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'76 Sprint V
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I've got low compression on my Sprint V (80 psi).

My ring gap is just over .63mm (the highest my feeler gauge goes).

Edit: one ring gap = 0.73mm, other one = 0.86mm

I need to order new rings but don't have a clue how to measure what I currently have in order to order the next size. Any help appreciated!
⚠️ Last edited by carelgrundlingh on UTC; edited 1 time
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Veni, Vidi, Posti
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You may need more than new rings, but start by measuring the piston diameter.
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'76 Sprint V
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Up to what ring gap size would ring replacement be sufficient, and when does a new piston etc come into equation?

Piston - 57mm
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Do you know how many miles are on the engine?
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'76 Sprint V
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'76 Sprint V
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Unfortunately not...speedo was busted when I bought it
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If you can take the cylinder to a proper machinist and have him measure the bore in several places as well as the piston, it would give you an idea if it needs to be bored out or just honed with new rings.
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T5s
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Re: How do you measure ring size?
a mirror or a good friend??
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'76 Sprint V
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'76 Sprint V
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Re: How do you measure ring size?
T5bitza69 wrote:
a mirror or a good friend??
Laughing emoticon
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Enthusiast
'76 Sprint V
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'76 Sprint V
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Quote:
If you can take the cylinder to a proper machinist and have him measure the bore in several places as well as the piston, it would give you an idea if it needs to be bored out or just honed with new rings.
Thanks, I'll do that tomorrow. Having no Vespa guys anywhere nearby, it will be a normal engineering/machinist shop. So they wouldn't be able to advise me on stock sizes and oversize numbers.

Which wear away quickest: the actual rings or the cylinder (grinding away at the bore)? In other words: can the rings wear to such an extent that it is smaller than originally or would it rather eat away at the cylinder? The two different size gaps (edited in OP - .86mm and .73) make me think the rings wear easier?

The top part of the cylinder, where the discoloring start (probably about 8mm), where the piston stops, doesn't have a large lip (lack of better word?). When measuring the rings in that area give both rings a 0.03mm smaller gap, so essentially 0.03mm of the original bore has been worn away....correct? Not much, so I would think only rings would be in order?
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If you can establish what size piston/bore you have consider going up one size on the replacement rings. i.e if you have a standard piston get 1st o/s rings (if you have 1st o/s get 2nd o/s rings etc) and adjust the ring gap accordingly by dressing the ring ends.

It'll be a little more difficult to get the piston/rings into the bore when mounting the cylinder so you'll need strong finger tips/nails.

Only do this if you're handy mechanically and can recognise piston rattle. It'll freshen the engine up considerably for a while but it won't overcome a worn piston/bore so listen out for excessive piston rattle/slap afterwards.
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'76 Sprint V
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Had piston and bore measured at engineering/machinist shop:

Piston: 56.87mm at bottom of piston where they normally measure other pistons

56.84mm at the Piaggio recommended area (5mm below last ring)

Bore: 57.03mm

So just rings then?? Stock size (57mm) or next size (57.2mm) with some filing down?

Any idea what the piston/cylinder clearance on Sprint Veloce should be?
I can only find the values for P150X i.e. 0.225mm.
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To be honest I'm a little out of my league. Most of my knowledge is about four stroke engines which doesn't apply here. I'm hoping TCC or others will chime in again. Meanwhile, I'll do some searching.....
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Looks like you just need rings, not a bore.

Ring gap is ideal between 0.25mm-0.32mm (these are my own evaluations). A gap value over 0.5mm and performance will suffer with things like poor compression, bad gas mileage, difficult starting, etc.
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'76 Sprint V
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'76 Sprint V
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Thanks to all for help...much appretiated

Now for the agonising wait for the rings...probably 4 weeks Crying or Very sad emoticon
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X10 350
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T5bitza69 wrote:
a mirror or a good friend??
French bean, carrot, cucumber, marrow?
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