126 Bis Piston info wanted

[damon500]

Hi, I am working through the challenges of fitting 650cc barrels to a 500cc Giardiniera engine. 
I am using a 126 Bis flywheel so need to know the weight of a 126 Bis piston to compare to the aircooled piston to determine balancing.
Does anyone have a 126 Bis engine apart that would be able to weigh a piston for me?
Thanks
Damon

[smallcox]

Hi Damon,

Having trouble working out what your after, A bis Flywheel is not balanced the same way as a air cooled engine. If your retaining your original crank and rods then you can retain the 500 flywheel as the bis flywheel will be well out off balance. 

The weight off the piston would not make any difference to the rotating balance if this is all your changing in my opinion.

Smallcox

[damon500]

Hi Smallcox,
Thanks for your reply, You are partially correct about the balancing, the Bis flywheel is balanced the same way as the 500 Giardiniera which is different to a normal aircooled as it is also a flat engine.
The reason I am using a Bis flywheel rather than the original is to upgrade the clutch to a 126 diaphragm type rather than the original coil spring type (clutch is larger and flywheels are different)
I would be interested to see what the difference in weight between the 126 bis, 126 650 and 500 pistons
According to the factory specs piston balance should be accurate to within 5 grams - it makes a huge difference to the longevity and smoothness of the motor if you get it right- I just don't know within 5 grams of what..

[gsxr126]

Within 5 grams of each other [-/

I'm pretty sure smallcox is right when he says the weight of the piston doesnt make a difference. As long as they're all the same weight you should be right balance wise. A piston change will change things like compression and height in the bore. As long as the internals are dynamically balanced, ie: pistons rod and crank you should be right.

As for type of flywheel, whatever you choose to run should be matched with the front bits off that model if that makes sense

[damon500]

Oh, believe me piston weight makes a huge difference... 126's aren't as vibratory as a 500 probably due to the much heavier flywheel. The piston weight variation to "each other" is not critical as both pistons rise and fall at the same time- it is the overall weight of both pistons and rods relative to the counterweighting of the crank (and flywheel) that is my concern.
When I used to rebuild 500 motors we would balance the pistons to within 1g of original weight and they all ran beautifully- my 500 with 126 pistons is still running strong over 20 years later. However I have driven several Fiat 500's fitted with VW pistons (and even rebuilt one- unfortunately) and all of those those engines were very harsh and regularly rattled themselves to pieces due to the extra piston weight.
I'm guessing that the combination of Bis flywheel, 500 Crank and 126 650 pistons will probably be fairly close to right but I would still like to know for sure...

[smallcox]

Hi again,

Just thinking, if you use a air cooled flywheel from a 126 the bis clutch fits straight onto it and the flywheel is as close to balanced for a straight swap. [-/

As for piston weight, the best wieght would be zero but that is impossible. It is only the pistons that fully riciputate "did i spell that right" and partionally the conrod. the lighter these cab be made the smoother the engine. there are also other factors that effect the pistons. ie pin offset, skirt type, type of ring.