Coolant Filler Plugs

Sometime in the 1980s, Land Rover replaced the brass radiator filler plug with a plastic equivalent. The reason for this was presumably one of cost, certainly there seems no technical advantage. The worst failure I've ever seen from the brass item was one where the head was damaged to the extent where it was difficult to get a strong enough grip to remove it. Many owners have tales of failures of the plastic item however. Land Rovers (and their owners) being what they are, a variety of tools will have been used to unscrew this rather important device, and peering under the bonnet of a strange vehicle it is not uncommon to spy a chewed distorted stump of a thing where the 21mm hex head once stood. If the plug fails, at the very least it will bleed pressure and coolant from the radiator. Worse still, in the case of the 300tdi type engine, there are two of the beasties, on an engine known to be very sensitive to coolant loss.

The worst design 'feature' of the plastic plug (Land Rover Part No. ERR4686) must be the small slot in the top, which merely encourages the use of a screwdriver. Until recently, I have always assumed that the numerous tales of catastrophic failure were due to the plug being cracked by use of this slot.

Since I fitted the TGV engine into my Ibex, I've been trying to find the source of a slow coolant loss (1-2 litres every 6000 miles). While this isn't drastic, and the coolant level sensor I fitted provides early warning, I still wanted to track down the source of the problem in case it heralded a more serious failure in the future. When the engine was fitted, a new radiator and new filler plugs were fitted at the same time. Because the engine is higher than the expansion bottle, I know that a reducing water level tends to leave the top galley of the engine empty. When I tried to unscrew the plug in the thermostat housing, the top blew off explosively, drenching me in hot coolant and spraying most of the workshop off the underside off the bonnet.

Fortunately I wasn't scalded, and managed to retain enough of my sense of humour to see the 'Home Improvement' style funny side. Digging around the workshop revealed a spare (used) plastic plug, and a 1/2" BSPP Pipe stop. This latter fitted perfectly (once the remains of the old plug were removed from the housing) with an O-ring acting as a seal. While this worked fine, the o-ring is likely to eventually creep out and fail as it is not correctly located. there needs to be a locating groove to hold the majority of the O-ring body, either in the plug or the pipe face. Alternatively, a flat faced seal could be used.

My final solution was to replace the plug with a similar part from RS, part number 727-755 described as a hydraulic pipe stop, G1/2 (1/2" BSPP), which has a large O-ring seal fitted into it. I'm carrying a spare one in the truck, waiting for an excuse to fit it to the radiator whenever I next need to open the cooling system for any reason.

RS Part 727-755

Free Plug! - Well no, you have to pay for it, but I don't get paid for it IYSWIM. Several companies offer aftermarket replacement filler plugs if you want a more shiny solution, including the excellent X-Eng.

(Article date approximate due to website updates)