To balance or not

[Tusayan]

51% to some is the same as 102% to others, the only difference is the convention by which you make the calculation.  For V-twins it is common to use only the weight of one reciprocating assembly (and balance to 102%) but for some other multi cylinder engines (e.g. vertical twins where the cylinders are in phase) it is common to calculate using the weight of both pistons etc. in which case the same balancing job would be described as 51%.

[Antietam Classic Cycle]

Hi, folks.

I'm not an expert on this topic in any way, but having researched many sources when rebuilding engines in the past, I recall that the recommended balance factor for our engines was indicated to be 52% (could possibly have been 51% -- it's been a while).  When I had the 949cc engine in my 850-T balanced, I discussed this at length with the balancing specialist (balancing is all they do), and this made perfect sense to them.   This engine is using 88mm Gilardonis.

The reason I looked into balancing is that the engine in my '74 Eldo police bike has 88mm iron liners and Venolia pistons.  While it runs very well, it doesn't seem particularly smooth.   I assume that the pistons are heavier than stock, and I was interested to see how much difference balancing would make.  Unfortunately, the T engine has not yet run, so I don't have my answer yet.

 I could possibly dig up the information I found on this subject, if anyone is interested. 

Cheers,
Shaun

My V700 crank was balanced to 52%, rods matched and balanced end-to-end, pistons were already matching. One thing to remember when balancing a Guzzi crank is to add weight for the oil volume in the sludge trap.
http://thisoldtractor.com/moto_guzzi_loopframe_crankshaft_balance.html

[Rough Edge racing]

My V700 crank was balanced to 52%, rods matched and balanced end-to-end, pistons were already matching. One thing to remember when balancing a Guzzi crank is to add weight for the oil volume in the sludge trap.
http://thisoldtractor.com/moto_guzzi_loopframe_crankshaft_balance.html

  So..do you think it's runs smoother than a engine as it was done at the factory? V8 engines ,not flat cranks, are balanced at 50%...Generally, a balance factor is often chosen that makes vibration least noticeable to the rider..

[Antietam Classic Cycle]

  So..do you think it's runs smoother than a engine as it was done at the factory? V8 engines ,not flat cranks, are balanced at 50%...Generally, a balance factor is often chosen that makes vibration least noticeable to the rider..

Like I've written previously, the engine isn't completely together yet and it will be a few months before I start it for the first time.

[lazlokovacs]

very interesting thread

currently have an 1100 cali engine on my bench, I'm planning on balancing it FWIW.

expect many questions as the project develops

(currently stuck trying to get the crank out of the case)

[s1120]

Great thread...  I come mostly from the car world..  but that being said, if im just doing a piston swap, unless there is a HUGE difference, Ill just balance them to themselves..  If they are close to the stock weight I will try to get them close to OE... I normally balance if Im changing to a non stock rod though.. 

[pyoungbl]

I'm really happy to read through this thread and get a clearer understanding of stuff like balance factor.  I had a Ducati engine built a few years ago.  We installed higher compression pistons with a slightly larger bore so it made sense to have the crank balanced.  Man, that engine was smooth!  Fast fwd a few years and I decided to install a big bore kit in a 250cc thumper.  That engine had a balance shaft and was pretty smooth (for a single).  The stock piston/ring/wrist pin assembly weighed 275 grams, the big bore kit weighed 330 grams.  I did not try to have the crank balanced because (a) this was a pressed crankshaft and I did not want to split the cases (b) everyone who did this job said that there were no vibration issues.  Maybe I'm just such a tender flower that I'm sensitive to these vibrations but I found the vibration level to be objectionable.  Where I once could clearly see objects in the mirrors now they were a blur.  I ended up with the stock setup.  Bottom line, if the engine is already torn down it's not that expensive to also balance everything and it can't hurt.  A 20% change in piston weight justifies the extra effort to balance.

[ttietjen]

Just received the Gilardoni piston/cyl kit for my Eldorado, weighed the piston, wrist pin, clips & rings to compare with the stock ones. The total wt. ended up being exactly the same ( 490 gr ). The new pistons were a little heavier and the rings a little lighter. It seems rebalancing should not be a issue unless you intent is to achieve better than stock results.