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Need A65 Crank Balancing Advice.. #779138 07/19/19 5:24 pm
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Tracey Spear Offline OP
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A local machine shop has my A65 Crank, Rods, Pistons, Pins, & Clips for balancing. The shop usually balances automotive cranks, but the owner checked with the manufacturer of the equipment and they had specs for A65 at 65% balance factor. I just checked on the status of the job. The owner of the shop was a little concerned by how far off the crank is, he stated that he's having to take off 300g and is starting to run out of places to drill.

Does that sound right?
Or are we messing up a good flywheel?

Thanks y'all

Tracey

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Re: Need A65 Crank Balancing Advice.. [Re: Tracey Spear] #779139 07/19/19 6:06 pm
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Tracey Spear Offline OP
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Update: Shop Owner said that when he first mounted the crank it shook the screen. He has taken off 220g and would need to take off an additional 91g to get the crank perfect. He said even 91g off it spins much smoother and is no longer shaking the machine.

I told him not to worry about the 91g, that for a street bike this should be a huge improvement, assuming we're on the right track by taking of 220g.

Again, does any of this sound correct or have we screwed up a good flywheel?

Thanks again.

Re: Need A65 Crank Balancing Advice.. [Re: Tracey Spear] #779141 07/19/19 6:51 pm
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kommando Offline
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Sounds wrong, unless the flywheel came from day an A50 there should only be the difference caused by the new pistons.

Re: Need A65 Crank Balancing Advice.. [Re: Tracey Spear] #779146 07/19/19 7:11 pm
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MarcB Offline
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I've had one crank needing two Mallory slugs added to counter weights and keep from having to drill too much into the flywheel. On another crank, the same shop shaved just the lightest amount off near the pin so as not to have to drill too much on the opposite side.

If it comes back looking like swiss cheese, though, I'd get it redone.

Re: Need A65 Crank Balancing Advice.. [Re: Tracey Spear] #779147 07/19/19 7:16 pm
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Allan Gill Offline
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Does the shop have the right balance weights?

A lot was taken off my crank, a bit like a Swiss cheese but it is a smooth motor. Doubt they took 300g off it though! But if it was that bad then cutting 2 half moons off the flywheel like on some late A65 cranks and the A70 cranks might have been better.


beerchug
Re: Need A65 Crank Balancing Advice.. [Re: Allan Gill] #779166 07/19/19 9:45 pm
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Tracey Spear Offline OP
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Originally Posted by Allan Gill
Does the shop have the right balance weights?


I have no way of knowing. I assume that if the balancer mfg recognized and had specs for the engine, the weights would be correct, missing something, either way it's a done deal.

The A70 rods were a little lighter and the Emgo pistons were a little heaver than original, +.20 due to rust in one cylinder.

Everything's apart, I'm building this bike to ride, balancing seemed a good idea. The thing is, since I acquired this bike in rust seized condition, and have never ridden a '69 BSA, I'm not sure I would know. At this point I recon it's an act of faith.

Re: Need A65 Crank Balancing Advice.. [Re: Tracey Spear] #779171 07/19/19 10:26 pm
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Stein Roger Offline
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I've seen quite a few "Swiss cheesed" cranks, and I'm thinking this can't be right? How the hell can they be so much out of balance when the actual dimensions are pretty close to the blueprints?
I built and rode a Norton 650SS with such a crank, and it vibrated just as bad as any other twin in a Featherbed...

Re: Need A65 Crank Balancing Advice.. [Re: Tracey Spear] #779173 07/19/19 10:46 pm
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NickL Offline
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Ask the bloke how he is calculating the weight. Not everyone uses the same parameters.
I've statically balanced several a65 and a50 cranks and never had to remove that much meat.
I use a set of factory weights.

Re: Need A65 Crank Balancing Advice.. [Re: Tracey Spear] #779180 07/20/19 12:07 am
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John Healy Offline
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The stock A65 balancing weights weigh 1.2975 kg or .64875 kg each.

I don't have the rod, bearing shells and piston weights on hand to calculate the factor, but he has those parts in front of him.
He can take those figures above and reverse engineering for the factor that gives him a figure close to 1.300kg. That is going
to be the figure he uses!

300 kg sound like an awful lot of metal to remove! Especially where BSA's specification for the depth of balancing holes is 3/16".
The alarm bells should have gone off that something was wrong with his factor when the balance weights were so heavy that
the flywheel machine started to vibrate so bad.

As a rule, with these twins you are much more interested in getting the side-to-side imbalance, than how close you get to their recommended factor.
Side-to-side rock is felt through out the rpm range, while changes in the actual factor will move the rpm where the vibration will be felt.

In reality you cannot "balance" a single or vertical twin. You can just move the vibration to a different point in the rpm range.

When we bought the weights to balance our own cranks they gave me a sheet of recommended factors for British motorcycles.
It was then I realized why I have had so many problems with people balancing my crankshafts. I can't tell you how many
flywheels I have had to replace because someone had a "sheet." All of the factors on the sheet given to me were no
way near what manufacture's recommended figures and some were totally "off the wall."

Good luck...
John


Last edited by John Healy; 07/21/19 5:58 pm. Reason: Comin' into Los Angeles Bringin' in a couple of keys Don't touch my bags if you please, mister customs man

Re: Need A65 Crank Balancing Advice.. [Re: Tracey Spear] #779228 07/20/19 3:41 pm
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gavin eisler Offline
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page B42 1970 manual "For balancing purposes a weight equivalent to 70 ( seventy ) percent of the reciprocating weight is attached to each crankpin journal" A65 part no 61 -3710 , 2 off, weight 22 oz 14 drms.

Not 65 %. 71 manual has the same 70 % balance factor.


71 Devimead A65 750
56 Norbsa 68 Longstroke A65
Cagiva Raptor 650
MZ TS 250
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Re: Need A65 Crank Balancing Advice.. [Re: Tracey Spear] #779259 07/21/19 1:39 am
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John Healy Offline
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22 ounces 14 drams is 648.475 g A bit easier to understand in today's world. Especially when you are communicating with someone balancing crankshafts. I told you the sheets that circulate around the commercial balancing world should be taken with a grain of salt Customer beware!

Last edited by John Healy; 07/21/19 1:39 pm. Reason: Kg to g

Re: Need A65 Crank Balancing Advice.. [Re: Tracey Spear] #779275 07/21/19 9:20 am
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NickL Offline
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650 grams is close enough for jazz. Or good enough for England.

Re: Need A65 Crank Balancing Advice.. [Re: Tracey Spear] #779301 07/21/19 4:51 pm
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DMadigan Offline
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In the first instance John left off the "k" but it should have been obvious that 1.2975 kg / 2 = 0.64875 kg, not grams.
The second instance you added the "k" to John's post, not a direct quote. 22 oz 14 dr = 648.495 gm according to the Machinery Handbook.

Re: Need A65 Crank Balancing Advice.. [Re: Tracey Spear] #779310 07/21/19 7:09 pm
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John Healy Offline
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What happened to pounds and ounces

[Linked Image]
An actual 61-3710 for balancing an A65

[Linked Image]

2 on my scale 1.3010 kg

And if the person doing is honest with you he will admit that the balancing will not be perfect. While small, there is going to be some variation from the calculated percentage and what you will actually get. Last time I checked most people I trust with my balancing are human. That's why I have weights to check that they give me the percentage I asked for. I am more interested in that they got both journal weights the same and they located them accurately in the center of the journal. It is the rock from side to side that really concerns me!

In reality, the factory balance figure is not a technical exercise, worked out with a slide rule or the like. It wasn't worked out in some dreary engineers office. It is a compromise chosen from observations made by test riders who mount these bikes and ride them. Like the customers who buy these bikes not all test riders are the same. It's like exercise where you throw out the comments of the road racer (Percy Tate was one of Triumph's testers) at one end and the comments of the likes of the tester referred to as Casper Milktoast on the other. Then from the average tester's comments calculate a factor that you think most of the buyers will like (or put up with).

Then the customer changes the handlebars, and their resonant frequency, and all the effort is for want. Take the Triumph twin down tube frame used on late non-units. It was noted for how much these bikes vibrated. Mount a looped crash bar across the twin down tubes and its a different motorcycle. Take a nice (reasonably) smooth T140 out of its native frame and put it in a Rickman road race frame and you can't hold on to the handle bars... No need for a tach. Just shift when you cannot hold on any more. If a balancing house says he knows more than the factory, personally, I wold run like hell! The factor is nothing more than an average derived by humans placing their butt on the saddle. John


Re: Need A65 Crank Balancing Advice.. [Re: Tracey Spear] #779322 07/21/19 8:53 pm
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DMadigan Offline
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What happened to your post between John Healy and NickL?
Pounds and ounces are only used in third world countries now.

Re: Need A65 Crank Balancing Advice.. [Re: Tracey Spear] #779327 07/21/19 9:41 pm
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kevin roberts Offline
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on the 1965-single-downtube-framed T120 that i race at high rpm, greg blagus in michigan balanced the crank at the 62 percent he does for the BSA cranks he makes.

it didn't vibrate any more at idle than my old (uncompensated) morgo T120, and at speed the vibration is not noticeable.

i only ride it for about 40 seconds at a time, but if 62 percent was really off i would surely notice.


every day you do not take a chance is a day of your life that you will never get back.
Re: Need A65 Crank Balancing Advice.. [Re: Tracey Spear] #779335 07/21/19 10:50 pm
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NickL Offline
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"2 on my scale 1.3010 kg"

As i said, close enough for jazz.


Moderated by  Allan Gill, Jon W. Whitley 

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