When the new-for-1975 Honda GL1000 was introduced, it was the world's first mass production water-cooled, shaft-driven flat-four motorcycle. Evolutionary and revolutionary, it employed proven practices in an entirely new package.

One of those practices was water cooling, up to then seen on only a few motorcycles, including the classic 2-stroke Scott and Suzuki's GT750 2-stroke triple. The Honda GL1000 was a hit, with some 100,000 sold in the U.S. during the model's 1975-1979 production run.

Honda followed the GL1000 with the 1980-1983 GL1100. Slightly larger and with more creature comforts, the GL1100 was mechanically very similar to the GL1000 and continued the GL's reputation for solid engineering and bulletproof reliability.

Yet as reliable as those bikes were — and still are — the youngest Honda GL1100 is now pushing 34 years of age and the oldest GL1000 39. Time flies when you're on two wheels. And as these were bikes made for touring, a typical GL1000 or 1100 today can easily be approaching — or exceeding — 100,000 miles. These bikes amass high miles with ease, but like any bike they have critical points of maintenance that are often overlooked.

On the GL, one of those points is the water pump. An obvious sign that replacement is nigh is a slight coolant leak from the "weep" hole in the front engine cover, directly below the water pump. When the water pump's seals start to fail, coolant will pass into the front cover and out this hole.

Another indication of imminent failure is a grinding mechanical noise in the front cover that changes with engine speed. Noises here can be from bad timing belt idlers or from a failed or failing water pump bearing, which is usually accompanied by coolant leaking out the weep hole. Either way, it's time to change the pump before it fails completely, possibly causing coolant to mix with the oil.

The water pump is housed in the front engine cover, which contains the transmission shift linkage and also serves as the mount for the engine oil filter. Oil is constantly circulating inside the cover and through channels feeding the oil filter. Additionally, the water pump is driven by the oil pump; if the water pump fails catastrophically, it can lead to a world of hurt.

Although not technically complicated, replacing a GL's water pump requires patience, so plan accordingly. More experienced wrenches can probably complete the job in under three hours, but if you're an average weekend warrior, make it easy on yourself and spread it out over two days. That'll give you plenty of time for cleanup and unanticipated issues.

Telltale "weep" hole under the bottom of the GL's front cover. The stain is from leaking coolant.

Whether a Honda GL1000 or an 1100, the process is basically the same. Some models have frame-to-engine clearance issues, requiring removing/installing bolts with the front cover. We drained the oil hot, then let the engine cool down before draining the coolant. We used a factory Honda replacement water pump and aftermarket seals. The pump was the correct updated unit, but the aftermarket seals bit us in the you-know-what. How? The replacement 56mm water pump O-ring in our kit was defective, resulting in a leak. We had to do it all over again, something we know you don't want to experience.

As always, a good shop manual is essential for necessary torque specs and to help guide you through the process.

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To start, put your GL up on its centerstand and place a drain pan under the front cover. Remove the oil drain plug followed by the oil filter assembly and drain the oil completely.

Remove the radiator guard, then the upper and lower radiator mounting nuts and bolts, and attaching plates. The hoses and upper stud bolts will hold the radiator for now.

Next, remove the coolant drain plug under the front coolant inlet and drain the coolant. Loosen the lower hose at the radiator, then remove the two bolts securing the coolant inlet and remove the lower hose complete with the inlet cover.

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As the coolant drains, remove the GL's "gas tank" assembly. With the tank off, remove the two hoses at the plastic coolant overflow tank and remove the tank. The tank is held by a bolt at the bottom, with a stamped steel strap bolted to the frame securing it at the top.

Now we're getting somewhere. Remove the two remaining bolts securing the intermediate water pump cover and you'll see the water pump.

It's a good idea to map out the front cover bolts, as they vary in length. We used our new gasket to make a pattern on a piece of cardboard, transferring the bolts as appropriate.

Disconnect the cooling fan electrical connector, located just above the cooling fan. Loosen the upper radiator hose at its connection to the engine and gently ease it off the engine while holding the radiator. The radiator should now lift out of the way completely.

With the radiator removed you can see the rest of the bolts you need to remove to release the front cover with the water pump. Loosen each bolt slightly, then remove them and transfer them to your cardboard bolt map.

Next, gently work the front cover off the engine. It can require a little persuasion, but it should come off fairly easily. We were lucky, as the gasket stayed with the cover; it's much easier to remove the old gasket from the cover than from the engine case.

Next, flip the cover over and remove the three bolts on the inside securing the water pump to the front cover. We've already removed two here, with the third just being removed.

Support the cover on blocks of wood. Using a suitable socket, knock the water pump out of the front cover. It should come out fairly easily.

Here's our old water pump (on the left) next to the latest Honda replacement pump. The original used a molded Bakelite-type impeller where the replacement has a stamped steel impeller.

Here's our gasket and seal set. It might look a little intimidating because of all the O-ring seals, but their location becomes pretty self-evident as you remove the old ones and prep the area for reassembly. Clean all parts thoroughly to prepare for reassembly.

The kit includes a new water pump to oil pump shaft seal. Note also the large O-ring and the smaller O-ring to the left. Pry the seal out with a screwdriver and remove the O-rings.

Clean the seal area and the O-ring seats. Install the seal using a suitably sized socket. It presses in quite easily. Locate and remove the other O-rings. Clean their seats and install the O-rings after giving them a light coating of silicone grease.

After ensuring the front cover is clean, dress the two water pump O-rings with silicone grease and install them on the water pump. Press the water pump into the cover by hand. It should go in fairly easily, and its offset design means it will only go on one way.

Bolt the water pump to the cover using new aluminum washers supplied in the gasket kit. Make sure all O-rings are in place and the engine and cover sealing surfaces are clean. Install the new cover gasket dry, without any sealer, on the engine. The locating dowels will hold the gasket in place.

Before you re-install the front cover, make sure the water pump shaft and the oil pump shaft are clocked the same. The oil pump shaft has a slotted protrusion that keys into the water pump shaft. Clock them both at 9 and 3 o'clock. Next, install the cover in place, locating it on the dowels.

Loosely bolt the front cover in place, followed by the intermediate water pump cover with a new gasket, again without any sealer. Tighten the cover bolts evenly. Factory torque is 9ft/lb. Install the drain plugs with new sealing washers and the oil filter assembly using a new filter.

We'd read that a NAPA 7733 radiator hose could be cut in two and used to replace both upper and lower hoses. As you can see, they line up pretty well. Purchased from Honda, the stock hoses list for $76.63; the single NAPA hose cost us $16.49, plus tax.

After cutting our NAPA hose in two, the new upper hose (the left of the right pair of hoses) looks almost identical to the original. The lower hose is straighter than original (the left of the left pair of hoses) but hot coolant will let it relax once it's installed.

Next, it's time to prepare the radiator for re-installation. We found it easiest to attach the upper hose to the radiator before installing. Your bike may or may not have the original radiator clamp protector as shown here to keep the clamp from chafing on wiring.

Hang the radiator on the fixed upper locating bolts. Attach the upper locating plate and nuts loosely to secure the radiator in place.

With the radiator loosely attached, slip the upper radiator hose onto the outlet at the engine and tighten the clamp. Check the position of the upper clamp; we had to adjust ours. Connect the cooling fan connector.

Attach the lower radiator locating plate and bolts. Tighten the upper locating nuts. Loosely attach the lower hose to the radiator and the lower inlet. Put a new O-ring greased with silicone on the inlet base and bolt the inlet in place. Tighten the radiator clamps, then reinstall the radiator guard.

Install the coolant overflow tank. Double-check mountings and hose clamps, then fill the cooling system with a 50/50 mix of distilled water and silicate-free antifreeze. Give the system time to "burp" then top off the radiator.

Refill the crankcase with fresh oil, then start the engine and run it for a few minutes. You'll probably have to top off the coolant a few times before it's full. Run the engine to full operating temperature and look for leaks and any sings of overheating. Fill the overflow tank to the marked line and that's it!