hmm, I have a RB20 in my S13 and I must say that the RB20 isn't as prone to overheat as quickly as the 25 or 26 so definately check your thermo and waterpump... As i have my rb20 in my car with the r32 radiator which isn't but like a 1/2 inch taller than the s13 radiator.. but the thing is that i am currently running without a fan as mine died (you know you can't fit the clutch fan) and mine hasn't overheated yet.. So make sure you check out your thermo and waterpump, one of those is most likely your problem. Good luck