In fact, there are numbers forged there, so it seems clear that material is not meant to be removed from between the ribs.

[pflaming]

Matt: Please note, nothing was removed from my RODS, the removal came out of the pistons so when completed, each rod/piston stayed together. The balancing weight included the rod from point 1. I would not tamper with the rods. But I'm no authority, just relating what I did. 

[DrDoctor]

   I’ve looked into this balancing thing, and learned that with engines having long strokes (such as the flathead engines being discussed herein), the necessity for balancing isn’t as pronounced as for engines with shorter strokes, such as the small-block Chevrolet. With that in mind, I’d agree with both Matt Skinner, and pflaming – balancing the pistons would probably be advisable, but taking material from the rods, which don’t have that much material to spare, is most likely not a good idea. Further, it was suggested to me by a machinist here that compromising the rods could yield in a catastrophic failure of a rod, with possible collateral catastrophic damage to the engine. Now, I’m not saying it will happen, only that it could happen. Now, it’s your engine, and thus your choice – how much of a gambler are you? Thot's to ponder . . .  Thx.

[Matt Wilson]

Hmmm, good points, guys.  I guess I really need to ponder this thing.  And pfleming, my apologies, I guess I should have re-read your earlier post before quoting you with an inaccurate quote.  :-)

[Dodgeb4ya]

Don't grind them ribs for sure....

 

 

Edited December 25, 2016 by Dodgeb4ya

[DrDoctor]

   If you’re dealing with rods as depicted by Dodgeb4ya, he’s 100% correct!!! Don’t, under any circumstances remove any material from the ribs, or you’re asking for a catastrophic failure of the rod, and most likely of the engine, as well. The rods pictured are “dainty”, at best. They do their intended task, but don’t expect to do it if you’ve compromised their only reinforcement. Again, Dodgeb4ya is 100% correct – don’t grind them ribs!!! Thx.

[Matt Wilson]

Thanks again, gents.  I've never seen a rod cap with 3 ribs, like the one pictured.  That's an interesting one.  Anyway, as advised, I will avoid doing anything to the ribs.

[Dodgeb4ya]

On 12/22/2016 at 9:46 AM, The Dr's In said:

   If you’re dealing with rods as depicted by Dodgeb4ya, he’s 100% correct!!! Don’t, under any circumstances remove any material from the ribs, or you’re asking for a catastrophic failure of the rod, and most likely of the engine, as well. The rods pictured are “dainty”, at best. They do their intended task, but don’t expect to do it if you’ve compromised their only reinforcement. Again, Dodgeb4ya is 100% correct – don’t grind them ribs!!! Thx.

I must tell you those rods are not dainty by any means they are heavy series six cylinder dodge truck rods

[Flatie46]

Is there a difference in the car rods and truck rods in a 218 or 230?

[Dodgeb4ya]

52 minutes ago, Flatie46 said:

Is there a difference in the car rods and truck rods in a 218 or 230?

The rods/caps I posted a picture of are of a 413 heavy series DT 413 flathead and use three ribs for strength. 

The 218/230 con rods are the same whether used in a car or truck.

[Elwood]

Since I'm finishing up balancing and polishing a set of 1554530 connecting rods for a T-245 230 engine, I can confirm (1) that there is a lot a weight variation among these rods, even ones that are clearly from the same forging dies, and (2) that those ribs on the bottom of the big end cap are for strength, and are not balancing pads, to be removed as needed.  I started with about fifteen rods to choose from, and the variance from lightest to heaviest was 44 grams, on a rod design that averaged about 805 grams.

Balancing rods involves matching them as closely as possible (my maximum tolerance is one gram, with 0.5 gram being the target) across the total weight, and the weights, measured using a connecting rod balancing fixture, of the big and small ends.  Balancing only the small end (either by removing material from the small end or the piston) takes care of the reciprocating mass, but does not balance the rotating mass, i.e. the big end.  And unbalanced rotating forces reduce smoothness, which reduces bearing and component life.  Even a small amount of imbalance can results in pounds of force, which increase exponentially as RPMs increase.  So yes, a slow turning engine such as our flatheads, with slower piston speeds, do not generate as much stress due to imbalance as a short stroke, high-revving engine, but they do benefit from balancing.

When I started my balancing and polishing project, I decided (1) not to remove any material from the big end cap ribs, (2) to remove little, if any, material from the area where the beam spreads out to meet the crankshaft bearing insert (the area where the rod is most likely to fail), and (3) to eliminate as much as possible rough edges and sharp corners, which are stress risers.  Looking at the rods, I was amazed at how crude they are in some respects: the area around the bolt bosses in particular is very poorly finished, with evidence of metal surface tearing where the bosses were broached (check the side flats where the odd/even cylinder number is stamped), and sharp corners.  And when I measured the rod caps by themselves (as a guide to where I needed to remove weight on the big end), I found an uncomfortably high amount of variation there, too: over 14 grams from lightest to heaviest.

Removing the forging flash from and then polishing the sides of the beams, smoothing the sharp corners around the bolt bosses, and carefully removing material from the top of the small end as needed brought the average rod weight down to around 770 grams, with a total variance of not more then 0.5 gram.  The small ends balanced out to about the same tolerances, but I was only able to get the big ends to within about 0.8 gram variance.  But balanced and polished, these rods are likely stronger than stock, and smoother with less stress on the bearings and crankshaft.

[Dartgame]

Assuming the rods are to be reconditioned with new bushings and the big ends resized. My thought is to balance the rods to (counter logically) to the heaviest or the middle of the set prior to reconditioning., by adding material to the big or small end as needed, with welding. Or by removing weight as Elwood describes above. Adding weight is Probably not practical and detrimental to the metallurgy - so this might be a completely stupid idea.

I certainly do not like the idea of removing ribs etc., that will weaken a given rod. The rods in these motors are spindly enough as is compared to a mopar small block.

Has anyone looked at the specs on a slant six rod ? I wonder if they could be a substitute- but I seem to recall that the 23 inch motors rods are offset some how ? So maybe not.

Edited May 4, 2017 by Dartgame

[Elwood]

10 hours ago, Dartgame said:

Assuming the rods are to be reconditioned with new bushings and the big ends resized. My thought is to balance the rods to (counter logically) to the heaviest or the middle of the set prior to reconditioning., by adding material to the big or small end as needed, with welding. Or by removing weight as Elwood describes above. Adding weight is Probably not practical and detrimental to the metallurgy - so this might be a completely stupid idea.

I certainly do not like the idea of removing ribs etc., that will weaken a given rod. The rods in these motors are spindly enough as is compared to a mopar small block.

Has anyone looked at the specs on a slant six rod ? I wonder if they could be a substitute- but I seem to recall that the 23 inch motors rods are offset some how ? So maybe not.

I've never heard of anyone welding onto a forged rod, either for balancing or repair, so I'd assume there's good reasons not to do so.  Generally, one wants to balance to the lightest rod in the group, because (a) it's a lot easier to remove material than to add it, and (b) lightening the rods results in less stress on the internal components (it also allows the engine to rev up faster, although that's usually not the objective with these flatheads).

I don't know that I'd describe these rods as "spindly".  Sure, they're not H-beams, but they also don't have to contend with the same stresses as a short-stroke, high revving LA V-8 engine.  They can be balanced without touching the two ribs on the big end cap, and by removing the forging flash on the sides of the beams, followed by polishing and shot peening, they'll definitely be stronger than stock.

The slant six rods (and the big block Chrysler/DeSoto/Dodge Truck flathead rods) will not fit, if for no other reason than the offset big end on the small block flathead rods that you mentioned.

[Matt Wilson]

Well, the good thing is that I don't have to worry about whether to remove material from the ribs or not.  I have the engine I'm going to rebuild now, and its rods have weight pads on the caps, and on the top of the small end of each rod.  What I have is a 251 that I'm converting to a 265.  The 251 rods and the 265 rods all have these pads.  I was concerned that they would just have ribs and no pads, like my 230.  Like Elwood, I noticed that the finish on the rods is not as good as I would hope.  There are rough edges and such.  I may try to smooth those out.

[Dartgame]

I am familiar with rod balancing under normal circumstances, as Elwood points out you match to the lightest rod. I've had rods balanced, shot peened, and reconditioned with new rod bolts for mopar small & big blocks in the past. They have the balance pads needed to make them a matched set.

I admitted my idea was probably unworkable, because of the potential for damage to a given rod. It was just a passing thought.

Since you can apparently balance them by smoothing out flash and so on that makes more sense. 

Elwood, are you able to get replacement rod bolts for these motors ? Eventually I see myself building one of them,and am curious about that.

Too bad the 218/230 rods don't have balance pads like their bigger brothers.

[Matt Wilson]

Yes, I'm also interested in getting replacement bolts for the 265 rods, if anyone knows of a source.  The bolts are very specialized for these rods.  The bolt heads have a triangular shaped protrusion that fits into a recess in the rod, so that the bolts won't rotate.  They are definitely not your common, off-the-shelf bolt.  My machinist says that most people just reuse their bolts.  He's not talking about old flathead rod bolts, but I'm sure it's fine, as long as mine are in good condition.  Still, I'd prefer to find some new (or NOS) ones if I could.  Any leads?