Vespa S50 > S125 > S200
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Joined: 20 Apr 2009
Posts: 4

Mon Nov 02, 2020 2:30 am quote
Hi there.

I want to share my small project with you and ask for some hints. I found in my garage one S50 2T tuned to 70 with engine dying. I was just ready to fix it, when I found rusted and ugly S125 4T for really few coins.

Changing the engine was on my mind since some time, but I was not sure about wiring, CDI and other components, so I decided to look for a complete Vespa. I was hunting for 125 ie, but it's quite impossible to find one cheap.

What are the differences?
1) Wiring. 4T engine has differences there and looking about both I think it wouldn't be such easy to convert that. The CDI is also different, but I believe you donít need the key or locks.
2) Fuel system. On 2T is completely different, and the fuel tank is bigger (no oil tank for 2T on right side).
3) Gas cable and rear brake cable.

As this was a straight forward swap and S50 was scratched a little, I decided to paint the body before reassembling. Tried to invent some crazy colour, but wife unfortunately made a clause: original fire red or nothing. Finished with glossy for deep colour effect, still not polished yet.


Joined: 20 Apr 2009
Posts: 4

Mon Nov 02, 2020 2:54 am quote
The engine.
Ok. So here we are. 4 stroke 125cc. 15 PS @ over 9000 rpm, 12 Nm @ over 8000 rpm. Quite a change even from tuned 70cc. Stage Pro claims peak power rise from 5 PS to 15 PS, but I believe that real was below 10PS (with bigger carb and Arrow exhaust).

LEADER engine range is specific however. I found that AC versions have only different piston and cylinder. Checked with part numbers - all the rest seems to be the same or almost the same. So why not to change for 200cc? There was one model I found (thanks to this forum) - Piaggio Liberty 200cc 4T, and the cylinder part number is 844113.

The 200cc Vespa should have around 20 PS @ 8500 rpm and 17,5 Nm @ 6500 rpm. So big bore, same stroke. That should give even more work culture than only going from 2T to 4T.

As the engine is still to be disassembled, it's a time for a search. 200cc cylinder is not common on the shelf, as it was used only in one model. Do you know any source, preferably in EU, where I can get one cheap?
Joined: 20 Apr 2009
Posts: 4

Mon Nov 02, 2020 3:10 am quote
The gears
Here I will need your advise. I will not drive Vespa out of town too much, but converting it to 200cc with low rpm top torque would require gear ratio change.

The stock 125cc ratios (thanks to
Primary - 15/46
Secondary - 15/42

There are some nice calculation here
I'm just not sure if only primary 17/44 will do the job, or I should consider also secondary 16/42 additionally. All thoughts welcomed, especially from those who installed that. The top spead with both would be better, but I'm not sure if 200cc will handle that.
2010 S50 Modified "Punkin"; 2003 ET4 Malossi 187 "ISO"
Joined: 18 Feb 2017
Posts: 668
Location: Deepinnaharta, Texas
Mon Nov 02, 2020 5:29 am quote

Top speed with 17/44 primaries on my ET4 187cc is 73MPH (GPS) at approximately 8500 RPM. The 200 should handle that ratio (about 6.6:1) easily.
Joined: 20 Apr 2009
Posts: 4

Mon Nov 16, 2020 2:32 am quote
Cylinder base gasket size.
200cc cylinder with piston arrived from Italy.

Ordering the rest of the parts, I found that it's compression is regulated by the cylinder base gasket - the head gasket is always the same. To choose the proper size, one need to measure without cylinder base gasket the difference between piston in TDC and cylinder top.

Now - when it comes to aircooled Vespa LEADER engines, there are only 125 and 150 cc. For 125 it should be 0-0,4mm and for 150 it should be 1,0-1,4mm.

I grabbed the manual for Liberty 125 - 200 cc and this shows only one general 1,0 - 1,4mm.

When I install my 200cc for check, the difference is 0,2mm. All this makes me think that 150 has different cylinder design (the combustion chamber seems to be 1mm deeper?). Or the crankshaft used in Liberty 200 is different.

Liberty 200 LEADER has 8411965001 crankshaft, when 125 and 150 LEADER has 8412985001. As some of you wrote that 200cc cylinder is plug and play, can anyone confirm that the crankshaft/conrod kit is the same, or I will have much higher compression?

vespa 125-150.jpg
Vespa 125-150

libery 125 - 200.jpg
Liberty 125-200

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