Polished Rods

[RichardL]

Wish I could find a "before" picture, but I guess these should do for the '57 barn-find A7.

They are within a gram of one another, which, I'm assuming, is close enough for static balancing. Anyone care to comment on this assumption? Not sure from where to remove the material if, indeed, needed.

Richard L.

[a10 gf]

That's a beautiful sight !

[Greybeard]

Blinging beautiful!

Stop polishing your rod; you'll go blind

[trevinoz]

Richard, you need to weigh the ends of the rods and remove metal to make them the same as each other.
Just suspend the rod with the end in question resting on the scales.

[RichardL]

Trev,

Thanks. I've been referencing the static balancing description posted earlier by KiwiGF, so I knew about weighing both ends. Maybe I need to go back to it and read again, but I don't recall instruction regarding from where the material should be removed.

Richard L.

[trevinoz]

Richard, usually from the top of the rod and the caps.

[Jules]

they look beautiful Richard, shame you wont see them once assembled!!

[Swarfcut]

 Richard.   Years ago I had a pair of rods polished and matched. Having access to a chemical balance I did a check weigh and found them not to be the perfection I had been led to expect. So back I went, to be shown the precision balance in use....An Avery Sweet Shop Scales, calibrated " Red for NO, Green for GO" My rods stayed well within the green but there was an obvious difference.

 It was pointed out that as soon as the engine runs, the accumulation of carbon on the pistons alters the weight anyway, so for all intents and purposes they considered the job to be fine.

 I had my doubts, but it proved to be a smoother runner than before, but in retrospect that could be more down to the new bearings, crank grind, rebore and pistons!

 You won't have a problem weighing to a gram, (close to perfection) . I reckon mine were matched to the nearest 1/4oz.

  Swarfy.

[KiwiGF]

Trev,

Thanks. I've been referencing the static balancing description posted earlier by KiwiGF, so I knew about weighing both ends. Maybe I need to go back to it and read again, but I don't recall instruction regarding from where the material should be removed.

Richard L.

For the bottom of the remove off (both) sides metal on the outer side of the bolts, this part of the rod is relatively safe to remove metal from, as the bolt reinforces it.

For the top of the rod remove metal equally around the top of the rod above the gudgeon pin hole, a linisher can be used, this will weaken the rod but not in an area likely to be short of strength.

[RichardL]

Appreciate all these comments. I'll be checking end-for-end weghts today and sharing the results before any material removal takes place.

Now, on to the potentially controversial or stupid part. No problem understanding matched end and total weights on any engine having a crankshaft with offset journals, but why care about it when the rod ends share the same centerlines (assumung the weight difference is not silly, of course)?

Richard L.

[Swarfcut]

Richard. It's all to do with rotational mass, moments of inertia and rotational flex axis and other more esoteric principles documented within learned engineering papers.

 My take is that the closer the reciprocating and rotational parts are matched, both for their  actual mass and the distribution of that mass, then the better the set up will be, and the greater chance of success of achieving the desired balance factor and a smooth running engine.

 It's a bit out of my comfort zone, having never mastered simple harmonic motion or calculus. Its more a case of fit and (try to) forget with my builds.

Swarfy.

[RichardL]

Swarfy and anyone,

I have about as much mechanical engineering schooling as it takes to get a degree in electrical engineering, so, just enough to get in trouble. This said, I'll keep the discussion going on this common centerline question to see if my thinking is askew.  As I envision the imbalance with a common centerline, the effective flexation would have to occur toward the flywheel, perhaps at the junction of the journal to the web, or on the actual centerline of the flywheel. So, should one think this potential induced flexation caused be a few grams of weight difference is anywhere near the potential flexation caused when an explosion occurs on one cylinder while the other is casually breathing out?

Richard L. 

 

[KiwiGF]

Appreciate all these comments. I'll be checking end-for-end weghts today and sharing the results before any material removal takes place.

Now, on to the potentially controversial or stupid part. No problem understanding matched end and total weights on any engine having a crankshaft with offset journals, but why care about it when the rod ends share the same centerlines (assumung the weight difference is not silly, of course)?

Richard L.

I’m not 100% sure I have understood your question but the underlying principle is that the small end weight is reciprocating mass, and big end rotating mass.

I guess its also assumed (when just matching the rods weight) that the crankshaft is balanced evenly, so if one rod weighed more than the other it would (technically) have a slightly different balance factor to the other, also it could introduce a “twisting” (horizontal/rocking) axis vibe, having said all that, these engines are a compromise and will always vibrate, some engines have inherently perfect primary balance (straight 6 and 90deg v twin) but the 360deg vertical twin is not one of those, and is really just the same as two singles bolted together with the pistons going up and down in unison.....

1 gram difference seems pretty good to me, you may find the piston/rings/gudgeon have more difference than that!

[RDfella]

I gave up trying to understand the theory of engine crankshaft balance decades ago. Made by brain hurt too much. Especially when even the best experts can't get it right. Was just refreshing my memory with 'Advanced engine technology' by Heinz Heisler and even he made the mistake of saying that in a 90* vee twin when one piston is at TDC the other is at half stroke. It is not and where it is depends on stroke and conrod length as Phil Irving explained so long ago. In a motorcycle vibration is perceived from the frame, as the engine and frame vibrate as a unit and just changing a mudguard or stay can affect the balance factor required for smoothness. The best-known example is the need to alter the factor when putting a triumph engine into a norton frame. I recall a famous TT rider (but can't currently recall his name) who suffered terrible vibration from his bike. He cured it by filling his expensive light-alloy handlebars with lead! Which is why I work by trial and error. When developing an engine I set the balance factor deliberately low and then add weights later to find the best compromise (I allow for, or make on existing engines, a window in the crankcase whereby I can adjust those weights without stripping the engine).

[RichardL]

RD,

"I allow for, or make on existing engines, a window in the crankcase whereby I can adjust those weights without stripping the engine." The only thing that comes to mind to say about this is, "that's some gansta' motorcycle mechanics."

KiwiGF,

Got ya' on the rotating vs. reciprocating, but on our engines, as I envision it, the balance factor would be the same regardless of the difference in rod weights. Maybe more obscure (to me) factors, like twisting, bending, etc. come into play, but, as I mentioned, these seem trivial compared to imbalance at combustion. Referring to your excellent post about static balance, the procedure, as I recall, has you hanging a single weight from the journals to test the counterbalance.

If, at any point here, it seems like I actually know what I'm talking about here, that would be a fortunate accident.

Richard L.