• Ciao Guest - You’ve landed at the ultimate Guzzi site. NEW FORUM REGISTRATIONS REQUIRE EMAIL ACTIVATION - CHECK YOUR SPAM FOLDER - Use the CONTACT above if you need help. New to the forum? For all new members, we require ONE post in the Introductions section at the bottom, in order to post in most of the other sections. ALWAYS TRY A SEARCH BEFORE STARTING A NEW TOPIC - Most questions you may have, have likely been already answered. DON'T BE A DRIVE-BY POSTER: As a common courtesy, check back in and reply within 24 hours, or your post will be deleted. Note there's decades of heavily experienced Guzzi professionals on this site, all whom happily give endless amounts of their VALUABLE time for free; BE COURTEOUS AND RESPECTFUL!
  • There is ZERO tolerance on personal attacks and ANY HYPERLINKS to PRODUCT(S) or other competing website(s), including personal pages, social media or other Forums. This ALSO INCLUDES ECU DIAGnostic software, questions and mapping. We work very hard to offer commercially supported products and to keep info relevant here. First offense is a note, second is a warning, third time will get you banned from the site. We don't have the time to chase repeat (and ignorant) offenders. This is NOT a social media platform; It's an ad-free, privately funded website, in small help with user donations. Be sure to see the GTM STORE link above; ALL product purchases help support the site, or you can upgrade your Forum profile or DONATE via the link above.
  • Be sure to see the GTM STORE link also above for our 700+ product inventory, including OEM parts and many of our 100% Made-in-SoCal-USA GTM products and engine kits. In SoCal? Click the SERVICE tab above for the best in service, tires, tuning and installation of our products or custom work, and don't miss our GT MotoCycles® (not) art on the BUILDS tab above. WE'RE HERE ONLINE ONLY - NO PHONE CALLS MADE OR RECEIVED - DO NOT EMAIL AND ASK QUESTIONS OR ASK TO CALL YOU.
  • Like the new V100, GuzziTech is full throttle into the future! We're now running on an all-new server and we've updated our Forum software. The visual differences are obvious, but hopefully you'll notice the super-fast speed. If you notice any glitches or have any issues, please post on the Site Support section at the bottom. If you haven't yet, please upgrade your account which is covered in the Site Support section or via the DONATE tab above, which gives you full site access including the DOWNLOADS section. We really appreciate every $ and your support to keep this site ad-free. Create an account, sign in, upgrade your account, and enjoy. See you on the road in 2024.

Piston Weights

PerryB

PerryB

Just got it firing!
Joined
Nov 5, 2010
Messages
21
Okay, I have received my new high compression pistons from Gutsi Bits and the combined weights for the piston+pin+rings+clips is different by about 1.1 grams. Is there a max difference published by Guzzi? Should I try to lighten one of the pistons? This is for a 1977 G5.

Is the preferred balance factor still 52%?
 
I have looked at the factory shop manual and it lists a maximum difference a 1.5 grams. Mine are in spec at 1.1 grams, but I'd like to get closer.

The interior (underside) of each piston is very clean--no casting ridges or "flash." I am too ignorant to know where to remove metal to balance the pistons. Online there are videos of people milling the inside of the pistons to balance them, but I lack the courage (or foolhardiness) to just start grinding. I guess that I will take them to a machine shop for this procedure.

Anyone else recommend either the 50% or 52% balance factor? Anyone have experience with balancing to either figure?
 
1.1 grams and you think that'll make some noticeable difference ? Heck the differing
carbon deposits will be greater than that after a short while . But hey , it's winter and
why not have a project . Have some fun and get back to us on this , I'm not too sure how
you can quantify the possible difference that may exist , but I've done a few things to
some of my bikes just to make me happy and to hell with measurements ! Peter
 
When I've had Guzzi engines rebalanced, 52% is what I used, on the advice of Charley Cole of Zydeco Racing. Here's another bit of advice from him: "Be sure to tell your balance guy to add 27 grams to the crank pin weight during static balancing. That is how much oil by weight is in your crank pin while your engine is running, but it is not there during static balancing. That's a pretty big balance error, if that is not included."
 
Thank you for all the replies.

The weight disparity is between the two complete piston+pin+rings+clips.

I am trying to fix a vibration problem that has plagued this bike ever since I rebuild it back in 2008. There is a thread that I started on this forum describing my attempts to diagnosis and repair this problem.

When I ride this bike it feels like the crank is out of balance, like there is a significant mass off-center swinging around at crankshaft speed. I want to re-balance the pistons, rods and crank. I want to do as much of the work myself as possible before I turn the crank over to a machine shop for dynamic balancing.

Does anyone have experience with balancing to 52% vs. 50%?
 
PerryB said:
Thank you for all the replies.

The weight disparity is between the two complete piston+pin+rings+clips.
Click to expand...
If you swap the pins you might get closer to the sets being the same weight. But if it is within the factory weight limit specs already, you might have other problems like worn out crank bearings, something with your clutch or even loose engine mounting bolts. Unbalanced carbs or ignition can make a bike shake like hell as well
 
This may seem totally out in left field , but have you checked your carden joint for play or for
looseness in it's support bearing ? This has been the only thing that has caused noticeable vibration
in any of the Moto-Guzzi's I've played with over the years . Peter
 
It is not out of left field. However, this is a problem that I've dealt with ever since I got the bike back on the road. Prior to returning the bike to service I replaced the bearing in the swing arm and the u-joints.

The vibration is present even when the bike is on the center stand and I rev it up in neutral. The vibration depends solely on engine rpms, not road speed.

I have swapped the piston pins so that the lighter pin is with the heavier piston. The 1.1 gram difference is after that swap.
 
Bent crankshaft.

Did this engine seize at some time in history or was the piston seized in the barrel?

The only thing that makes sense to me from what you have written, is a bent crankshaft.
 
PerryB said:
Does anyone have experience with balancing to 52% vs. 50%?
Click to expand...
Yes. But you already got 52% from Amboman.

I have always used weight of the rotating mass added with 50% of the weight of the reciprocating mass.

I still think your crankshaft may be bent from your statements.
 
Hmmmmmm. I bought this bike as a non-running derelict with the intention of turning it into a cafe project. When I got it home, I spent about ten minutes trying to get it to start before I just decided that I was going to rebuild it anyway and began ripping it apart. It cranked over fine. I concluded that it was bad gas in the carbs and didn't spend much time past that.

When I took the motor apart, there was wear, but no damage, certainly no evidence of a seizure. I was able to have the rod and main journals polished and they were still in spec. The machine shop that did that work also balanced the crank and didn't report any irregularities. The main bearings showed no unusual wear patterns.

I have to say that the bike runs really well. I have checked and re-checked the valve timing and clearances, the ignition timing and carb sync about a billion times. No adjustments made the bike vibrate less. One of the tests that I did to try to diagnosis this problem was to run the bike down a steep hill in 5th gear, compression braking at about 3,000 rpm (50 mph) and hit the kill switch. With the ignition off and no combustion taking place but the engine still spinning, it still vibrated. This told me that it wasn't a tuning issue, but an internal balance issue.

One odd thing is that I had originally used a lightened flywheel/ring gear combo but later switched to the original unmodified one to see if that would make any difference. While the lightened one performed better, the vibration was EXACTLY the same with the heavy one. In terms of vibration it made no difference which flywheel was installed.

The new pistons from Gutsi Bits are about 35 grams lighter than the ones that are in there now, so I will have to have the crank re-balanced anyway. I will use the 52% factor when re-balanced.
 
Well, it sounds like you would have uncovered this by now, but loose motor mounts will cause significant vibration.

Jason
 
Have you checked the cardan joint and carrier bearing in the swing arm as moto uno suggested? I had a cardan go bad on a CAII that caused vibration.
 
You must log in or register to reply here.
Back
Top