Don Terrill’s Speed-Talk

Just wondering, on a stock engine with original component weights, how far off has anyone seen the stock OEM balance job, GM, Ford, Etc.. thanks

One of the worst offender I've found has been SBM. Have run into rods from the same engine that were 20+ grams out. They are bad enough I use a band saw to trim the balance pads instead of grinding 'em. Not unusual to see stock rotating assemblies 50-70 grams out when spinning them up. The late model stuff is considerably better.

Just weighing stock components on V8 GM's I have seen 15gr + difference in parts many times, I can't tell you how much correction it took to balance the assembly as I don't keep track of how much the assembly is out of balance. On V8 engines we find they are out quite a bit yet some 4 cyl engines will hardly be out 3 grams stock. Older V6 Buicks were out as much as 25gr. on components.

440 6 pack rods. 42g difference from light one to heavy one.
pistons were within 15g.

406 ford aftermarket old school pistons. 30 ish grams off
looked someone home cast them and machined.

I had a BBM 440 , 6 pack set of rods that one was out a total of 90 grams, IIR corectly it as 35 gm heavy on the pin end 55 heavy on the bottom. Also saw where 2 sets of 6 pack rods were completely ruined, because the guy wanted to lighten them up , just by how they looked. I could not make a balancable set out of the 16 rods. That was about 10 years ago....the guy still gets a red face when I "mention" them .

Wow,that's some difference.Did these engines run reasonably smooth before balancing? The often wrong but often correct Smokey Yunick observed high speed speed film from running engines with see through oil pans.There's a mass of oil clinging to the crankshaft and it changes position constantly. Yunick assumed this oil weight,maybe a pound or more,will affect engine balance so precise balance isn't possible or getting it to the tenth of a gram isn't necessary.Is this just more Smokey tall tales?

wil8115 wrote:440 6 pack rods. 42g difference from light one to heavy one.
pistons were within 15g.

406 ford aftermarket old school pistons. 30 ish grams off
looked someone home cast them and machined.

With respect to Chrysler mfg, these are accurate numbers, not as high in every unit, but some.

It was more "normal" to find between 25 and 30 grams difference in conn rods in almost every RB unit. This was very common.
Pistons were somewhat closer.

Thanks, Gary in N.Y.

P.S. From all my factory balancing experience Chrysler was without a doubt the worst. Having said that and having been with them for a few years (engine specialist) there were no "vibration" complaints ever in the factory units. Only the Hemi's held closer tolerances within each unit.

GOSFAST wrote:

wil8115 wrote:440 6 pack rods. 42g difference from light one to heavy one.
pistons were within 15g.

406 ford aftermarket old school pistons. 30 ish grams off
looked someone home cast them and machined.

With respect to Chrysler mfg, these are accurate numbers, not as high in every unit, but some.

It was more "normal" to find between 25 and 30 grams difference in conn rods in almost every RB unit. This was very common.
Pistons were somewhat closer.

Thanks, Gary in N.Y.

P.S. From all my factory balancing experience Chrysler was without a doubt the worst. Having said that and having been with them for a few years (engine specialist) there were no "vibration" complaints ever in the factory units. Only the Hemi's held closer tolerances within each unit.

Back in '71 I had a 340 Duster that would buzz the shifter and clutch pedal when hitting 4000 RPM. Would definately relate the buzz to engine out of balance since you could dupicate with the car standing still. I didn't complain to the dealer but still considered it as an issue.

I've balanced engines, afterward I'd get compliments they never had a engine that ran so smooth. If a balance job is noticable to a layman, can't figure why at the factory level Mopar engineers never caught on. Remember reading in the old Direct Connection books, engine balancing wasn't recommended since they were balanced very close at the factory....Hahahaa

FWIW, Packard claimed the industry-best balancing on their '55-56 V8s. The parts were statically balanced, crank dynamically balanced and then the assembled engine was run on a balance stand and the front damper/flywheel drilled and/or tabs welded on the flexplate, as needed. Must not have been cost-effective, as Packard went out of business and no one else ever did the assembled balance AFAIK.

The other thing I've learned about forged rods is one cannot look at the amount of material left on the balance pads and guess which one is heaviest.

On my performance Packard V8 rods, there seems to be a lot of extra metal not adding any strength, so I do a lot of machine work to lighten them. They're wide and the outsides of the big and small ends are rough, so I make them round in the lathe. Then, I grind off most of the balance pads top and bottom and install ARP bolts prior to balancing. On one particular rod, which looked like all the rest, it ended up so light I had do a lot of polishing on the sides of the beams of the others to get them down to the weight of that light rod. Could never determine where it was light.

Has everyone else noticed the powdered metal rods are pretty much dead nuts on?

jack vines