CBR600 f3 fork cartridge mod into 5th Gen forks.

Hi Guys.

I know this is diggin up an old thread, but I'm unable to find definitive info for this fork mod as to which combination of valves are best to use.

Anyone done this with good results?

Did you use the full F3 components or a mix of F3 & 5th Gen Valving & install them into the F3 cartridges?

Are the 5th Gen valves identical to F3's?

If so, are they shimmed any differently to the F3's?

Decent used F3 forks are like rocking horse shit over hear in the UK.

Found a pair @ £60, 50miles away, collected during winter months, stripped em down last week, one brass rebound adjuster screw was seized solid, needed a good soak in (Deblock) release fluid, bit of heat from blow torch & the little shit broke free with some gentle back n forth coaxing.

All parts fully stripped/cleaned polished/re-assembled & ready to rock along with a fork service kit.

May need to upgrade the coils, don't know what's in there as yet till I pull my 5th Gen legs apart, all being well, previous owner has fitted some heavier linear coils?

Any input would be most appreciated.👍

What sort of valve bodies are in the F3? If they are the same HMAS units as the 5G (and they probably are) then my advice would be to replace them with Racetech Gold Valves or similar that have better flow. This is more beneficial in the compression valve as that is where the jolting in the stock fork arises. If they are the older-style Showa 3-port valves then these are not a bad starting point.

The original shimming in a 1997 CBR600F3 is apparently as follows:

Fork comp. 4 port HMAS

(1) .10 x 12

(2) .10 x 17

(3) .10 x 16

C : .20 x 8.50

Fork rebound 4 port

(1) .10 x 17

(2) .10 x 16

(2) .10 x 15

C : .20 x 8.25

Compared to the stock VFR:

Compression 
HMAS 4 port
.10x12 bleed
(2).10x17
(2).10x16
.20x8 clamp
 

Rebound:
HMAS 4 port
.10x17
.10x16
.10x15
.20x8.25 clamp
 

These look to be basically identical except the CBR has additional rebound shims which will stiffen that  compared to the VFR, but these might also be to offset the adjusting screw effect as well.

Both compression stacks use a .10 x 12mm spacer shim to create a low speed bleed. If you do replace the valves with aftermarket parts you will need to drill a fixed bleed to match this. 

I have something similar in my 99 with VTR1000 fork caps with rebound adjustability, 0.95kg/mm springs and Gold Valves in compression and rebound.

Cheers for the info Tez.

I've worked on a fair few sets of forks & shocks over the last 40 odd yrs or so, modified a few to work on other bikes, even straightened a few with decent success, but never had the interest to play around with OEM shim stacks until now, aftermarket valving is off the table, silly money.

Done a ton of internet searches, can't find any definitive info on this subject, seems to be a lot of conflicting info of using F3 or VFR valves or a combination of both, drilling out stock valve ports, that may or may not work?.

I've read F3 & 5th Gen stock valves are identical in design, will confirm for defo when I get the forks apart tmora.

I'm hopeful someone who's done this mod with success will jump in with some known shim stack recipes that have been tried & tested to work.

Try finding a pair of used VTR1000 forks here in the UK, near impossible, I'm used to & quite like the linked braking system, so for now it's staying.

Pls feel free to chime in with useful info & pics if poss.👍

The forks are one year only, 1994.   The rebound damping is a bit odd, as it is metered up at the cap, at the end of the hollow tube.  The caps are also fairly rare, partly because of production numbers and for reasons you found out...  frozen and then subsequently buggered up into junk.

I was able to combine parts from 2 sets of caps for a set of forks waiting to go on a RestoMod 86 VFR.  And the caps for my other set of 94 F2 forks are soaking, right along side a set of F3 caps.   Luckily the adjusters are still good....   just very frozen.

(Don't know how this post got above the one I responded to.  Delete and re-post?)

Lottery ticket time, just discovered what I thought were CBR600 with HMAS 4 ports, are in fact the better earlier Showa 3 port valve

Mucho sought after by the VTR crowd.

 Copied from VTR forum:

Showa 3 port valves can benefit from a filing mod.

File a large chamfer on the back side of compression ports (check valve side) & a slight chamfer on the back side of refill ports.

This is done to reduce oil flow turbulence, any turbulence effectively shrinks the orifice size, hence reduces flow.

............

Time to spend a few hrs in the garage stripping & cleaning.

22 minutes ago, Captain 80s

The forks are one year only, 1994.   The rebound damping is a bit odd, as it is metered up at the cap, at the end of the hollow tube.  The caps are also fairly rare, partly because of production numbers and for reasons you found out...  frozen and then subsequently buggered up into junk.

I was able to combine parts from 2 sets of caps for a set of forks waiting to go on a RestoMod 86 VFR.  And the caps for my other set of 94 F2 forks are soaking, right along side a set of F3 caps.   Luckily the adjusters are still good....   just very frozen.

Yeah seized rebound adjusters are a royal PITA to free off.

Apparently a mix of ATF & Dot 4 fluid 50/50 mix makes a potent release fluid, it's nasty shit tho with Dot 4 in, but good to soak stuff away from the bike in a tub etc.

I've used an Ultrasonic cleaner in the past to vibrate the crap out, (Innotec Deblock release fluid) with a bit of added heat is also awesome, aint cheap, it's by far the best stuff I've ever used.

36 minutes ago, Gaz66

File a large chamfer on the back side of compression ports (check valve side) & a slight chamfer on the back side of refill ports.

This is done to reduce oil flow turbulence, any turbulence effectively shrinks the orifice size, hence reduces flow.

............

Time to spend a few hrs in the garage stripping & cleaning.

pictures are worth a thousand words here! My VTR forks are currently blown apart on the bench and I am very interested in this journey of yours

https://www.peterverdone.com/archive/highspeed.htm

Tons of info in this link. 👍

The photo below shows a Showa 3-port valve and that is the HMAS valve. And then for comparison a recent Gold Valve (hint: it's the gold one). It is easy to see that the Showa 3-port has a lot more flow area than the HMAS, and the Gold Valve has a bit more again.

[image: IMG_1367.JPG]

[image: IMG_3164.JPG]

Ughandi, 4/25/2025 at 5:40 PM

pictures are worth a thousand words here! My VTR forks are currently blown apart on the bench and I am very interested in this journey of yours

Done a bit of chamfering to the 3 port F3 valves, not as radical as some I've seen online, it's a very fiddly process, my needle files are not that sharp anymore, need to re-stock my files.

If anyone is planning on doing any valve chamfering, I suggest buy some quality flat & round needle files before starting, it'll make it less of a PITA job.

Suppose you could use a tiny tungsten burr in a chuck on a flex shaft running off a dremel type tool, never seen any small enough for this job, one slip tho & valve would be ruined, hence I opted for files.

From the info found so far, any chamfering is a plus to reduce turbulence & give the oil an easier path to flow, also plan on giving the F3 shim stacks a try as a base to work from.

I've read about taking the valve surface down flat to remove the tiny bleed notch then drill a 1mm hole thru valve side, can't find enough info to move forward on that, so valves are staying stock 3 ports with a mild chamfer for the foreseeable.

I measured all shims & gave em all light hone on both sides on some 1000 grit wet & dry, this took off all the marks & crap stuck fast that brake clean wouldn't shift.

Next task is pull my 5G forks for a strip/clean & swap out the guts.

5G cartridge tube.

5G oil lock spacer.

F3 inner piston rod / rebound adjuster rod & top cap.

F3 rebound & comp valves.

F3 shim stacks as base setting.

[image: Chamfered comp.JPG]

[image: F3 shim stack.JPG]

You do need a fixed bleed of some kind through the compression shim stack and that is what those little cutouts provide. If you calculate their total area they are probably similar to a 1-1.3mm diameter hole so they may as well stay.