Inliners International Forums General Discussion Engines Connecting Rod - Weight and Balance

Greetings . . .

Spent some quality time with a triple beam balance and a set connecting rods I purchased off ebay several years ago. They are machined for insert bearings - and were sold as ready to go.

I am weighing them for balance and was surprised by the variability:
Assembly...Rod....Cap...Pin Bolt..Rod Bolts..TOTAL
A..........475.0...202.5...21.9.......88.7......788.1
B..........456.8...172.0...22.5.......88.8......740.1
C..........461.5...221.6...22.0.......88.1......793.2
D..........445.4...198.3...22.5.......87.6......753.8
E..........458.3...186.4...22.0.......89.4......756.1
F..........466.8...185.0...21.9.......88.7......762.4
Average..460.6...194.3...22.1.......88.6......765.6 grams

Max Var....29.6....49.6.....0.6........1.8..... 53.1 grams

Ave Var..6.426%...25.5%...2.711%....2.033%....6.936%

Abs Var..6.646%...28.8%...2.740%....2.055%....7.175%

With so much variability I am wondering if they are worth any more investment in time. Perhaps I should pursue another set?

I guess I will grind the parting lines off the beams and weigh them again. But the one 'runt' cap will cause me to have put the others on a 25% diet . . . seems extreme . . .

Thoughts?

Thanks in advance.
stock49

Do you have more rods to choose from?

All I have is the babbitted set that came out of the car. I had put them aside with the stock crank because the bearing surfaces are still serviceable. The rods were babbitt serviced by Federal Mogul . . .

Might be worth a second look . . .

Do you have any or much varaibilty with pistons? Just thinking light weight pistons/heavy rods might help lessen the difference.Merry Christmas Jay.

Thanks Jay . . .

I am stroking 3/16 with a Hi-Torque crank. The pistons are custom built from Ross. I expect them all to weigh the same . . .

I would weigh them too . . . but I had them drop shipped to the machine shop so the machinist could final hone the bores to Ross' spec and directly measure clearances . . . the shop wouldn't have it any other way.

You'll need to find another set. Removing the parting lines will only take off around 5 grams, so it won't be that much of a reduction. Also, the babbitted rods are real flimsy in the beams and everywhere else so its doubtful you could trim enough off of the ones that are the closest in weight, much less the heavier ones.

Thanks CNC-Dude . . . I figured as much. I am going to spend some time with some mics trying to understand where the additional weight is coming from. There has to be a reason for the variance . . . they all have the same part number.

I am not going to pretend that I know anything about this engine.I think that the pin is too big to use the 261 rod? but what about later model 235 rods? Will they work? There is not much going on right now, give me a number that you are looking for and/or what other engines' rods will work and I will keep my "eyes open" for parts. Jay

Since he is limited to using the babbited rods, that is the only style he can use, as the later insert rod is not compatible with the early crankshafts.
Stock49, the weight variation in the rods, even with the same part number is just the nature of the beast. It is normal to have a 10-15 gram difference in a matched set of rods, except in aftermarket rods where the manufacturing processes are much more precise.

Just did a little work on the parting lines of the heaviest rod. You were spot on about those 5 grams!

My mics have revealed other variations. There are flats ground into the rod side and cap sides - the big end balance pads. There is a lot more meat left on the heavier rods. Also the pin bolt boss' are ground differently. I am going to even these up and see where I get.

I have also found quite a bit of variation in the dimensions of the caps. The shoulders that extend off the caps (that mate with the troughs in the oil pan) are all different. I am going to true those up.

You are right about sticking with Babbitt style rods - they are the only kind that will work with the jet oiling system . . .

Before you get too deep into this, you might want to verify that the housing bore of the rods is correct for the journal size you have ie, standard crank/standard rod housing bore, .010" crank journal/.010" housing bore, etc... An undersize housing bore will have more babbit in it, thus more overall weight, and that could be why there is such a weight difference between some of them also.

Hi CNC-Dude . . .

Just to clarify. These are Babbitt 'style' rods - but they have been machined for insert bearings. A must for any boost in compression ratio. So we are talking the weight of bare rods and caps . . . worth some tinkering no?

As for journal sizes. The crank is an NOS Hi-Torque just magna-fluxed, polished and balanced at the machine shop. So the bearings will be standard not oversize.

The Babbitt angle seems to be confounding the thread. 'tlowe' asked if I had any more rods "to choose from". In fact I have two sets: the machined for inserts set that I am working to 'balance' and a set of babbitt lined rods that came out of the previously running engine. This latter set would require machining for insert bearings to enter the fray . . . $

regards,
stock49

When doing an insert conversion you have to also get pressurized oil to the rod journals, since splash oiling cannot provide adequate oiling for a bearing. How are you going to do that?

Looks like you need to replace rods B&C with rods that weigh closer to the rest. Ideally rods that are babbitt type that have been cut for insert bearings. Is this correct? I don't know of any around here right now but will be on the lookout at swap meets ect. Maybe someone else here or at stovebolt has a set of rods with the same problem, the 2 sets could be combinded to get both parties clooser to equal weight. I would love to see a picture if possible of one of the rods. Wish I could do more to help. Jay

The babbitted rods are going to be a lot heavier than the finished machined insert rod, so its going to be difficult to estimate what rod weight you'll need to start out with to arrive at the inserted rod weight. As keep in mind that 60 years ago when these rods were made, the set you have may have been considered to be a matched set and close enough for GM at that time, since they didn't weigh things in grams back then, only pounds and ounces.

The use of insert bearings with the jet oiled system has been a rebuild option for quite sometime now - for practical reasons. There are very few places that pour babbitt anymore.

When setup correctly the jet oil system is good for around 15 psi at the rods. This limits the RPM and C/R in a jet oiled engine. But the thickness of the babbitt linings is also a factor.

Roger Huntington discussed early mods to babbitt rods in his book. The shops would first pour brass as a 'shim' and then pour a thinner babbit lining. This allowed for higher C/R loadings that would have distorted the normal stock spec babbitt linings.

I am following Huntingtons specifications for jet oiled clearances but with insert bearings.

Grams versus ounces is certainly a factor here . . . but I have since read that GM typically would match rods/pins/pistons in pairs for balanced placement on sister journals (1-6, 2-5, 3-4) . . .

Guess I will have to work in pairs as well . . .

Thanks Jay . . . for now I am going to try to work with this 'set' . . . using the term loosely. I bought them for $100 seven or eight years ago. In hindsight I should have bought two sets - I would have more to choose from!

Chev's of the forties has a pic of a machined rod:

Funny how VW in Germany in the same time period (well from the 50's at least) picked their rods to fit within 8 grams of each other, if one was heavier, it was rejected and put in a pile of "next weight group" with other heavy rods. You saw three different weight group rods produced through the years and it is even noted in most decent manuals.
I match mine to within 1 gram, and caps to each other and big ends to each other. Then wrist pins and pistons, if I have a "heavy" piston, it is sometimes easier to remove metal from inside the wrist pin at the end, than it is to reduce the weight of a piston. You don't need to shave nearly as much steel compared to aluminum.

The end result is not only matched weights, but matched centers of weight (If that is how you call it) on the length of the rod/piston assembly.

Haven't checked my Nova rods, maybe I did when I first had them out and replaced the pistons, pretty sure I did.

That will work for a 4 cylinder that has 2 opposing cylinders to counteract the imbalance, but because of the 120° crank angle of a 6 cylinder crank, it takes 3 cylinders to counteract the imbalance because they never have another cylinder 180° away to oppose the weight difference. So you will need(2)sets of 3 R&P, not(3)sets of 2 R&P to do that.

Just woke up and read that and did not understand a thing I'm sure it makes sense but not to me right now, being sick and barely awake.

In VW land, the different weight group rods go in different engines. Not mixed in the same engine. All 4 are within 8g.
So one engine has rods from the same weight group only.
I have found it fairly easy to go from 8g to 1g accuracy.
I know race engine builders that go to 0.1g accuracy... I think that's overkill but if you have a scale that can do that and time to kill, go for it.
We built 10.000rpm aircooled VW engines that ran just fine with 1g

Thanks Stock 49 for the photo. To me that rod looks thin compaired to todays product,yet millions of Chevies ran with them with no trouble. I also understand that when new these rods came with poured in bearings and just splash oiling. Can you tell me what a jet oiling system is and what is a safe rpm/compression limit for this kind of set up.Just trying to learn more about these engines. Jay

Those millions of chevies also made no power or compression per cylinder to speak of

This is one cheap way to make wrist pin oilers, just notch the rods. I have done this on VW rods with no ill effects, a well known trick and some engines are made this way. Wrist pin lubrication is still only splash oiled, but this adds a more directed, additional splash aimed towards them.
http://www.youtube.com/watch?v=7wuNuuEoN7Y

Edit: looks like the rod in the picture above already has a squirt hole where the neck begins, 12 o'clock.

Here is another way to make a stationary oiler/squirter jet by robbing oil from the main bearings instead: http://hotrodenginetech.com/pin-oiler-prep/

I believe BMW used poppet valves at th eend of each squirter in some of their engines (I watched a VW builder adapt these jets to an aircooled VW case) which helps maintain good oil pressure at idle, when oil pressure drops down, and the poppets would only open when there was more oil pressure available.

Here's another way to do it with add-on pressurized hard lines aimed directly at the writ pins/piston tops:
http://mngforce.typepad.com/nc450vdev/engine-modification-piston-cooling-oil-jet.html

Hi Jay . . . 'splash' oiling is a misnomer. There are in fact hard lines to jets in the pan that supply oil under pressure to the rods. There is an opening in the bottom of the rod cap and dipper to catch the stream of oil.

The 1937 short "Riding the Film" documents fully what was then the cutting edge of GM technology:
1937 Short by Jam Handy

'37 was the first year for the 216. Based on the writings of Roger Huntington the stock oiling system is good to 150hp with a redline in the low 4000s. One needs to convert to full pressure to rev higher or put out more ponies . . .

regards,
stock49

The stock oiling system supported 105hp in the powerglide cars of '50 thru '52. California Bill Fisher's speed manual and dyno testing was all done with the stock oiling system delivering 125 to 150 HP depending on the mods.

The pin oiler hole is part of the stock system. Some shade tree guys would peen the hole over thinking it would raise the oil pressure . . . only to starve the pins . . . :-(

Many did experience problems with these rods, but not because of the babbited end so much as it was the pinch bolt design that resulted in failure on these rods. Thats why the GMC rod conversion was so popular, even for just mildly modified Stovebolts. They swapped right into the babbited engines with no mods required to the rods or crank. For the later full pressure engines there was some minor fitting that had to be done, but you ended up with a bulletproof set of rods by doing it.

Stock 49, that film completely explains the rod oiling system. The only question for me is why anyone would go to the trouble of oil jets and dippers when all you have to do is drill the crank,a very easy job at the factory IMHO compaired to all that plumbing! I am probably missing something but can't a machine shop drill the crank and get full pressure to the rods? Maybe the way the crank is made there is not room inside it for the oiling holes to be drilled without "breaking through". CNC-Dude, am I correct in beliveing that GMC rods are pressed on at the pin instead of bolted with a pinch bolt? If so it's pretty easy to see a big improvement in strength. Thanks everyone for taking the time to help me learn about these engines. Jay

A question that can only be answered by some long since dead engineers from 1930's GM . . . or perhaps the accountants from the same time period.

Transferring oil pressure from the mains to the rods involves so called 'rifle' drilling from the crank journals to the rod bearings . . . this must have been more expensive then the plumbing . . .

For me I am just working with what GM delivered at that time . . .

Great video, I know nothing about the older chevy engines and I didn't realize there is no pressurized oil feed to rod bearings.... amazing... that it works at all

All the GMC rods are full floating with a bushing. They also have full oil pressure to the wrist pins as well. Many top shelf aftermarket rod companies have now begun to incorporate that feature into some of their rods as well.

Just trying to learn something. How hard are GMC rods to get? Are they big $$$ ? IF Stock 49 used them would the engine have to be converted to full pressure oiling? Would the expence be worth it or are you (Stock 49) trying to stay with the original system on purpose, just like some people convert to an open driveline while others want to keep the torque tube. Jay

GMC rods are cheap and plentiful. This swap began long before the insert rods were available for the Stovebolts, so they were used successfully in the pre-full pressure blocks. The GMC rods are also longer than the Stovebolt rods, so there are a few benefits associated with that as well for using them. You also have to remember that the average hotrodder back in the late 40's and early 50's was straight out of WWII and had a craft or trade skill they had acquired on Uncle Sam's nickle, most everyone had welding and machinist type trade skills. So doing something like that was no big task for them to undertake.

The big truck engines that come to our shop all have full floating pins with bushings. The rod is drilled from the big end to the small end so that the pins get get full oil pressure. I have never seen a pin failure or any visable wear. We do not check them because the piston,rings,pin,and liner are all just replaced with new and reconditioned rods are put in. On the newer engines the pistons are 100% steel. How they stay together being that heavy amazes me. Are GMC rods drilled through the center also? Jay

Yes, the GMC rods are drilled through the center.

Thanks CNC-Dude. I think this would be a good upgrade to a jet oiled engine. Having seen pictures of some of Stock 49's engine and parts here (they look like works of art) he could easly do the conversion if that is desired. IMHO no one can see inside the engine and there is no way to tell if this has been done by the sound ect. makes using GMC rods the way I would go for increased durabilty. To each his own. After all a lot of people think I am crazy for running an intergrated cylinder head on purpose. Jay

Over a decade ago I decided that I would not transplant an engine into my '49 fastback. I decided instead to pursue a California Bill spec 216 'hop-up' that preserves the stock chevy oiling system. Nothing has deterred me.

The planning is long since done and the build is now underway. The pistons have been forged. The block, crank and head are back from the machinist . . . and I am ready to start assembly.

The disparity in weights of the connecting rod set (currently in hand) is but a minor setback. I have just purchased 3 NOS rods for 10 bucks a piece. I will send these off to the machinist to have the big ends prepped for insert bearings.

With 9 rods to choose from the bench mark I am targeting for this aspect of the build is:

Rod: ~450 grams
Cap: ~185 grams
Pin bolt: ~22 grams
Rod bolts: ~89 grams
Piston: ~535 grams
Pin: ~94 grams

Total weight: 1375 grams

Stock 49, I know what you mean when you say "Nothing has deterred me". I am the same way when it comes to using a intergrated cylinder head on my 250. IT would have been easy to transplant a 235/261/ or a later six into the 49. Can you please post the the mods done to the engine? Iknow about the stroker & the dual carbs but nothing else. Where can I get a copy of the Speed Manual? Thank you for the good opertunity to learn about these engines. My Aunt had a '50 powerglide 2 door that I remember riding in as a kid. Found out about a year ago that my cousin (who is still alive) got it as a first car! I will have to talk to him to find out more. Jay

This new book should be out at the end of Jan. 2014

Looking forward to the new book Scott.

I like the idea of building a 216. They have been beaten up for years mostly by those who don't understand the oiling system. For longevity and dependability I'd take a 216 over a Ford flathead every time. In fact I did several times.

Thanks Beater. Im starting off the new year with(2) brand new books. I'll have a printed proof for my first book next week, and then the first 200 books will be shipped within 10 days of that. Then the Stovebolt book will follow afterward. I just bought (10) ISBN numbers so they can be distributed through major retailers. I plan to write and publish the remaining (8)books in 2014 as well.