ST 125 - the first Aprilia road bike

In the 70’s and early 80’s Aprilia was mainly known for its small capacity and competition off-road motocross bikes, then at the Milan Motor Show in 1981 Aprilia debuted the all new ST125 (Strada) road bike to excited crowds. On sale in late 82, but categorised as a 83 model, the ST featured a single cylinder liquid cooled 2 stroke engine produced by Italian company Hiro Motori. Design wise it leant on Italian market bikes such as the HRD and Malanca and also Japanese bikes such as the Yamaha RD/ Kawasaki AR models of the era. The Hiro motor was pre-mix only for 2 stroke oil, maybe this was due to its motocross based engine. For 1984 the ST remained in production alongside the new updated STX 125 model which featured a nose fairing and auto-lube, with the ST gaining the auto lube and is recognisable by its Aprilia-branded crankcases. As with the STX, the ST had a new exhaust system with a separate silencer end-can was used in 1984, replacing the long black pipe. The last evolution of the ST came in 1985 with the AS125R, this model featured a significant upgrade in the form of a Rotax 127 engine with the RAVE powervalve, and a large jump in performance.
The ST 125 was identified by the type code ST 125 and had DGM 50803. Chassis specification: 32 mm Marzocchi fork
APS (Aprilia Progressive System) with Sebac rear shock
Grimeca hydraulic twin piston front brake with 240 mm disc and a 160 mm drum at the rear.
18” wheels Engine specification: The engine was produced by the Italian company Hiro Motori s.a.s.
Reed valve intake system called H.I.S. (Hiro Induction System).
Forced circulation liquid cooling
Dell’Orto PHBH 28 AS carburetor.
Six-speed transmission.
Maximum power: 18.07 hp at 8250 rpm
Top speed of 126.8 km/h (78.78mph)
400-meter acceleration tests from 50 km/h (31mph), 23.7 seconds/ 98.800 km/h (61.39mph)

Brochure translation:

The 125 ST APRILIA is a motorcycle that goes beyond conventional, focussing on himself the skills of a Gran Turismo and a Sports Car. Elegance of lines, absence of vibrations, comfort, fluidity of driving and low consumption blend perfectly with speed, recovery, road holding, braking and safety. The liquid-cooled engine with internal circulation pump, allows homogeneity of temperature, consistency of performance and maximum noise containment. The front fork with advanced stud and the Mono-shock absorber rear with A.P.S. system (April Progressive System) they guarantee driving comfort and safety in curves with any background. The upgraded electrical system, equipped with a 130W generator with all battery services, offers greater lighting power, as well as supply consistency even at low rotation speeds, ensuring more safety in night driving. The saddle is foldable with a safety lock to access the object compartment. The instrumentation ideally positioned and also equipped with a "fuel reserve light" is truly functional and complete. In this way it allows constant control of every part of the bike without creating problems or distractions driving.

1988 AF1 Sintesi Replica Swiss version

Sometimes the rules on motorcycle homologation are a little bit silly, an example of this in the 80's was that to sell a bike into the Swiss market it could only have one headlight...

Aprilia were no exception to this so had to produce a revised fairing with a single square headlight to be able to sell a bike into the Swiss market. Honda also produced a single headlight version of the JC20 NSR125 which looks equally odd. Other examples worth checking out are the Swiss RC30, OW01 and even the 916!

My Bike at the Classic Motorcycle Mechanics Show, Stafford 11th - 12th October 2025

After finally getting my bike fully assembled and in one piece, it was time to pack up and take it to the 2025 Classic Motorcycle Mechanics Show in Stafford.

I entered the bike as a private entry, and it was allocated a space in the Retro Hall, just off the main exhibition area. Unfortunately, the layout wasn’t clearly marked, so exhibitors were left to choose their own spots. The space I ended up with didn’t really do the bike justice — the lighting was poor, and the location felt a bit tucked away. To make matters worse, there were several empty spaces in the main hall’s private entry section where the bike would have been far better showcased, both in terms of visibility and presentation.

Despite these challenges, the weekend turned out to be a great success. The bike stood out and managed to pick up a ‘Highly Commended’ award, which I was absolutely thrilled about — especially considering it had only been completed a week before the show!

Throughout the weekend, I had the chance to chat with a number of visitors who had owned an AF1 back in the ’80s and ’90s. It was fantastic hearing their stories and memories of this iconic Italian pocket rocket — the nostalgia and enthusiasm they shared really added to the experience.

In addition to my AF1, I also entered my Honda MBX on the Retro Mopeds stand. You can read more about that experience here.

along with my Honda, safely lashed down in the back of a van

My bike, tyres and final assembly

 I bit the bullet and ordered new tyres as I just couldn't bring myself to re-fit the old Avons as they are around 7 years old. I chose Bridgestone BT45's partly because of availability and also I used to run BT92's on my old RGV250 and they were amazing tyres. I used my local bike shop again the Bike Shed in Waterlooville as they helped me out with tyre removal and recycling, give them a go!

Next up was to just fit everything that was left over. A special mention to the fairing being a faff, but it looks like the fairing mounts had bent, so long term I would like another look at these to get the best possible fit.

So the bike is together and it looks brilliant - you will have to wait until the Stafford show for the full reveal (unless you are on the FB page), but there are a couple of jobs that need sorting before it can go back on the road.

*Front wheel bearings need replacing
*New fuel hose.
*Re-fit some of the over flow hoses
*New battery

My Bike, paint, bodywork, assembly

This weeks update is just the collection of parts that were out for painting, prepping some ready for assembly and starting the final assembly.

First off the nose fairing came back after having a full re-paint and having a mystery hole filled by my brother-in-law

Secondly the tank and seat unit were done via Dream Machine Race Paints

Assembly, paint match and wheel prep for new tyres

Faring, mirrors, indicator fir plus tank parts

bearing fit plus cleaned disc surface

seat and rear light fit + add Dunlop decals

paint match check

My bike, wheels 2

I picked up my wheels from the powder coaters today, I got to say they have done a great job as usual and most importantly got rid of all the corrosion and given it a finish that should last another 30 years!

I will have to do a little prep on them as there is an area on the rear wheel that should not be painted, so i'll be super careful and remove this myself.

The conundrum now is do I fit the old tyres, which look good but ate 5 and 7 years old, or fit new tyres? What would you do???

My bike, seat covers

 This last weekend I managed to finish up fitting the seat covers. I had ordered these covers from an Italian company called Volcano Industry  based in Sicily (probably named after Mt Etna!) who I originally found on ebay. 

The kit turned up fairly promptly and had a run of elastic sewn on the edge which you can use to staple it to the seat base. The kit is very tailored and fits very snuggly, so a little effort is needed to pull it around and get it in the right place, an extra set of hands would be useful! 

The end result is as good as the original if not better, the cover follows the curve of the foam and looks factory. Volcano also can offer alternative colours for the back pad for Sintesi use.

Bottom line: I am super impressed, Volcano have over delivered in quality, fit and finish and I can't recommend them highly enough. If you are restoring a Sintesi, buy this kit.

Full disclosure: I paid full price for this kit, I just believe in praising and recommending a great product when it warrants it. 

https://www.volcanoindustry.com/shop/seat-cover-aprilia-af-1-sintesi-1989-1992/