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Help with pistons.....

MotoWally

Tuned and Synch'ed
GT Contributor
Joined
Nov 5, 2013
Messages
24
Location
Bartlesville, OK
Hello all,

I need another piston, pair of pistons, and rings like this one pictured.

Supposedly, these came off of an 81 G5 1000. Inside markings say Mondial piston,2639.2, and the top markings are Guzzi, SC, an arrow, and C. I find it odd that this piston is full round, with no cutout around the pin area.

The jugs are marked with a C. SO far, internet and actual travels to shops have not netted any goods. I thought I could get one piston, and two sets of rings, but no luck.

I also need to get a weight of an old stock 850 piston, so I can have a point of reference for balancing.

I am kinda committed to using these jugs, so I am stuck with getting pistons to fit. Saw some buzz somewhere about using VW pistons, but that's almost as bad as trying to find these to fit.

Thanks, guys.
 

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Tonerjockey

High Miler
GT Contributor
Joined
Oct 28, 2008
Messages
726
Location
San Jose
Hey Wally,

I believe I have the mate of that piston:



Piston_zps5af1f361.jpg


I was gunna swap out my G5 pistons but only found this one... over a year ago.

Lemme know if you want to match them up.

Mondail 2639.2

Alex
 

MotoWally

Tuned and Synch'ed
GT Contributor
Joined
Nov 5, 2013
Messages
24
Location
Bartlesville, OK
Well, "B" is closer than "A", let me check the clearances, I may still try and use it.

So, hang on to that thought......
 

john zibell

Moderator
Staff member
GT di Razza Pura
Joined
Oct 28, 2008
Messages
8,511
Location
Huntsville, AL
MotoWally said:
Hey, Toner, let me know what you want for that piston.

You can email me at [email protected]

I'm gonna try it, looks like my only option right now. Thanks !

It may work, but the piston to cylinder clearance will be a little large. At least you are not trying to put a "C" piston, into an "A" cylinder which wouldn't work at all.
 

Tonerjockey

High Miler
GT Contributor
Joined
Oct 28, 2008
Messages
726
Location
San Jose
I sent ya an email.

Bassically we settle on price, I ship to ya, if you like it (looks right etc) send $$ if no send it back.

Ciao 4 now,

Alex
 

MotoWally

Tuned and Synch'ed
GT Contributor
Joined
Nov 5, 2013
Messages
24
Location
Bartlesville, OK
John, my measurements showed that the "B" piston was only .03 MM smaller than my "C" piston. Do you think there will be a noise problem aka piston slap ?
 

ghezzi

High Miler
GT Contributor
Joined
Jul 7, 2011
Messages
836
Location
Brisbane
More importantly, have you measured the cylinders accurately?
0.03mm = 0.00181", basically that is just over one thousandth of an inch.

As long as everything is within factory tolerances, put the biggest piston in the biggest hole.
 

MotoWally

Tuned and Synch'ed
GT Contributor
Joined
Nov 5, 2013
Messages
24
Location
Bartlesville, OK
I have 2 "C" jugs, one "C" piston (came with the jugs off of eBay) and the "B" piston I got from Toner. Haven't ordered rings yet. Honed out one side of the case to fit the jug, working on the other side. And, I'm still trying to find a weight from a stock 850 piston so I can start on getting the rotating weight close to stock by machining the pistons & rods. So far, no help with that, bought three manuals, but no piston weights. Lots of specs, though.

Feel like I could go into the Guzzi repair business after all this. LOL
 

Tonerjockey

High Miler
GT Contributor
Joined
Oct 28, 2008
Messages
726
Location
San Jose
As far as weights, in the past I have used a gram scale and balanced one side with the other.

I compared pistons,
Then piston with rings, wrist pin and clips,
then rods with new bearings,
then the whole schmere. One side vs the other.

This procedure told me what wieghed what, and where any deviances were coming from.

Since I didn't want to remove anything from rods (or otherwise damage Carrillo rods) I removed any needed weight from the pistons if needed...
Once I got one side as close to the other as I wanted to get, I cleaned out the crank trap (reinstalled plug) and took everything (plus flywheel, clutch assy etc, marked for assy with all bolts etc etc) to a shop nearby (Gromm) and he balanced the crank and complete assys to whatever degree I was looking for.

This process created a smooth running engine.. as least as smooth as balancing rotating Guzzi parts can get me.

Are you sending the jugs to get Nikisil'd (sp?) ? I have used Millenium (in Wisconsin) twice now and have been very satisfied. Finish the pistons and send the pistons (and rings) to them with the cylinders. They'll make a great fit.

Good luck.

Alex
 

contractor20

Cruisin' Guzzisti
Joined
Oct 28, 2008
Messages
306
Location
Bairnsdale Victoria. OZ (west of NZ)
All I can add is a small amount of data from the official V7/Sports,850T workshop manual (with LeMans and Convert supplementries)
No listed piston or conrod weights for any model
Permissable weight variation...piston 1.5gm, conrod c/with bolts and nuts...3gm

Static weighted crankshaft balance as follows
For the two 750's.with 82.5mm bore X 70mm crank ..........1.471 - 1.601kg (31/4 - 31/2lb) big range or mis print tho it lists the range in metric and imperial measure ?
850T.- 83mm bore X 78mm crank ........ nothing listed
Convert. - 88mm bore X 78mm crank..........nothing listed
LeMans.- 83mm bore X 78mm crank .......... 1.586 - 1.616 (3.497lb - 3.563lb)
 

MotoWally

Tuned and Synch'ed
GT Contributor
Joined
Nov 5, 2013
Messages
24
Location
Bartlesville, OK
I'm using the stock Eldo 850 rods and crank, but the engine I have came with no pistons, just flaky jugs, and gnarly heads. I got a set of jugs and one piston (88 MM) off of eBay, supposedly came off a 1981 G5 1000. My thought was that I could do the "big bore" convert very inexpensive. ($1300 + when I called out to Cali for a kit price) They are iron-lined, but look to be in good shape, plan is to hone them lightly, and new rings, along with whatever new stuff I can get for the bottom end. I know, it's kinda Frankenstein-ish, but this is the road I chose. My thinking is to balance the piston/rod assemblies as close to stock specs as possible, without the expense and trouble of balancing the entire assembly. That's why I need the weight of a stock 850 piston, and I can go from there. Surely someone has one laying around a shop somewhere ?
 
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