'89 KX125

The old bikes like to wear out the shock body bore.

Nice job on your restore! I hope you are planning on racing it?

Those bikes only wear out the "shock body core", if the shock oil is never changed. The 89 KX's had great suspension components. The 44mm KYB shock was used thru 1992, Yamaha, Kawasaki and Honda 125. Do not put a gold valve in your shock. Keep the stock piston and have the parts replaced. That would be the best for your shock.

The real weak link on the 89 KX 125 is the motor, as the power is very flat. The hot ticket was the Weidler Dynamics KX 125 motor mods. It was very expensive to get those bikes to run competitivley. If I remember correctly it consisted of:

85 KX 125 black box
85 KX 125 stator and flywheel
86 KX 125 piston (domed piston and single ring). The stock piston was a flat top and I think a dual ring.
Modified stock pipe and a shortened stock silencer.
Modified air box. Carb is also jetted.
Head is recut to accommodate the domed piston.

The cylinder mods were extensive. The center KIPS vale was removed and a plug was installed. The center KIPS valve was so useless. It did not lower the exhaust port effectively and got in the way of the bike making power. The tunnels in the transfer ports had epoxy added to reduce the volume. This increased velocity and gave the bike good bottom and midrange. power.

With all of this said, the Honda's were still faster! Yea the "good old days". It is fun to revisit.

On the older bikes, (pre 86/87) the gold valve is a good option as it provides for an isolated outward bending compression and rebound shim stack. Most of the late 80's stuff is very tunable. In your case the 89 KX's were some of the best handling bikes ever made. It has modern suspension components. Gutting out your internals and replacing them with a gold vale for your bike is a waste of your money.

I recommend the following:

Replace the endless piston band and corresponding energizing o-ring.
replace the seal head assembly.
replace the bladder
replace the bottom-out bumper.
check for wear in the body. have the shock body hard anodized.
Have the spring changed for your weight.
Have the shock revalved for your ability and weight.