How To Rebuild Your Vespa GS Front Shock
Before You Get Started
It would seem that the GS front shock is a bit of a rare bird as I was unable to find a good-looking replacement for my bike. After I rebuilt mine it came to light that you can buy a new reproduction shock from Bill Drake and from Pascoli. Guess I should have looked a little harder!
Bill Drake is a veteran in the UK Vespa scene and Bill's shockers have been remade and copied from a pristine original Italian original stock item. You can expect to pay about £120. You can contact Bill Drake via email (email@example.com)or by phone 02380 617316 (Evenings only).
M. Pascoli makes a reproduction as well. You can find this item for 100€ on the Pascoli website at http://www.mauropascoli.it.
Removing The Shock Cover
I will assume you have removed the shock from your bike as this makes it a lot easier to work on. Once the shock is removed from your bike you will want to seperate the top cover from the main body of the shock.
Use a flat-head screwdriver and turn the top of the shock-bolt clockwise while using a vice or spanner to hold the hex shaped portion of the cover. This will require a skinny screwdriver as it will need to go through the hole in the top of the shock cover. Once the cover is removed the spring should drop out.
Bottom Rubber Bushing
I was also unable to find a replacement rubber bushing for the bottom of the shock so count yourself lucky if yours is still in good condition. It was suggested that you can use black silicone adhesive sealant (or some sort of 2pak screen adhesive) to make your own bushing (special thanks to Paul Casey on the Vespa-Sports Yahoo list for this awesome idea! It's inexpensive and works!). Since I had no bushing left on my shock I had to get creative and this sounded like the perfect solution so I was off to my local auto store where I found some sealant.
After I cleaned up the old bits and bottom mounting hole I had to figure out a way to hold the inner-bolt tube in place while I applied the sealant and it dried. This took some creativity. I used a vice to hold the shock level and then I used another vice and a scredriver to hold the metal tube level in the sealant. Once I had this figured out I was in business. I put a LOT of sealant inside the bushing tube and then gently pushed the metal tube into the hole while avoiding the creation of air pockets in the sealant. This was very messy but once in place I put everything in the vice and let it dry over night. I came back the next day and it worked! This is not exactly the best solution but it seems to work. I will report any issues with this once I put a few miles on the bike.
Once the cover is removed you will need a special tool to remove the top cap of the fluid reservoir. Using a tool my friend made from an old piece of metal and 2 screws I had laying around (or you might want to try some circlip pliers or adjustable pin wrench if you have them around). You can easily buy the parts for this tool from a local hardware store.
I drilled a hole in the center of the metal that is a bit larger than the diameter of the shock-bolt allowing me to slide the metal to top of the resevior cap. I made 2 marks where the holes in the fill-cap are located and drilled them out. I found 2 screws that had the right threads and the right length and attached the tool to the reservoir cap.
Once the cap was off I drained the oil and replaced the rubber o-ring. This o-ring is still available from ScooterWorks.com (part #90151) and cost $3.45 with shipping.
Choosing The Right Oil
Choosing the right oil for the job was my next big step. Basically you need a shock oil with a viscosity of 13 that has "seal swell" properties as the seals in our shocks are natural rubber based.
The Vespa manual says to use Univis HVI 13, Esso (Exxon) Univis J 43 (which is a different brand that uses an outdated numbering system for viscosity making 43 the viscosity), Shell Tellus 13, and Mobil Fluid 62 (thanks Mike Clarke of Mike Clarke Scooters!). Shell suggest Morlina 10 as a substitute for Tellus 13. Morlina 10, like Tellus 13 is a 10 W SAE viscosity oil and they can sell 20 Litre packs to all those interested. Since you only need 50cc's Shell suggest that motor cycle fork oil of 5 W SAE is a reasonable alternative, but will provide a greater damping force - Possibly no bad thing for a GS/SS front end! (Thanks Dave Dry from Vespa-Sports forum for doing some research!). This is all very confusing and it turns out that modern versions of these oils don't have seal swelling properties so you can't use them as they will actually destroy seals that require seal swelling!
I should have tried eBay as Paul Barnes from the Vespa-Sports list claims he found some there for $20! Nice! Paul purchased some of the Bel-Ray 'Seal Swell' oil pictured below and bought a bottle of 10 weight and 15 weight from McBrannCycles.com. Paul mixed his oil in the proper ratio to get 13 weight oil.
The other crucial element to choosing the correct oil is the getting one that has seal swelling. Using the correct fluid is critical to the function of the seal because it makes the seal swell slightly, thereby providing the sealing to the shock inner wall and around the shaft going through the center of the seal.
And a tip from Dave Dry: Don't over fill the reservoir or you will lock up the hydraulic action!
You will note in the picture below that I don't have any 13 viscosity fork oil. I used a combination of 10 and 20. Make sure you mix them in the proper ratio!
Filling The Reservoir
I measured out the correct amount of oil for the shock as noted in the Vespa manual (50cc or 1.69 ounces) and began to fill the resevior. You will need to move the shock-bolt up and down while you do this so that the oil gets through the tiny holes and into the resevior. Add a little at a time until you run out and then replace the reservoir cap making sure to put the seal in first.
Once you tighten the cap you can replace the spring and put the cover back on. Voila! You're done. Now just put the shock back on your bike and enjoy!
A Final Note From Paul Barnes: I've heard about this other odd thing for awhile but have just come face to face with it. There are actually TWO types of internal for our front shocks. The first type is type illustrated in the factory parts book. The other is not. Instead of the 28mm x 8mm x 4mm oil seal used in the first type, the second type uses a smaller seal, (20mm x 8mm x 5mm), and a separate o-ring to seal the top of the shock. The top of the inner tube (the part the shaft slides up and down inside of) is made differently to accept this smaller seal. The smaller seal is a readily availbe seal from any seal/bearing shop, as is the o-ring. I don't know which design came first, but I'll bet it was the seal/o-ring type, then came the single seal.
Special thanks to all the chaps on the Vespa Yahoo group that gave their advice, experience, and research. It made my job a lot easier. If you have anything to add, edit, complain about just use the CONTACT US link at the bottom of the page.