Crankshaft Balancing

[Ohio HD]

I'm putting this is General as this would apply to all models of Big Twin motors, as well as Sportsters.

Rather than add a lot of traffic to another posting in Twin Cam, I decided to make a new post.

I'm using the S&S balance work sheet as it best defines what is measured and what the weights that need to be added to each flywheel to balance each one. The reason for the post is to see how much difference in balance factor a change in piston weight has on the crankshaft.

The sheet is using basic Evo motor component weights. And then gives the weight that is needed to attach to each flywheel to balance them statically. Then I added another column with piston weight changes to see the effect on balance factor.


The jest of the sheet shows that with pistons weighing 1230 grams (both pistons) to reach a balance factor of 60% we need to add 1273.9 grams of bobweight to each flywheel so it can be balanced. Then by changing the piston weights to 1118 grams (both pistons) and we add the same bobweight value of 1273.9 grams, the balance factor is now 64%.

I'm pretty sure this is accurate because the rods, crankpin, etc. weights are irrelevant if we don't change them. We're looking for a balance factor and what weight is needed to reach that.

Anyone that's balanced HD cranks, I welcome your feedback to see if you agree with this.


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[jmorton10]

I think what you're asking is correct.

I trued & balanced my own flywheels for years.  I still have the S&S balancing kit & an old Harley truing stand & a digital scale (& a Snap-On TQ multiplier I used to tighten down the crankpins) We used a 60% balance factor on almost everything.

I haven't used the stuff in years & just keep it around for nostalgia reasons.  Years ago, I was half owner of a custom Harley chopper shop (my partner who is dead now was an H.A. member) & I did all the flywheel truing & balancing for tons of stroker motors we built back then.

~John 

[turboprop]

I'm putting this is General as this would apply to all models of Big Twin motors, as well as Sportsters.

Rather than add a lot of traffic to another posting in Twin Cam, I decided to make a new post.

I'm using the S&S balance work sheet as it best defines what is measured and what the weights that need to be added to each flywheel to balance each one. The reason for the post is to see how much difference in balance factor a change in piston weight has on the crankshaft.

The sheet is using basic Evo motor component weights. And then gives the weight that is needed to attach to each flywheel to balance them statically. Then I added another column with piston weight changes to see the effect on balance factor.


The jest of the sheet shows that with pistons weighing 1230 grams (both pistons) to reach a balance factor of 60% we need to add 1273.9 grams of bobweight to each flywheel so it can be balanced. Then by changing the piston weights to 1118 grams (both pistons) and we add the same bobweight value of 1273.9 grams, the balance factor is now 64%.

I'm pretty sure this is accurate because the rods, crankpin, etc. weights are irrelevant if we don't change them. We're looking for a balance factor and what weight is needed to reach that.

Anyone that's balanced HD cranks, I welcome your feedback to see if you agree with this.


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Another way to examine this is to run the numbers for piston A to achieve a balance factor of X. Then change the piston weights and do a reverse calculation. What you should see is something like piston A = X% balance factor. Pistons B have a balance factor of Y% using the same crank and the delta is Z%. This is used often during rebuilds to determine if a re-balance is required.

[Ohio HD]

Thanks, I was brain locked on reversing the calculation, you made me look at it again. I think I have it correct.


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[billbuilds]

     Brian, I'm a little confused by a couple of numbers on the balancing example that you posted. On the left side under CP it reads 421. On the right side under the front piston weight column it reads 470. Please ignore my ignorance but what does the 421 denote? This is the figure that shows as piston weight on the spec sheet. Perhaps I'm missing something and it is actually 470?

[Ohio HD]

Bill, no your not missing anything. The 470 was a weight for any standard piston. I was just trying to get the calculation to give me a 60% balance factor. Then the 421 was just a note for me (your CP piston weight) and the 559 is your total weight based on what you measured. The 615 across from the 559 is the other piston weight total.

The whole exercise was for me to determine what a piston weight change would do to a given 60% balance factor. It doesn't matter what V-twin motor it is once you have some established values. And only change the relevant piston weight. I wanted to use the S&S weight of 1230 for two early TC piston weights to establish the 60% factor.

Turboprop jogged my brain and I created the second calculation, only seeing what the piston weight does to an established 60% factor. All of the other calculations on the first sheet are to come up with bobweight values to actually allow you to balance the flywheel halves. The piston weight is of course a part of that.

Don mentioned earlier that CP may be able to supply a heavier wrist pin for your pistons. That's why I mentioned to weigh the original piston, rings, etc. to see what you really have. Those two weights are really the only two values that you can calculate what you may need to do. See what the difference really is. Remember that your piston will weigh a little less when they come back from Fuel Moto.

[billbuilds]

     Brian,,
     OK, I see now. Obviously best to try to ring that balance factor back down closer to 60%. Campbell Industries opens in about another 40 minutes (weather permitting). I may give them a ring and ask about heavier wrist pins.
                                       Much appreciate your and Turboprop's time, Bill

[Admiral Akbar]

So you are going to balance the crank at 60%?   On a rubber mount I'd do around 52.  60% is good for a solid mount un-counter balanced motor as it directs some of the vibes horizontally.

[Ohio HD]

No, no balancing going on. I'm just looking to see what effect piston weight changes have on an existing balance factor.

I used to use the DOS version of Accelerator Dyno program, it had the crank balance program in it. I don't have a PC that'll run a DOS program anymore. 

[Admiral Akbar]

The numbers you have are correct then.

           Old BF x Old Recip
New BF = ------------------------
               New Recip

Note that for me the rod ends weighed 227 front 248 rear 2000 TC crank.

[Ohio HD]

Thanks, I appreciate your input on the calculation.

I used Evo component info just to get a balance factor. I don't have the weights of TC crank components.

[Admiral Akbar]

I understand.. Just providing more info..

Also, Stock 2000 fxdx crank looks to be balanced at 52%. At least that is what I got.

[billbuilds]

    Brian and Don @ HD Street Performance,

    Max asked a balance/weight question on the TC page on the CP 98" piston thread that I started. He noted that
even with the 131 gram wrist pins we're still 43 grams shy of the 600 gram weight of the stock piston assembly. I replied with a question that probably belongs on this thread so I'll ask it here. I found some .927 X 2.250 wrist pins on the Summit Racing site that are .250 wall thickness and weight 153 grams each. They are Diamond brand and made out of H-13 tool steel. With reference to Max's question about stock 88" piston weight I wondered if the heavier wrist pins would put me closer to stock balance? Thanks, Bill

[Ohio HD]

Bill, I agree with Max. The 0.200 wall pins are not going to equal the weight of 600 grams per piston. Below is what I get, it matches what Max said. I also calculated the 0.250 wall pin area and come up very close to what they told you, a weight of 153 (I got 155.237). This gets you close to the 600 gram weight.


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Then I used a 52% balance factor supplied by Max as a known early TC balance factor of a 2000 FXDX

With the 0.250 wall pins the balance factor goes to 53.3%. This should be just fine with the 0.250" wall pins then.

I still would like you to weigh the old pistons and rings so that you know what you really have. We're relying on a 600 gram piston weight as a standard. It would be good to know what the old pistons assemblies weighed.


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[JSD]

Good work & interesting Thank you

[billbuilds]

     Brian, Thanks for your time plugging in these figures and helping me to understand balance factors and thanks to Max for his keen eye. Just for clarification, you're talking the original 88" pistons, pins, keepers and rings, not the RevPerfs, correct? I will try to find a more accurate scale to firm up the weight of those pistons. Your help is greatly appreciated.

[kd]

Bill, if you are going through the trouble to reweigh one set on a known accurate scale, I would weigh both complete sets (as is) so you know what you have and how to get where (or close) to where you want to be.

[Don D]

Bruce if we are to ignore the factories balance factor what do you think is an appropriate factor for a street performance Twin Cam?

[Ohio HD]

Brian, Thanks for your time plugging in these figures and helping me to understand balance factors and thanks to Max for his keen eye. Just for clarification, you're talking the original 88" pistons, pins, keepers and rings, not the RevPerfs, correct? I will try to find a more accurate scale to firm up the weight of those pistons. Your help is greatly appreciated.

No problem at all. It's been a good learning process along the way. And Max added some good info for us as well.

Yes, the 88" parts. Just to be certain is all. It's one of things that would keep me up at night because I didn't know. 

And as Kirk stated, if possible weigh the new set when you get them back, just to be certain. 

[billbuilds]

     Kirk and Brian,

     I've been having a difficult time coming up with an accurate scale to use but luck may have changed. I'm seeing a triple beam balance scale for sale on Craigslist that's about a forty-five minute drive from me. Still has box and instructions and looks pretty much new. The guy is only looking for $50. Would a scale such as this be satisfactory for measuring the weight of these pistons? They have digital kitchen scales at Walmart but I don't really trust their accuracy. Thanks, Bill

[Ohio HD]

If the triple beam is in good shape and a quality brand, it's going to be very accurate.

[billbuilds]

     It's an Ohaus brand with a 2610 gram limit. There are quite a few on ebay so seems his asking price is pretty fair.
                                            Thanks for the quick reply, Bill

[Ohio HD]

Ohause is a great brand. I'd buy it. I used to have one a long time ago.

[kd]

With a beam scale you can confirm the accuracy with weights in / on the pan.  The beam will have an adjustment to set 0 and in some cases you can add things like lead shot to.  Most producers of beam scales aim for accuracy and a good name brand usually won't risk their reputation with crap.  You may be able to take it in to a gold buyer or like business and check it against or with test weights.  Once done it will be good to go provided the knife edges and pivot load bearing surfaces are protected in transport and handling.  IMO They are a good choice because they don't depend on things like battery power level.   

[Ohio HD]

As far as the electric, battery powered, mine shows when low battery, and it shouldn't be used at that point until new batteries are put in the unit. You can buy an AC power supply for it.

My standard is a CP piston. If it reads what CP says in the documentation, then it's correct. It always has matched their piston weight.