....

 

You need a 1.3cm radiator cap, stock honda caps are 1.1cm

 

0r you could file down the lips of the radiator.

 

I have a nissan cap on mine. That's what somebody sent me that said would work. It doesn't screw on all the way but it holds.

 

Your stock one is a Type D (Denso) fitment. The Koyo needs a Type F fitment (88-91 Civic, 90-93 Integra, 97-01 Prelude, and most aftermarket radiators). You can get one from Honda for onr of the above radiators or an aftermarket one (Mugen, Spoon, etc) with a higher compression (stock is 1.1kg/cm2; Mugen is 1.3lg/cm2; Spoon is 1.35kg/cm2). The higher compression rating allows the water to get hotter without boiling, but sometimes the stock plastic radiators crack under this pressure.

 

i use a koyo cap