Headshok experts - please help


I have a Fatty D headshok in pristine condition and always worked well - had frequent services in the past done by cannondale mechanics (usual oil and seal replacements). Since I no longer have a bike shop nearby that can maintain this fork I decided to do the oil replacement myself - bought all the tools and seal kit.

I followed all the instructions from the Cannondale workshop manual and after replacing the oil for some reason the lockout does not work anymore (it was perfect before). There was no oil leak before nor after the oil change so I didn't change the seals . In unlock mode the fork runs perfectly smooth. When I lock it, after a few cycles of stroking the piston the resistance builds up up until what appears full lockout then when I continue pushing, the lockout 'gives up' and the fork goes back to normal suspension mode with no resistance.
I am completely puzzled by this. I repeated the bleeding procedure 5 times thinking trapped air may be the issue - made no difference

The lockout mechanism opens and closes the visible tunnels/holes in the lockout chamber (in the middle of the piston) so I can't see any issues there.
I noticed that the lockout chamber has several seals around it - could this be the issue? - (but if so why would these fail during an oil change?....) The cannondale manual makes no mention of replacing these so I'm reluctant to touch them and I'm not even sure that the seal kit (HD 166) contains these.
Again I did not replace any seals (lower cap is the recommended one mentionned in the manual) since there is no oil leak and I'd prefer to save them for a day when I really need them - not sure if this could be the issue also?
Any help would be much appreciated!

jay clock

Massive member
Hampshire UK
I have different issues with my Headshok so if you find anyone who can service them let me know


Oil lockouts work by having the right amount of oil.
If it's low it won't lock out properly, only part lock or nothing

I'd check oil level in the fork first.

jay clock

Massive member
Hampshire UK
that is not my problem. Mine is that i got it out last year and tested it for an MTB course I was going on and it all seemed fine. Next day (day before the course) i got it out again and the suspension had zero resistance and was banging like a couple of hammers in a bag and totally unrideable. I borrowed a bike for the course and put mine back in the shed. Got it out yerserday and bouncy suspension again

How do i fill oil? and what oil?

PS just checked and lockout not working but I can live without that for now
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