Motorcycle Fork Oil FAQ

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I am still in the process of rebuilding the front forks on Cal (my 1972 Harley-Davidson FLH Shovelhead), and I thought I would put together a quick FAQ about motorcycle fork oil – nothing ground shattering here, but certainly a list of reasonable questions that people sometimes ask.1

1) How Often Should You Change Your Fork Oil?
ANSWER:  First, check your manual – there should be an interval listed.  However, if there isn’t, a good rule of thumb is to change your fork oil whenever you change your engine oil.  This might be slightly overkill, but it is easy to remember.  At the very least, you should change it once every year or two, but increase the frequency for hard or dirty riding conditions (i.e. off-roading)

2) What Happens if I Don’t Change My Fork Oil Regularly?
ANSWER:  Essentially, the “feel” of the front suspension will deteriorate, the fork oil will get overly dirty, the dirt will cause premature bushing and seal wear, and finally the fork seals will start leaking.  Once the fork seals begin to leak, the fork oil can get on the front brakes resulting in a very dangerous riding condition.

3) Can I Change My Fork Oil Without Removing My Fork Legs?
ANSWER:  That depends.  Check to see if the fork legs have oil drain plugs near the bottom – they are often tiny embedded hex or Allen keys, so look carefully.  If you have a drain plug, you are in luck and can change the oil without removing the fork legs.  You will need to remove the drain plug, then push down on the front fork while holding the front brake.  This will compress the legs and squeeze out the last of the fork oil.  If you don’t have a drain plug, you will need to remove the legs by loosening the triple tree pinch bolts and sliding each leg out the bottom of the tree.

4) How Much Fork Oil Do I Need?
ANSWER:  Check your manual or the Internet.  There is usually a wet specification (the amount needed for a quick change) and a dry specification (the amount needed after a complete fork leg rebuild).  Some bikes require you to fill the legs and take a distance measurement from the top.  Finally, although imprecise, you can measure the amount of oil that came out of the fork legs upon draining – refilling with a similar quantity, keeping in mind potential leakage and loss over time.

5) What Viscosity Oil Should I Use?
ANSWER:  Again, check your manual.  However, keep in mind that greater viscosity oil provides greater dampening.  So if you have found the damping a little weak, consider jumping up to the next heavier viscosity oil

6) Is There a Difference Between Engine Oil and Fork Oil?
ANSWER:  Absolutely, they are formulated differently and should not be inter-changed.

That’s it folks – a simple fork oil FAQ.  I hope you learned something and found it useful.

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