I'm after peoples opinions on how close would be to close for an engine driven fan. I'm looking around 10mm at this stage.
Its on an L20B in a B310 Sunny and the fan is a 7 blade nylon construction donated from an unknown source. It will be solid mounted. The radiator is Aluminium 3 row.
Thermos are not an option.

10mm is a tad too close, extreme temps can cause the blades to soften & load on fan will pull it towards the rad core.

Water,as in a deep puddle, will also pull the fan into the core.

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A little knowledge is a dangerous thing.
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"R.I.P. Baz. 29 April 2022. Thank you for all your contributions to the Datsun community over the years. You will be missed." - OZDAT

We used to use a L20b with 6cyc water pump, triple-core radiator and plastic fan for rallying - all in a 1600, leaves about 1cm between fan and radiator. Couldn't change the fan belt without removing the radiator to get the new belt in - you can always get the old belt out easily with cutters. Needless to say you make sure you're using a good quality fan belt of exactly the right size.
Put a grill, some sort of square mesh on the back of the radiator, about 1/2 or 3/4 inch squares (going from memory).
Cut the leading corners off the fan at 45 degrees - can be done in the car so you can flex the blade onto the grill and see if it's going to rub across the grill or dig in.
The blades will flex forward and rub on the grill instead of digging a big circle in your radiator.
This has even saved radiators when you have that brain-fade and stop a bit hard against something solid or in a ditch and the fan and water pump would have probably just punctured the radiator.
Don't use the steel fans if doing high revs or your've got a real "revvie" engine that changes revs rapidly, Ive seen a blade break off and come out through the radiator top pipe and bonnet, then the car feels like it will shake itself to bits in about a second. I've seen little bits and pieces come off the plastic fans, but not fly apart.

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'71 Datsun 1600, SR20DET red-top
Mostly OK, needs typical rust and panel work.

I have destroyed a good radiator with 10mm clearance and a plastic fan.
12-15mm seems about the safe minimum

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Circuit Race 1600 in the build

Thanks everyone for your opinions and experiances.
I expecially like the idea of the radiator guard as I have been in an accident before (someone pulled out on the red at a set of lights) and the force of the impact made the engine move forward. Back then I had the bluebird viscous clutch water pump (no fan attached as I didn't have the room). So in short the Hub slapped the back of the radiator luckly only took the paint of the radiator

At this stage the I have a standard "no AC" waterpump on and the bolts that hold the pulley on are about 20mm from the radiator. I can move the radiator forward another 8mm as it has spacers on its mounts. The fan will be mounted so its around the pulley hub rather then infront of it. The fan I have is almost perfect but I'll be visiting the wreckers for a better fit. Otherwise there will be some thining of the fan required.