The T5 is one of my favourite Vespa engines, particularly when fitted with a Malossi or Polini kit, but these engines haven’t always been easy to get spares for.
One of the previously hard-to-find bits was the cylinder head. These weren’t previously supplied with the most popular kits, so many standard heads were opened out to suit the 172cc Malossi kit.
Jumping from 125cc to 172cc means going up 10mm in bore size (from 55mm to 65mm), but it is not sufficient to simply reprofile the head to match or the compression ratio is far too high. It is also vital to open out the bowl of the combustion chamber in order to reduce the chamber volume to a sensible level. Previously this was the sort of engineering work entrusted to specialist scooter tuners around the country but these people are being lost and the pool of skills and 2-stroke tuning knowledge is shrinking.
As such what the world really needs is a bolt-on cylinder head that will match the kits, as Malossi and Co should really have supplied in the first place, as well as a replacement for the original Vespa 125 head.
Enter SIP Scootershop who’ve gone to the trouble of casting a brand new head from scratch with a combustion chamber and squish band shaped specifically to match the Malossi or Polini kits, or the original Vespa 125 cylinders. Admittedly there are already CNC-machined solutions for the T5 kits (SIP also sell CNC heads from MMW and MRP in Germany) but SIP’s is the first dedicated cast solution to fit the kits.
Off the head
These new heads feature:
- An O-ring in the head to improve sealing
- A pre-made threaded hole to suit the CHT temperature sensor of the SIP speedo
- A target squish (pinch point) of 1.2mm
- Claimed compression of 10.5:1 – 12.5:1
- Use of original spec long-reach plug (some heads modified for kits used short reach plugs)
SIP sent us a Malossi version to try, so we compared it to a standard head that had been modified originally by Chiselspeed in the early 1990s and later by another tuner to suit the Malossi kit.
The most noticeable difference between the two was the width of the squish band, which is the area around the combustion chamber which should match the profile of the piston crown. The SIP head has a wider squish band whereas the modified standard has a much narrower squish band.
Despite this difference, when we tested the volume of the two heads (using a drilled CD case siliconed to the gasket face as a flat plate to fill up to) they matched almost perfectly for volume.
Putting the Malossi head on was easy. We cleaned the surface of the gasket face (which had faint visible machining marks) by flatting it on glass plate with fine abrasive paper taped to it. It took very little effort to remove all the machining marks proving that the surface was already very flat.
The T5 head is intended to fit on only one way around (the four stud holes are not evenly spaced). SIP include the arrow on the casting which must point outwards, away from the frame of the scooter. The head also includes recesses so that dowels may be used to align the head
When we checked the squish using a cross-shape of solder, the readings were very uniform in the region of 1.38 – 1.42mm. This is a little larger squish than the quoted target, but certainly it’s ok to use and the piston is never in danger of touching the head. You can expect some variation with different barrels, casings and gaskets so we’d say that this is within acceptable limits for a plug & play item.
Given that the volume of both heads was within 0.5cc (SIP head slightly higher compression) we weren’t expecting any great variation in performance, however it was a pleasant surprise that the engine ran noticeably better with the SIP head through the entire rev-range, as you can see on the dyno sheet above.
The test engine spec is a typical bolt-on configuration to a standard motor with a Malossi kit and Scorpion expansion chamber.
- Pretty much Plug & Play
- Works well right out of the box
- Version available for standard engines
- O-ring sealing is a useful upgrade compared to standard
- Temperature sensor thread is in a silly place at the top of the head. This means that you need to make a hole in the cowling to fit it and must remove the fragile sensor every time you take the cowling off.
VERDICT: If you need a T5 kit head then this is currently the most cost-effective solution for the Malossi & Polini kits and also the only source of cast standard-style heads.
Price: £106.71 (standard), £115.67 Malossi/Polini 172
For more info: SIP Scootershop.