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Nov 15

MOTO International

Sharing Dave's ol' tech articles, episode 4

by MOTO International

 
 

A fork seal is a difficult piece to design. It must reliably seal in the fork oil, ward off contaminants, and induce as little drag on the fork tube as possible. This last point may seem minor but I have seen forks that operated very harshly all because of excessive friction in the seals.


 

The available fork seals for the various models all work very well, especially if you get the most recent versions. This is not as simple as looking them up in the parts books. Many have been updated and some of the updates aren't immediately apparent as the older seal has sometimes remained available for use in other applications. For example, the most common fork seal (9040 3449) has the same inside and outside diameter as its earliest predecessor (9040 3547) which is still used as the output seal for all big-twin transmissions and in small-twin rear drives. The dimensions are also shared with another seal (9040 3548) used exclusively to seal the front of big-twin 5-speed transmissions. Here then are my recommendations (actually Moto Guzzi's) for the various forks:


 

MODELS

PART NUMBER

QUANTITY PER FORK LEG

V7, Ambassador, drum-brake Eldo

9040 3550

1

V50, V50 II, V50 III, V50 Monza

9040 3144

1

T-5, V65TT, SP II

9040 3857

1

LeMans IV & V, Cal III, 1000S, Strada, SP III

9040 3952

1

V65 SP, V65 Custom, V65 Lario, T-3, disc-brake Eldo, V7 Sport, 850-T, Convert, 850 LeMans I & III, SP 1000, G-5, CX-100, Cal II, Mille GT

9040 3449

1

Daytona

3053 0502

1


 

Some bikes, notably late 70's models, originally had two seals per leg. This was back in the days when Moto Guzzi and most other manufacturers used the available seals which were really designed for rotating, not sliding shafts. To overcome their deficiencies, Guzzi chose to stack them. All current seals (except for the drum-brake Eldo, V7, and Ambo seal which hasn't been updated) are designed first as sliding seals. They have shorter, stiffer lips and an additional small lip facing upward to reduce the amount of debris getting to the actual seal. These seals are sometimes slightly taller than the old-style seals. Although it may be possible to use two per leg, one works very well and causes the least friction.


 

There are many aftermarket seals available but I wouldn't recommend any of them. I'm most wary of those which claim to be leakproof. To me this means that the manufacturer's design criteria is skewed away from the necessary balance between a reliable seal and low friction. Those I've tried have proven to be very high in friction which contributes to a poor ride.


 

There is another reason, specific to Moto Guzzi, to avoid aftermarket seals. Guzzi-made fork tubes (used on all models except the V65 TT and Daytona which use Marzocchi forks) are smaller in diameter relative to their nominal sizes than most forks. Other manufacturers make their tubes to the chosen diameter, say 35mm, then bore their sliders to 35mm plus the allotted clearance. Guzzi bores the sliders to the designated size and grinds the tubes smaller for the needed clearance.


 

What this has to do with seal selection is that aftermarket seals are most likely designed for slightly bigger tubes that those used on most Guzzis. For example, the current Guzzi 35mm seal, which is blue in color and carries the Marzocchi brand name, is marked as 34.74mm. For some reason, I still see some older-style seals coming through from Moto Guzzi. These are also made by Marzocchi but are black, shorter, and not sized to the smaller diameter of Guzzi forks.


 

On a few of the earliest models, Guzzi fit a circlip above the seals. Don't worry if your new seals don't fill the allotted space as the circlip has proven to be superfluous -- by far the majority of models don't use them.


 

New seals should always be driven into place against the bottom of their receptacle. This insures they are in line with the fork tube so as to induce the least possible drag. If you don't have an appropriate driver, use one of the old seals (after it's been cleaned), then pry out the old seal. Please resist the temptation to leave in the extra seal, thinking it will help prevent a future leak. Fork seal failures are very uncommon the Moto Guzzis. All the extra seal will do is add friction causing a harsher ride.


 

If you are having repeated problems with seal leaks you may do well to look beyond the seal to find the problem's cause. I find most seals leak because the surface they ride on is damaged, not because the seal "went bad." In the case of forks, this means checking the fork tubes to make sure they are straight and that the chrome surface is still in good shape.


 

One last point -- regarding the fork wiper or dust cap. These are designed to limit the amount of dust and debris reaching the seal by lightly wiping the fork leg just above the seal. Nothing much goes wrong except that older ones can become cracked and loose their elasticity. When this happens they no longer administer an even but light cleanse of the tube.


 

Dust caps have become a particular problem for the V65 SP, V65 Custom, Lario, T-3, V7 Sport, 850-T, Convert, 850 LeMans I & III, SP 1000, G-5, CX-100, Cal II, and Mille GT. The part was recast by Guzzi in the late 80's. The newer version doesn't actually touch the fork tube and so is completely useless. Your old dust caps may be worn and crusty but they're still better than the new ones. As an option, I recommend using the accordion-style gaitors which give better protection to the seals, shield the exposed portion of the fork tubes, and induce less friction on the movement of the fork.



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