Don Terrill’s Speed-Talk

The vintage Chevy inline 6's and GMC 228-302 engine have staggered main bearing sizes on the four mains. From front to rear each main is a bit larger in size.I don't know if some other engines are like this......Since is more expensive to machine the block,it there a functional purpose to this?

Without diving into the SAE literature, the reference below suggests that this was an attempt to pre-emptively dampen nasty straight six harmonics.

https://en.wikipedia.org/wiki/Engine_balance

Different main bearing sizes on a crankshaft create plane imbalance in slide friction.

Many stovebolt owners ask to have them line bored all to the same diameter as the later 6-cyls and used those bearings.

When toyota used the chebby 235 as inspiration for their own straight 6 that was in the old land rovers, they did keep the staggered mains!
The Jeep too, the........ah, what did they call it? ah, the GO_DEVIL! also 3 different sizes.
that's four american ones I that were very common, and I line bored quite a few. I don't think I've done the GM tho.

Don't forget porsche/vw. Was the vanagon the last engine to use different sizes? Darn good trivia question.
Airplane engines probably still do have different sizes. Many are just big old flyin Volkswagens.

i assume they did it for the same reason they do with cam bearings bores. Why did chevy do it? I'd guess it would be a way to speed up production with whatever method they were using to bore the main housings, but I'm not sure, I don't know how they did it. Before my time. But It may have been something more like a giant reamer rather than line boring with single point tools.

The majority of old 3 bearing fours, and four bearing sixes.... each bearing pair is unique anyway, so, they might as well make them different sizes. Why not? I think the only reason they made the crank slightly different sizes is so they use the same thickness bearing shell material.
The top of the line engines at that time usually had all the same size, like packard. Packard straight eight had aluminum block with steel bearing shells. I guess when you have 9 main bearings then we make them all the same size LOL

And no ist isn't more expensive to line bore. It takes me the same amount of time, maybe an extra 10 minutes, but I don't have to hone it! It does require more faith, but I don't think faith is billable.

Kevin Johnson wrote:Without diving into the SAE literature, the reference below suggests that this was an attempt to pre-emptively dampen nasty straight six harmonics.

https://en.wikipedia.org/wiki/Engine_balance

Different main bearing sizes on a crankshaft create plane imbalance in slide friction.

That's an interesting possibility.
Another possibility is this principle would also be true in line-boring.

The journals could be made smaller as you go toward the front because they would have to handle progressively less torque. A weight saving technique?

https://etda.libraries.psu.edu/paper/18878/17294

Kevin Johnson wrote:Without diving into the SAE literature, the reference below suggests that this was an attempt to pre-emptively dampen nasty straight six harmonics.

https://en.wikipedia.org/wiki/Engine_balance

Different main bearing sizes on a crankshaft create plane imbalance in slide friction.

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Interesting link.

1 - I believe Considering friction (an "internal" force) in any inertial balance calculation is misplaced.
" Dynamic balance refers to the balancing of these forces and forces due to friction."

2 - That statement is also contrary to the Vibration Club for Men's definition of dynamic vs static unbalance.
Section 3.8 here -
https://www.iso.org/obp/ui/#iso:std:iso ... ed-2:v1:en

Graphical image here showing accepted definitions of dynamic unbalance (not restricted to reciprocating machines) -
http://johnmaherracing.com/wp-content/u ... alance.jpg

I think typical basic models of first mode crankshaft torsional vibration show, when a heavy flywheel is attached to the "rear" end of a crank, that end is a "node" acting as an anchor, and the motion there is near zero. The full +/- 0.5 degree twist-untwist happens at the "anti-node" snout/damper end . In that case the extra torsional damping that //may// be provided by the 4% larger diameter journal is at the wrong end. But, back in 1937 Chevy applied a torsional damper right at the anti-node, the most effective place for it to be.
http://chevy.oldcarmanualproject.com/ch ... o/3729.htm

Kevin Johnson wrote:https://etda.libraries.psu.edu/paper/18878/17294

Dissertation.gif

Figure 4-29.gif

Page 121.gif

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I believe that dissertation is about radial shaft motion and vibration, not torsional.

The sub title " A Dissertation in Acoustics" has me scratchin my head a bit, too.

Dan Timberlake wrote:

Kevin Johnson wrote:https://etda.libraries.psu.edu/paper/18878/17294

Dissertation.gif

Figure 4-29.gif

Page 121.gif

==============

I believe that dissertation is about radial shaft motion and vibration, not torsional.

The sub title " A Dissertation in Acoustics" has me scratchin my head a bit, too.

The text suggested by David arrived:

On page 68, William T. Thomson wrote: 3.7 ENERGY DISSIPATED BY DAMPING

Damping is present in all oscillatory systems. Its effect is to remove energy from the system. Energy in a vibrating system is either dissipated into heat or radiated away.

Sometimes you need to shift frames of reference and identify different degrees of freedom of motion (think about how torsional vibration is introduced and how the passage on page 121 suggests the fluid film can dissipate it as heat). Wish I could spend more time on this. Lots of work to get done.

The illustration below is from the dissertation:

Just to clarify the discussion to me: Are we talking different amounts of clearance in the bearings front to back or a different nominal journal diameter or both?

Dan Timberlake wrote:

Kevin Johnson wrote:Without diving into the SAE literature, the reference below suggests that this was an attempt to pre-emptively dampen nasty straight six harmonics.

https://en.wikipedia.org/wiki/Engine_balance

Different main bearing sizes on a crankshaft create plane imbalance in slide friction.

=============

Interesting link.

1 - I believe Considering friction (an "internal" force) in any inertial balance calculation is misplaced.
" Dynamic balance refers to the balancing of these forces and forces due to friction."

2 - That statement is also contrary to the Vibration Club for Men's definition of dynamic vs static unbalance.
Section 3.8 here -
https://www.iso.org/obp/ui/#iso:std:iso ... ed-2:v1:en

Don't invoke ISO if you are wanting to subsequently address torsional movement against a rigid construct.

Gotta go.

strokersix wrote:Just to clarify the discussion to me: Are we talking different amounts of clearance in the bearings front to back or a different nominal journal diameter or both?

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The crank's main journals get progressively smaller. Big at the flywheel end, small at the snout.
1/16" steps in the three main bearing (!) six in the 30s.
http://old-carburetors.com/1935-Chevy/i ... 04-600.jpg

~ 0.03" diameter steps in the later 4 main job used into the 60s

I did not look at clearance variations, although with the shimmed cap and manual cap centering adjustment field installation methods I think the true resulting clearance is open to serious (mis) interpretation.

If you take a Chevy 37-62 inline six apart you'll notice the two inner main bearing caps have no location dowels or register ground into the block...They install relying on the bearing to journal clearance to align them.

http://chevy.oldcarmanualproject.com/

Various pages and illustrations below to cover both the spray/splash lubricated version and the positive pressure version. A later technique for measuring drag on the bearings (ammeter) is also covered.