850 (non-Lemans) Pistons – Partial or Full Skirt

[BigDen]

Hi all,

I’ve been following this forum for a number of years, but this is my first post.  I have a 1974 Eldorado I got from Guzzi Paul in 2014 and returned to the road after motor, transmission, and fork rebuilds; a 1976 850 T3 I got last year as a salvage bike and returned to the road after I repaired the fork damage; and a 1954 Harley Panhead I have owned since 1970 that has been through a number of rebuilds, which is currently a period bobber.  I am now rebuilding the T3 to install new seals in the motor and transmission and refresh the engine bearings.  When I got the T3 it already had Gilardoni cylinders and pistons installed.  I have read a number of forum threads about re-balancing crankshafts after installing Gilardoni kits, which got me to researching T3 pistons.
 
When I installed Gilardoni cylinders and pistons on the Eldorado, I measured the difference in piston weights and found the Gilardoni bare pistons weighted about 22 grams more than the stock Mondial bare pistons (370 gm vs 348 gm, or 500 gm vs 478 gm for pistons with rings, wrist pin, and pin clips).  I did not have the crank re-balanced, and I believe I can perceive a slight difference in vibration compared to the stock pistons.
Since the T3 came with Gilardoni pistons installed, I do not have the original pistons to compare to the Gilardoni piston weights.  The T3 motor to me is perceptibly smoother than the Eldorado.

From my research on 850 T and 850 T3 pistons, I have found that Moto Guzzi installed both full skirt and partial skirt 4-ring pistons in these motors.  As best I can tell, stock Eldorado pistons were partial skirt 3-ring pistons, with wider ring grooves.  From measurements and calculations, I estimate the missing skirt material to be about 8.9 cc, which at 2.7 gm/cc for aluminum would be about 24 gm, close enough to account for the 22 gm difference in piston weight.

My question to the group is, does anyone know when Moto Guzzi changed from partial skirt to full skirt pistons on 850 T/T3 motors?
 
For a bit more confusion, see the attached photos from a recent eBay posting for a pair of cylinders and pistons from a complete, unwrecked 11/74 (1975 model year) Moto Guzzi 850T with 23k original miles.  Note that one of the pistons has a full skirt and the other has a partial skirt.  I recall reading that Moto Guzzi had a practice of using up older parts after a design change, but it is a bit surprising to me that they would mix full skirt and partial skirt pistons in the same motor!







Any comments or observations the group has regarding original piston skirt configurations in 850 T/T3 motors from your rebuild experience would be appreciated.

Thanks, Dennis

[Chuck in Indiana]

I recall reading that Moto Guzzi had a practice of using up older parts after a design change, but it is a bit surprising to me that they would mix full skirt and partial skirt pistons in the same motor!

Welcome to the wonderful world of posting  on WG. As far as when they changed, I'm clueless as usual. I have a hard time thinking even Luigi would put an engine together with two different pistons, though..
(bump to the top so maybe someone that knows can answer your question..)

[Perazzimx14]

I'm not sure I'd take ebay postings as gospel. I'd also not rules out two differetn pistons being used.

[s1120]

I really dont see a real manufacturer installing two different style pistons...  Some of these companies have done some scheky things... but I cant see that.. Plus if you look at that picture of the underside of the pistons you can see pretty clearly that either A) one piston has not been run as much, or B) they are from two different motors all together.  The carbon is totally different on both of them and frankly so it the wear and staining on the jug..

[BigDen]

It appears using the photos from the eBay posting in my original post was a poor choice as it may be diverting the focus of my post.  I did not intend to disparage either the Moto Guzzi factory or the eBay seller.  The intent was to illustrate as simply as possible the difference in the two piston skirt styles.  The frame tag on my T3 has a build date of Jan-1975, and the motor number matches the frame tag.  I’m hoping those of you with experience rebuilding 850 T/T3 motors may know if/when the factory transitioned to full skirt pistons during production.  If my motor was factory balanced to the weight of full skirt pistons, it would help explain why it runs smooth with Gilardoni pistons.