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Is it necessary to balnce crankshaft?

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ReggieV View Drop Down
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    Posted: 18 Dec 2017 at 11:57
I'm soon to be starting the rebuild of an engine which came to me disassembled.
The crankcases for this engine were originally for an 850 engine, and I am fitting 950 barrels and pistons, and the pistons are very slightly heavier 516g  - 950 against 490g - 850, so 5% heavier. When I purchased the bike, it was dissembled and I can't be sure that the crankshaft is definitely from this 850 engine but it has the correct stroke of 78mm, so isn't from and 750, and will be from an 850 or 950.
 
Although to have the crank balanced would be the sensible option at over £200 all in, I just wondered how accurately they were balanced at the factory and how forgiving they were if all was not 100% correct. In the day when altering the capacity from 850 to 950 was more common, was the advice to get the crank re-balanced or fit and forget?
 
I will be selling this bike (as the loft won't take much more weight and I've run out of space) and don't want to spend unnecessary money as it probably won't be reflected in the price I get for it, but if the outcome of mismatched crank to pistons will be horrible, then it will be a no brainer and I'm prepared to do it.
 
What do you think? What would you do?
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cyclobutch View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote cyclobutch Quote  Post ReplyReply Direct Link To This Post Posted: 18 Dec 2017 at 14:35
I upjugged my LM II from 850 to 950, though I never weighed any of the components.At first it felt very lumpy and I thought I'd fallen foul of balance factors. However, as I put the miles on it it smoothed right out. So I guess it just needed running in - or after all my teeth fell out I just didn't notice any more.
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theone&onlymin View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote theone&onlymin Quote  Post ReplyReply Direct Link To This Post Posted: 18 Dec 2017 at 17:48
On Dynotec website they say this...

Think about this!
The most important thing for engine optimization is for it to operate from low-down upward. The crankshaft should be well prepared for the forces that come with higher revs. (Bear in mind that many Guzzis now sport lots of miles). Accurate axial play and a perfect balancer are the prerequisites for long engine life. The newer the models are the less important the balancing becomes. Balancing must be done to T3 models and Lemans 2 and older.
Crankshaft electronically fine-balancing

First the pistons are bought accurately at the same weight. The conrods are weighed out and adapted according to their rotary and osscilating masses. After this, the master weight for the crankshaft is calculated and the crankshaft made. This master weight is then balanced, any errors being compensated for. This procedure protects small tolerances and supplies the best automatic result.

So they do.

Cheers
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ReggieV View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ReggieV Quote  Post ReplyReply Direct Link To This Post Posted: 18 Dec 2017 at 19:53
theone&onlymin wrote;      "So they do."
 
I'm not surprised that this is the best advice, although I had hoped to read that many people didn't bother balancing the crank and there were no issues, although cyclobutch says his was ok. In fact I recall reading that information some time ago but had forgotten it.
Looks like it'll be hand in my pocket then.....Cry Cry Cry
 
Thanks for the advice.
 
 
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Mike H View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Mike H Quote  Post ReplyReply Direct Link To This Post Posted: 18 Dec 2017 at 19:58
My workshop manual talks about balancing one piston & conrod against the counterweight webs, by having the main lying bearings on knife edges, and it shouldn't roll.

Can't remember if it's stated or assumed that both pistons and conrods should weigh equal, like for like.



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Dave P. View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Dave P. Quote  Post ReplyReply Direct Link To This Post Posted: 18 Dec 2017 at 20:56
[QUOTE=Mike H]

Can't remember if it's stated or assumed that both pistons and conrods should weigh equal, like for like.



Certainly should.

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italianmotor View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote italianmotor Quote  Post ReplyReply Direct Link To This Post Posted: 18 Dec 2017 at 20:57
All I know is that Guzzis that I have ridden that had the crank/pistons balanced were so much better to ride. Basset Down is a name I recall we used to use and the results were excellent. If you're selling on, just factor in the cost to the sale price and 'big it up' in the description when you come to sell - money well spent.
Guzzi lover doing my own thing: http://www.italianmotormagazine.com
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ReggieV View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ReggieV Quote  Post ReplyReply Direct Link To This Post Posted: 18 Dec 2017 at 21:33
Italianmotor wrote;  If you're selling on, just factor in the cost to the sale price and 'big it up' in the description when you come to sell - money well spent.
My current Moto Guzzi, not the one I'm building now, has had its crank balanced by Basset Down about 5 years ago and I have to say that it is remarkably smooth engine. It cost about £250 if my memory isn't failing.
I suppose that by not cutting any corners and "big up" things such as this is the best direction to go. I am starting to find that I am spending a lot of money on this bike though......it's not at the scary stage yet but it may be soon. ConfusedConfusedConfused
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