Anybody know the piston weight tolerances?

Hi,Everyone! I'm sort of an exacting type guy when it comes to rebuilding an engine and I've looked all over the net for a chart that would tell me how close the piston assembly of the Polini kit I ordered should match the weight of the old piston I'm replacing,percentage- wise.(piston,pin,clips,and rings).I know it's pretty critical on Automobile engines,but I don't want to drill or add weight unnecessarily.I haven't received the kit yet,just planning ahead.I own a refrigerant scale which measures to 1/4oz. and a gunpowder scale which measures EXTREMELY accurate up to 2 oz.Would a postage scale work?Just as a guess I wouldn't think it would be too critical on a one cyl.engine that fires every stroke,but I'd like to hear from Fred,Ree,Ron,Jim C.,Jamie,Doug D., or someone who might know the tolerance allowed.The Franco-Morini runs so smooth now I'm sure I'd be really dissatisfied if I pick up a vibration when I install the new top-end.Thanks for any info....I just couldn't come up with any and was probably looking in the wrong place.

Re: Anybody know the piston weight tolerances?


I don't know the answer to your question. However, I can tell you that I haven't experienced any problems with vibration after installing a Polini kit. They manufacture the kits to extremely close tolerances and my bike runs smoothly with the kit installed. Of course, you've got me curious. I also have a new kit in the mail right now and I'm going to weigh it in comparison to the old one when it arrives. My best advice is, relax. You're going to like the extended power band. IMHO, the kit is the best bang for the buck to increase horsepower while also giving you a fresh, rebuilt engine. It also doesn't come with the noise and high rpms of a performance pipe. Your current pipe will keep a lid on the rpms.

Chris S

Re: Anybody know the piston weight tolerances?

I doubt you will like my answer Don.... but.... the truth is... it doesn't make any difference.. because as you said... it is a single cylinder engine.

Any 'balance factors".. are purely (and theoretically) arbitrary.

In multi cylinders it will make a difference.

But not in an uncounterbalanced single.

and ... by the way... I have talked with single cylinder experts about this.

Admittedly... they are more after hp... but they will tell you what my 1st paragraph says.

And... I think you give the original engine manufacturers engineers too much credit to start with.. for these 'old' engines.

maybe it will be smoother.

Re: Anybody know the piston weight tolerances?

Ron Brown /


As near as I can tell, Fred is right, as usual. Everything I have been able to read on tuning 2 strokes talks about things like adding spacers to crank throws, trimming piston skirts and other weight changing operations. Nothing is mentioned about compensating for this.


Re: Anybody know the piston weight tolerances?

Hello Don-Ohio, You may be opening a can of worms if you start changing the weight, I`m would think that the Polini piston would be in closer specs than the Tomos by far! With 1 piston you dont have any to balance with like Fred said, is it a 1 ring like the other kits? Good Luck, hope it does you a good job... Doug D.

Re: Anybody know the piston weight tolerances?

By the way Don... I give great credit to the Japanese for outstanding engineering ... year after year.

And yet.. they have built engines that customers will complain about having lots of annoying vibes... when the same manufacturer has a very similar model that isn't known to vibrate badly at all.

So ... I think the science of vibration (that reaches the rider)... is not as cut and dried as some may like to think.

I almost took a job several years ago as a 'balancing technician' where I apparently would have run crankshafts on a test machine to evaluate its 'state of balance'... and then done some calculations and trigonometry and drilled holes in certain locations to achieves some degree of better balance.

I always wished I would have done it for the experience... even though I figured I would learn everything there was to learn about the subject in 3 or 4 months.

Re: Fred's right


A single cylinder engine doesn't have another rotating or reciprocating mass to set an equilibrium to.

And as Fred also states, a "balancing technician" is actually a "harmonics canceller", where you are removing or adding to a mass so it is not out of balance in relation to another mass. The object is to minimize the difference between the centrifugal forces within the final rotating mass.

Then you get to dynamic vs. static balance. Imagine hanging a yardstick from a string at the midpoint. If you spin the yardstick (rotaing mass), it will rotate reasonably close to the point where the string is attached. Now tape a penney to one end and spin it again. The yardstick is all over the place. To "balance" this new rotating mass, do you remove the penney, or add weight (mass) to the lighter end? It depends on a lot of parameters.

That's why the balancing technicians make the big bucks.

Just enjoy the new performance Don. After a careful break-in of course.


Re: Anybody know the piston weight tolerances?

Hey Don, not to change the subject, but I bought one of the Tomos sidestands (chrome) from mopedwarehouse, they are pretty nice, I see they have part of there catalogue on-line. Doug D. (VA)

You're all great...Thanks!

I appreciate your vote of confidence for Polini ,Chris.It raises my hopes.Fred,I like your answer.I'm relieved not to have to worry so much about it.I wasn't kidding about near zero vibration though Fred.I think I may be spoiled.I NEVER have to re-tighten a bolt on the Sebrings and I don't use too much of the blue Loc-tite except on the exhaust mount bolt at the rear.Now,my ANKUR,....that's different! It has some annoying vibration which I suspect may be due to the dry type clutch.I wish you had taken that job Fred,but you're a wealth of knowledge as it is.Ron,that's what I gleaned also,about not replacing removed weight.But being a worry-wart and like I said,an exacting sort of guy,I was afraid I had missed something.Jim C.,I liked your explanation with the e.g. of the penny on the yardstick.It reminds me of those old tire balancers that did such a good job(Hunter,I think)that balanced the tire on the car.The weights applied were not usually in symmetrical order as in the case of a bubble balancer,but when those Hunter balancers were used correctly,they compensated for brake drum weight also.Of course,a smart person marked the wheel in relation to the rim for future removal,and tire rotation wasn't so big back then in the 60s and early 70s.And Doug D.,I think it will be a 2-ring set-up but I'm not positive.And like you I think I'm gonna enjoy dealing with `MOPEDWAREHOUSE' in the future with the new catalog.THANKS,EVERYONE,for the responses!

Re: Fred's right

Thats a great way to explain it Jim!!

Ya see a lot if tech talk in balancing 4 cylinder engines because of the weight/firing order Nothing to do with mopeds but I thought I would just throw in some interesting Chrysler technology. They had a tremendouse engine vibration problem with their 4cyl engines (K cars and such) answer: softer engine mounts with a silicon guey stuff in them. Didn't help the engines but it was a super cheap fix.

I bet you are going to love the new kit.

I would say post the performance and outcome of the new kit but I know you will (if you have time to get off of your bike once you get it set up:)

Re: Fred's right

Thanks,Jamie! I am surely waiting with `bated breath' for that kit.I'll probably ride a lot if I can,but `ol man winter' is fast approaching and I'm gonna hear it from Linda about that building I was supposed to build for storing our excess stuff that we must relocate from another house.You bet I'll be whining and tryin' to get my way though on every sunny day off I have.Got 4 days off starting at shortly ,but I'm afraid the kit won't get here by tomorrow or next day.Happy `pedding!

Re: Anybody know the piston weight tolerances?


I always thought that in a sincgle cylinder engine the weight of the piston was ballanced by the off ballance weights on the crank shaft. I agree it doesn't make as large a difference but I think having these two weights in ballance would help reduce vibrations (probably not as simple as simply the same weight theres probably some weird torque calculations and the like)


Re: Anybody know the piston weight tolerances?

You can try all you want... but there is no way to fully "balance" a reciprocating weight.... with a rotating weight.

Can't be done.

you can reduce it... and you can transfer it.... and create 'different' vibes

but you can't fully balance or eliminate it.. unless you have an equal and opposite reciprocating force

like a BMW boxer engine

Re: Balancing the Vespa Grande

The only way to balance mine is to cheat and put down the centerstand.


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