Conrod balancing

[whistler]

I'm trying to have my crankshaft /flywheel etc balanced and my local company can only balance the conrods as a total weight instead of end to end which I have been advised to do. If i don't have end to end, just total weight balance, will it throw the rest of the balanced components out. Engine is overbored 1098 which will not be revved over 6000rpm.

[dklawson]

Ideally the rods should be balanced for both overall weight and end to end. However, the impact of not having end to end balancing depends largely on how you will use the car/engine. Consider for a moment that your car's engine left the factory in running condition with limited effort placed on balancing. Balancing all the rotating components will go a long way to reducing vibration and increasing smoothness. Not balancing the rods end to end may reduce that but it's not the end of the world either.

For reference, see the link below.
http://www.eatonbalancing.com/2007/11/1 ... ng-part-4/

[Spider]

The parts that spin - yes, I agree here they can well benefit from balancing, but they need to check it at a few different RPMs.

Now, the parts that go up and down.

The Con Rod Big Ends should be balanced (and you can do them yourself quite easily), so they have near equal weights.

One way to do this is by bolting the little end of 2 rods together, with a wide large disc in between them and the rods poking out each side, so they can see-saw. You need to go through each rod with all others to find the lightest Big End to work the others to.

As for the parts that go up and down, the Rod Little End, the Pistons, Rings etc, it is a total waste of time. I've tried once before to explain this here, however, louder voices won out that time.

To explain this;-

Firstly, when the crank Big Ends are at 90 degrees to the Bore, the pistons are lower in the Bores than 1/2 the stroke and when say 1 & 4 is at 1/2 stroke, then 2 & 3 will be way lower down the bores that that 1/2 way point. You might need to draw a diagram and think about that. What I'm getting at here, is even before we consider what happens when the engine is running, the reciprocating parts never balance out just spinning over.

Secondly, in operation, there's 100's of pounds of force on ONLY one piston at a time, the next one is trying to squeeze some gases, another exhaling gases and the last is drawing in gases, so the forces across each piston / rod widely vary by HUGE amounts, 100's if not 1000's of pounds. Do you really think 50 grams difference in weight between them will really matter ? (not that you're likely to see as much as 50 grams).

This is in part why most engine run Crank Dampers and there is Tortional Vibration that occurs when the engine is running.

It's also why in some modern engines they run 'Balance Shafts', not that they do make the engine actually run in Balance, but they can reduce vibration output from the engine assembly in to the mounts.

Sorry for the long winded explanation, but when I mention it, most people don't want to hear it as 'this is what the top line racers do' and that's not at all correct.

Coming back to your predicament, I'd actually be looking for another firm to do the work. If they just want to bang the Piston & Rod on a set of scales and get them all to weight the same, that could actually make the Rotating Balance worse as while as an assembly, they may all weigh the same, the Big Ends could all well weigh considerably different to each other.

If they can't be bothered getting this right, then what are they doing with the rotating parts ?

[360gts]

This is a home made rod balancer...used it for many years...rods come out to within 1-2 grams....very simple to make...use a good digital scale for weight.

[agricola]

slightly of the topic. I have a very accurate set of scales. I have 2 1275 being prepared: one a 1970s pre aplus no tappet covers the other an spi aplus.. so I thought I will weigh the rods + pistons the pre a plus weighed 1.168 kgs the later 1.067. I havent weighed the cranks to see if they reduced the weight of the cranks . Perhaps I should
The weight of steel rods + minispares swifttune pistons was 0.9 ish. I asked one of the most experienced engine builders in the UK about lightening the crank which he could do but his advice was if it was mine I wouldnt bother.

[whistler]

This is a home made rod balancer...used it for many years...rods come out to within 1-2 grams....very simple to make...use a good digital scale for weight.

I made one of those but when I placed a rod on it the scales woudn't settle on a weight. My next stop was to use some bearings instead of the wooden discs that you show but then sought advice and decided that for a road engine it really wasn't worth the hassle, so just going to try to balance the big ends using the seesaw method.

[mk1]

when I placed a rod on it the scales wouldn't settle on a weight.

How do you mean? Just curious, because everything usually reaches an equilibrium sooner or later.

[360gts]

when I placed a rod on it the scales wouldn't settle on a weight.

How do you mean? Just curious, because everything usually reaches an equilibrium sooner or later.

Could be Mark that the wooden discs that the other member used were just a shade too tight in the rod....just a thought!

[whistler]

when I placed a rod on it the scales wouldn't settle on a weight.

How do you mean? Just curious, because everything usually reaches an equilibrium sooner or later.

Could be Mark that the wooden discs that the other member used were just a shade too tight in the rod....just a thought!

If I put the whole rod on the scales the weight shown after about 3 seconds was steady/stable but when using the jig the weight fluctuated from the inital reading downwards and continued fluctuating after 10 seconds or more.

[Vegard]

Balancing rods overall weight is an absolutely waste of time. Who actually approves of this?
I know ACDodd weights the caps and "rods" and think that works, but it doesn't either.

[Exminiman]

Balancing rods overall weight is an absolutely waste of time. Who actually approves of this?
I know ACDodd weights the caps and "rods" and think that works, but it doesn't either.

Just wondered why you think it’s a waste of time

Does it not matter how out of balance they are ?

[Dearg1275]

Taking both sides of the argument, I’m sure the factory set tolerances on weights, so if you want to balance all the reciprocating parts you are just tightening those tolerances. At the end of the day it may be a waste of time, but it can do no harm. Now, reducing the reciprocating mass definitely has benefits. Less mass to accelerate means less force required. Less force needed = less energy expended.

D

[carbon]

when I placed a rod on it the scales wouldn't settle on a weight.

How do you mean? Just curious, because everything usually reaches an equilibrium sooner or later.

I've tried a similar set-up for measuring end to end rod weights, with no success.

Using a typical digital kitchen balance reading to 1g the repeatability of the end-to-end measurements was plus/minus a couple of grams. When I tried measuring the little-end weight using a 0-500 gram balance reading to 0.01g I got similar scatter. I came to the conclusion there was either too much friction in the pivots or I was not centering correctly in the big-end or little-end eyes.

As the overall weights of the rods (a factory set) were within 5 grams I decided to leave them well alone.

[OzOAP]

Bought these off a company I worked for, have used them since 1987, measure to 0.1g. Always repeatable.
Measures big ends on jig, then overall rod against rod.
Got loads of mandrels for BMC, Ford, Porsche, Vauxhall rods.

[whistler]

Bought these off a company I worked for, have used them since 1987, measure to 0.1g. Always repeatable.
Measures big ends on jig, then overall rod against rod.
Got loads of mandrels for BMC, Ford, Porsche, Vauxhall rods.

Nice bit of kit there.

[Vegard]

Balancing rods overall weight is an absolutely waste of time. Who actually approves of this?
I know ACDodd weights the caps and "rods" and think that works, but it doesn't either.

Just wondered why you think it’s a waste of time

Does it not matter how out of balance they are ?

Overall weight is not important. End to end weight is important. In my experience 1275 rods are not usually that far off, but 998 ones are absolutely rubbish.

[YMJ]

Up to 8000 and over, end to end balancing is essential. However, I’ve just built a 950 chugger with a massive flywheel and no power and although I did end-to-end, I do wonder if it was worth an entire Sunday dicking about with the linisher and the scales

[Exminiman]

Balancing rods overall weight is an absolutely waste of time. Who actually approves of this?
I know ACDodd weights the caps and "rods" and think that works, but it doesn't either.

Just wondered why you think it’s a waste of time

Does it not matter how out of balance they are ?

Overall weight is not important. End to end weight is important. In my experience 1275 rods are not usually that far off, but 998 ones are absolutely rubbish.

Are you saying that all four rods do not need to be balanced to same weight or the actual overall weight of the rods is unimportant?

I would of thought the lighter the better and the more balanced the better, to improve reliability, power and acceleration....