Balancing Gilardoni pistons to Guzzi crank. what I found.

[bigbikerrick]

Hello folks, about 3 years ago, I replaced the chrome cylinders on my eldo to gilardonis, but did not check the weight of the forged pistons in comparison to the oem pistons, so I decided to do that. I have put about 10K miles on the engine, and always have noticed a bit more vibration with the giardonis.
 Luckily I still have the stock pistons rings, etc. I took out so I could weigh everything and compare.
Here is what I found:
OEM piston assembly . Right side =487g Left side= 485 g.
Gilardoni piston assy.   Right side = 493 g.  Left side = 492g.
 
both oem and gilardoni piston pins all weighed exactly 96 g.
 
I averaged the weight of both oem pistons to 486g.
Basically I had to remove about 6-7 grams of weight from the forged pistons, to equal the weight the crank was balanced to.
A dremel made it pretty easy to remove material from the inside of the pistons, mostly from casting ridges, and a thick reinforcing ridge along the bottom piston skirts, on the inside.
I was able to get both pistons to exactly 486g. of weight including the pins, circlips, and rings.

 I also had a local machinist install new small end bushings in the rods, and they were reamed to fit the pins, and also re ground all 4 valves, and lapped them as well.
I removed a total of 13 grams of excess weight from the pistons!
Last night I took the bike out for a test ride, and all I can say is WOW!
What a huge difference this has made in smoothness! I can notice the difference at almost all rpms, but at my usual cruising rpms of about 3500-4000 it is buttery smooth, the mirrors no longer become "fuzzy" at different speeds.
I am very happy with the results, and it was kinda cool to take a peek at the engine innards to see how everything is doing after 10K miles.
I want to thank all you folks that so patiently help me , with my questions, especially Charlie! 

Rick.

[rjamesohio]

Very cool Rick. Well documented and great info!

Enjoy that smooth ride.


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[mtiberio]

were they 83mm or 88mm replacements? (844 or 949cc)

[TN Mark]

Many years ago when I lived in Southern California I rebuilt a G5. I brought the crank, rods, pistons etc to a speed shop in Costa Mesa to be balanced. As I remember, they spun balanced the crank and matched the weight of the other parts. That engine was amazingly smooth at every rpm the motor was capable of. I think that bike also received a lightend flywheel assembly I purchased from Moto International. The flywheel was so beautifully machined it was a shame to bury it in the transmission bellhousing.

Thanks for sharing your experience BigBikerRick.

[John Croucher]

I read in an engine building book that being exact is not necessary.  The reason, the ounce or 10ths of and ounce difference is insignificant relative to the massive imbalance caused by the fuel/air mixture explosion and many other friction factors that can very by many pounds between cylinders in a running engine.  This sounds  very reasonable to me. Close is good though.

[Tusayan]

This topic has been a source of interest to me for a long time.  Several factors come to mind, one is that even with the piston 'perfectly' balanced in relation to the crankshaft counterweight the piston is driven by a crank/rod mechanism that doesn't move it in simple harmonic motion...  As a result, unless the con rod is infinitely long the balance forces aren't perfectly in opposition, in reality. The other factor, as mentioned, is that there are other sources for engine vibration, torque pulsation around the crankshaft axis being the obvious one, and at lower RPM they tend to dominate.  Balance related vibration forces increase as the square of RPM.

I remember talking with Ian Gowanloch about this once, the Australian Ducati dealer and expert.  I was concerned that pistons I was ordering were the correct weight (they were, when delivered).  He told me he had balanced engines all over the map, significantly away from the perfect balance factor that is correct for a 90 degree twin, and he hadn't noticed much difference.  None the less, when I read a report like this one it makes me think that maybe there's something real going on.

[bigbikerrick]

were they 83mm or 88mm replacements? (844 or 949cc)

I have the 83mm Gilardonis, Mike.
Rick.

[bigbikerrick]

FWIW, I noticed the increase in vibration right away when I first rebuilt my engine with the Gilardoni kit.
  I even thought  maybe,I should have sent my original cylinders to Millenium to have them nikasiled, and re used the original Guzzi pistons, instead.
 There were a couple of points in the rev range where the vibration was very apparent, at around 2400-2800 rpms, and again at around 3500-4000 rpms.
  I would have the bike on the centerstand in neutral, and slowly increase the rpm, while looking at the left handlebar end and mirror, as it would "do its dance".
 After the balancing, I tested it the same way, and could tell immediately things were different, smoother all around.
 
I also made sure both  slides were  perfectly balanced  at open throttle as well as at idle. The way my engine behaved previously, made it seem like the synch was slightly off, even though it was spot on.

I always felt my 3 square head Guzzis had much smoother engines than my loop, I thought it was just "the nature of the beast" , but after the balancing, I think the round head is just as smooth as the others.
Rick.