Piston rings and CC

[ronnie66]

Hi All,
I am looking at putting new rings in on my Vespa and was wondering how to size them.
Using a micrometer the piston measures 57.5mm at the crown.
Please can anybody advise on What size rings i need to fit?
Also does this help to detect if my engine is a 125 or 150cc or does that depend on the length of stroke too?
Any advice welcome (as I'm still learning!)
Regards
Ronnie

[shipscat]

you have a 150 cc barrel and piston
A 125 cc is 52.? mm
i think pistons are usually marked if they are over sized +1 +2 +3 (someone will correct me if i'm wrong)
have look at parts suppliers they have lots of info

important !!!

when you buy new rings usually you need to file the ends a little to get the correct ring gap the reason you need to file them is because as they get hot they expand (the gap allows for expansion) , if the gap isn't enough the ring ends touch and expand then push into the ports with often catastrophic damage to cyinder especially on aluminium cylinders and sometimes the rider "sudden rear wheel lock up at speed cos the ring jammed in the exhuast/inlet port"
always cover you clutch !!!

[ronnie66]

Thanks for the advice chaps,
so My engine looks like a 150, it only has a 3 Port head and i noticed that the bottom ring is slightly longer than the top one is this normal?
Regards
Ronnie

[me]

Both rings should be the same length unless you have a dykes top ring - can you post a picture -

A new set of rings will be about £10 max for a standard piston.

When measuring pistons you measure them at right angles to the gudgeon pin and half way down, all good quality pistons are slightly tapered and oval this is to do with expanstion due to heat there smaller at the top so when the crown gets hot it grows and oval around the pin for the same reason.

Slide the rings down the bore and push down square with the piston till the rings are about 1/4 way down then with a set of fellers measure the gap between the rings this will then determin the amount of wear on the rings/barrel - then remove rings from barrel and slide piston down bore (without rings on) then hold up to the light see how much light you can see around the gap between the piston and the bore - the piston should block about 1/2 the light - this is a very rough guide to barrel wear but does give some indiction - then around the top edge you'll see a ring where the piston rings stop at TDC if there is a definate ridge you can easily feel with your finger then your looking at re-boreing time....

[henri]

i'm not aware of any major differences between heads on a 5port or 3 , stud spacings an profile are the same as far as i'm aware , as above ,always check ring gaps n adjust new rings to spec , an if replacing rings but not barrel its important to "de-glaze" the bore so rings can bed in , i use a hone ,but it can by done by hand with wet/dry paper , you just take the shine off bore very lightly an clean afterwards to be sure of leaving no grit behind .if not done rings wont run in properley an wear fast n be prone to seize . as for coverin ya clutch in case of seizures , ive mentioned before/elsewhere,i always ride with 2 fingers resting on clutch lever , not for seizures,just like flickin V's at random strangers/car drivers , saves time when they pull the inevitable numpty move , H

[vespasco]

You should really measure therjng gap before you order anything, just to see what rings/piston/rebore you may need.
As above, rings in cylinder, push down 25mm with piston, etc etc, measure the gap between the rings.
I only have the figures for a cast iron 200 but ring gap/clearance on assembly should be between 0.25 - 0.4mm. With 2mm being 'admissible after use' . Use feeler gauges.

Also, Is there a reason you think you need new rings?

[ronnie66]

Thanks for all the info everyone.
I have followed all the advice given and these are my findings:
Piston measures 57.7mm (90 deg at gudgeon pin, same at crown and base!)
Rings are 1.9mm thick (2mm?)

It is a three port, 150cc. With very little (if any) wear on the bore (no wear lip at tdc etc), the engine has done very little mileage and I am only replacing the rings as on removal of the head I had noticed the top ring partly stuck in its groove with signs of blow past on the piston.
In comparing the rings the top ring looked corse on its underside (cheap, inferior quality?)
I then noticed the gap difference between them,
In the bore off the piston 1/4 way down the bottom ring had a gap of 1mm, the top ring had a gap of 3mm.
Hence, I feel it best to replace them with a quality set, (any suggestions on the size, best make/supplier?).
Apologies for the lack of photographs as I still can't work out how to upload them from photo bucket (file too large for site seems to keep cropping up each time, any suggestions?)
I appreciate all of the quality feedback.
Thanks
Ronnie

[me]

as long as the new rings are the correct size to fit your piston - if you can look at the SIP site they have the best choice. Genuine Piaggio would be good if it fits your piston.

Are you sure the odd size ring isn't snapped at the end - it should have a small semicircle on each end to lock into the piston peg.

[ronnie66]

That was a thought I'd had, if it had snapped or been 'filed' down at some point.
I've not owned it long and I am just starting to work through my list of 'things to put right'!' As it has been slightly neglected in the maintenance department in the past few years.
I bought it as sort of a 'rolling resto' so am expecting to do quite a lot of work on it to bring it up to a decent standard.
I Love working on it, it's part of the fun of owning one and the only way to get to know them.
Cheers
Ronnie