I think you all bypassed the fact that he has mentioned "factory gauge still reads normal" my guess is that you have bought a cheapish (super CHEAP auto) aftermarket gauge which is causing the problem.Its the same story covering any engine,its designed to be reliable not fast not powerful,most of all its reliable,as soon as you start aftermarketing your engine you run into trouble,dont get me wrong I have webers a cam elec dizzie ect and when I put my foot to the floor its sick fun,but its in the shed in peices often too. btw my tacho varys day by day off its head and its SCA

wanted parts : good tacho.

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I can turn any hexagon into a perfect circle,pass me the multi-grips will you?

^how do you know the factory guage is reading correctly?

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You don't know what you are talking about. I have built many hot motors and have never had any real problems. All other competent mechanics, engine builders and mechanical minded folk will say exactly the same thing. Every so often something fails but that is what mechanical things do. They break down every so often. Too often do we see some guys blame the parts or there tools when simply it is there lack of thought and patience when putting in the wrench time. Everyone makes mistakes however what separates the guys that enjoy reliability and the ones that that do not is double checking every step you have made. Quite simply really

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Datsun 1600 CA18DET 185kw @ 16 PSI Engineered
http://ozdat.com/forum/viewtopic.php?f=59&t=6322&start=135

do they both run on the same sensor?

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My garage:
180b sedan daily (needs some TLC)
180B sss (under restoration)
180b Wagon daily
http://ozdat.com/forum/viewtopic.php?f=33&t=10489&start=30

Hi all,
Just wanted to let you know it's all sorted. Bought a candy/cooking thermometer and stuck it in the radiator neck and ran the car for about 30min. Factory gauge read 1/2 way (about 185 F) thermometer read 175 F and after market gauge 225 F. Factory gauge went one needle width past half and thermo fans switched on, bought it back to half way quickly. I watched this cycle several times. Since then the aftermarket gauge reads 250 F as soon as the ignition is switched on. The aftermarket gauge was on it's way out, just took a while to become obvious.

The gauges ran separate senders, although both are located in the lower T/stat housing. Haven't checked if it's the sender or gauge that's dead yet, just pleased that things are running at the correct temp. It's now got 1000km on the new engine and all is good.

FWIW the aftermarket gauge was an electric VDO. Quite old but reliable until recently. Not exactly a cheap POS, but not top of the wozza either. My engines have always been reliable, the last L20 did 16+ years of rally, autocross and daily driving and was still going good. It's modified but not what I consider hot. 4 x 40mm Keihin sidedrafts, extractors, re-graphed electronic dizzy, 5 speed, lightened flywheel, mild home port job etc. As MYDATO said, it comes down to being meticulous with assembly, correct tuning and vigilant servicing.

Thanks for all the positive suggestions, cheers Shaun.