GSF1200S K4 Fork oil change and improvements.

[linuxrob]

Another 10000 miles and 3 years flys by, so another fork oil change.

The STD Suzuki springs were changed for Hagon progressive ones last time and the 5W oil supplied by Hagon was used. I stuck with the Hagon recommended 130mm air gap. Although an improvement over stock I felt the forks need more damping.

STD Suzuki oil is 10W, quantity is 509cc per leg and air gap of 105mm, the level is measured from the top of the fork leg fully compressed without a spring fitted.
Simple enough to remove the forks. Supported the bike on the lower frame rails with the front wheel in the air. Front spindle nut, brake caliper mount bolts loosened and removed. 2 M8 spindle clamp bolts loosened from the right hand fork bottom. Brake calipers pulled back and hung with bungee cords out of the harms way, then front spindle removed and wheel dropped out. Removed the 4 front mudguard bolts then it was moved down and out. The top fork nut is now lossened while the forks are clamped to the yokes. Each fork is held by 2 M8 lower yoke clamp bolts and 1 M8 upper yoke Allen bolt, with these loosened the fork is slide down and out.

Before removing the fork top nut with it preload adjuster, the said adjuster is wold off to it minimum position. This makes the nut a breeze to undo without excessive spring pressure causing damage to the threads. When the nut is all the way out the preload adjuster is wound all the way down and 2 14mm open ended spanners are used to detach the adjuster from the rod.

With the adjuster removed the retaining collar, spacer and spring are removed before lifting the fork spring out. To remove the oil just turn the fork upside down and let the oil drain into a container.

As an experiment I intend to use Hydraulic oil in the forks. My reasoning is this:

From what I can find out, not every manufacturers 5,10, and 15W oil is the exactly the same as every others, you could have a situation where manufacture A’s 10W oil is almost the same as manufacturer B’s 5W oil.

All Hydraulic oil is listed with the ISO standard and has to be the same. ISO 32 is approx 10W, ISO46 15W and ISO 68 is 20W. Smith and Allen Hypower 46 an ISO grade oil was chosen as it is near 15W and should give the forks a lot more damping. A 5LT bottle was the smallest I could get and cost £17.00, if this works well I will use this oil in my other bikes.

 

Made a simple level gauge from a syringe, funnel and small length of plasic tube, seen a few level gauge types on the net but had these bit in the shed.

To start with 500ml was poured into the fork leg. I intend to us a smaller air gap to add some extra preload to the system. I set my “level guage” to 120mm.

After pumping the fork 10 times or so to prime the system I let it settle for 30 minutes then with the fork leg compressed and gauge inserted I drew off oil to obtain a level of 120mm below the top of the fork. (around 25ml of oil)

 

Fitted the spring, I choose to put the closer coils to the bottom. Then the washer. On top of the washer goes the spacer. The damper rod is then pulled back up the tube and the with the preload adjuster screwed all the way in then spun on to the damper rod. this is then  sercured with 2 open ended 14mm spanners, Adjuster is now wound back and fitted in fork.

With the top nut lightly tightened the forks were bounced up and down few time and they did indeed have more damping with the tubes returning slower than before.
While the forks were out it was an ideal opportunity to get some Red Rubber Grease onto the fork seal to keep it as supple as possible, these seals are now over 60000 miles and 15 years old. They show no signs of cracking or leaks.

A thin smear is all that is needed round the bottom of the stanchion before pumping the forks a few times then wiping away any excess grease and refit the dust seal.

 

forks to refit then wheel and mudguard. 

Did my usual YouTube video

 

 

[jedibiker]

Im put hagon in my 650 and I dont remember seeing them recommend a bigger gap. They sent 7.5w oil which wasnt enough. Im sure you commented on a thread I was on about it anyway.

Did you like the hydraulic oil?

Ive tried 10w and now 15w all at the proper gap of 104mm the 15w doesnt feel right but as you said, different brands differ. Cornering was harder it seemed. So may go back to 10 and just take it the forks will be as they are. Using nearly all travel.

preload set with 2 full bars to go.. to far and its too stiff