CX/GL 500/650 Removing & Replacing the Water Pump & Mechanical Seal

The Yamaha replacement mechanical water seal

The first section of this page describes the problem, and the Yamaha alternative seal. The second part covers how to change it and service the water pump / impeller.

NOTE that the Yamaha mechanical seal should not be used with the 27mm rear casing aperture (details further down).

HOWEVER a recent experiment showed that the Yamaha seal DOES fit the 650.

When your CX or GL starts weeping ominous brown (seriously old antifreeze) , green (the wrong kind of antifreeze) or pink (correct type of antifreeze) fluid, or engine oil, from the water pump area onto the top of the alternator casing, and then down the left hand side of the engine onto the H-box, you start to worry about all sorts of things being wrong. This leak is almost always caused by a special seal, called the mechanical water pump seal, having failed. This seal is located in the rear engine casing, behind the water pump impeller, and it prevents oil lubrication from the camshaft area (which drives the water pump) from mixing with the cooling fluid. When the seal starts to fail, it allows cooling fluid to escape from a small overflow hole.

However, if you have coolant leaking in this area, it might be something as simple as failure of the O-ring at the water pump end of the chrome water transfer pipe. This is easy to diagnose. Run the engine and shine a lamp into the area, watching carefully. If the mechanical seal is leaking, coolant comes out of a small drain hole directly under the water pump casing. A failed O-ring causes drips directly around and under the top end of the chrome pipe. If in doubt, stuff kitchen roll all round the area and see which part is wet. If it's a failed O-ring, remove the pipe (photos below) and change the O-ring. Read the part at the end of this article about one problem associated with the reassembly.

The weep might also be caused by a damaged or cracked coolant feed pipe. This is a slender rubber tube which runs across the top of the engine from the thermostat to a stubby pipe on the top of the water pump housing, being secured with a simple spring clip. The tip of the this hose at the water pump end can crumble or degrade, or the nasty spring clip can just wear out. Some kitchen roll stuffed around this area will easily show a leak - if this is the culprit, and there is slack on the pipe, cut off the last 1/2 inch and secure the fresh end to the stubby pipe with a proper Jubilee clip. Or, replace the pipe.

Another weep cause can be that the dome nut which retains the water pump impeller has become detached. This can be easily seen once the water pump cover is removed, and is fixed without dropping the engine.

The mechanical seal is in three parts (left). A rubber seal and base, which locates into a recess in the rear (flat) face of the water pump impeller; a ceramic seal like a large Polo Mint that abuts to it, and a shaped metal seal which locates into the hole where the camshaft pokes through, and which is spring-loaded against the ceramic washer. The ceramic washer turns with the water impeller - but the spring loaded part doesn't. So, wear and eventually leakage between these two components is inevitable.

Note that the metal thrust washer which fits between the flat face of the impeller and the camshaft is shown here still in place on the end of the camshaft, just below the ribbing.

It is this weeping that causes the wet leak. Fortunately, failure is usually slow, allowing plenty of time for the problem to be addressed; and no harm is done unless the water loss is enough to lower the level in the radiator. You can ride home provided that you keep topping up the radiator, and you don't let the temperature gauge go over 80%, or into the red. Remember that you should not remove a hot radiator cap without a thick rag over it, as when the radiator pressure is relieved, the remaining water boils instantly and squirts very powerfully out of the filler cap. You can sustain serious scalds. If in doubt, wait until the engine has cooled.

So, is the solution a straightforward fix? Well, yes and no.

No, because:-

(1) the mechanical seal is what's called an interference fit in the crankcase.

An interference fit means that the rear engine casing must be stripped bare and then heated to at least 150C to expand it. This allows the seal to be very carefully driven in. The actual seals are both ceramic and particularly fragile. Most of us wait until the wife is out, and then put the degreased rear casing first in the dishwasher and then in the domestic oven.

(2) whilst you can theoretically get at, and change, the seal without removing the engine, in practice this is so difficult (through lack of space) that the time lost in removing the engine is easily won back again. Also, more importantly, leaving the engine in the frame does not allow you to replace the inner oil seal, which should be changed at the same time.


(3) a recent mechanical seal change job went off the rails when we discovered that there are two subtly different models of rear crankcase cover fitted to the CX and GL engines. Whilst some crankcases have the 28.3mm aperture, some engines have the smaller 27.8mm size.

27.8mm or 28.3mm mechanical seal aperture?

It has proved impossible to determine which models have which sized aperture, due to the fact that so many bikes have had entire engine or rear case transplants.

After a great deal of discussion with fellow spannermen, I have to advise readers of this page not to use the Yamaha seal with the 27mm aperture. Most attempts have failed, and if you have the smaller aperture, the Yamaha seal being a mere 0.7mm more in diameter than the Honda one, is just too big to be persuaded to fit.

The Yamaha seal is fine with the 28mm aperture and the Honda mechanical seal fits either size.

The Honda mechanical seal I bought in May 2005, for a CX500A, measures 1.129" (28.66 mm) in diameter and its Polo Mint was 0.905" (22.99mm) diameter, 0.156" (3.95mm) thick. Without compressing the spring, the seal was 0.634" (16.11mm) deep. The inner diameter, measure at the metal end which goes into the aperture, was 0.5635" (14.32mm).

The Yamaha "Polo Mint" ceramic seal, which sits in the rear of the impeller, is 0.9035" (22.95 mm) in diameter, and is 0.156" (3.96 mm) thick.

It is suggested that you scribe "27mm" or "28mm" on your rear casing, once you know the aperture size.

You have to remove the engine and rear crankcase to do this job. But you don't need any special tools (you're not forced to remove the alternator rotor). All the same, it's an ideal time to pop off the alternator rotor and check the camchain tensioning apparatus, alternator stator and water pump, whilst you are in there. Especially if the cam chain and/or tensioner have done more than 20,000 miles - I strongly recommend that you change the chain and tensioner blades, whether the manually adjusted (all 500s except Eurosport) or automatic (late model GL500s, all 500 Eurosports and all 650s).

This is the top part of the rear crankcase (left) after removal from the engine, seen from the outside, and with the water pump impeller removed. The snake-like passages are where the coolant is pumped out to the cylinder heads, and the central hole is where the end of the camshaft sticks through. It is to the end of the camshaft that the impeller is bolted, and between the two goes the mechanical seal.





This is the Yamaha mechanical seal, part number 11H-12438-10 (right) in its three component parts. The ruler is in inches. From left to right, the rubber base which sits in a recess in the rear (flat) face of the water pump; the "Polo Mint" ceramic washer; and the mechanical seal body.

This view would be from the water pump impeller end.

If you unthinkingly separate the rubber base from the ceramic seal, note that the blue flash on the ceramic seal faces outwards, towards the rear of the impeller and towards the rear wheel.



The same view (left) but the opposite sides of the items. Note the orientation of the ceramic seal, with the blue flash not visible.

This view would be from the camshaft end.




Mechanical seal - the face that locates against the ceramic seal which sits on the flat face of the water impeller. From this angle you can't see the spring loaded part of the seal.







The mechanical seal edge-on. Here, you can see the spring loaded mechanism. The blue band is a built-in sealing compound.








A close-up of the mechanical seal's face which is pushed into the camshaft aperture, towards the inside of the engine.






The mechanical seal placed, but not inserted, into the camshaft aperture.







Where can I buy this Yamaha item?

Any Yamaha dealer! I bought my two seals mail-order from:-

Fowler's Motorcycles, Retail Mail Order Dept., 2-12 Bath Road, Bristol, Avon UK BS4 3DR. 0117 977 0466. The total price for two seals including postage and VAT was £21.97.

Part number 11H-12438-10.

David Silver quotes £20 + postage and VAT (approx £23) for the genuine Honda mechanical seal.

Servicing the water pump, impeller, and changing the mechanical seal

Skill Levels explained.

Skill Level : 3a. Personally dirty : 3a. Work mess : 3a. Tools : 3a. Space : 2.

All nut and bolt sizes are given for the spanner size required to fit them.

Engine out - or not?

If you have coolant weeping from the drain hole under the water pump casing, there are two things you can try before delving deeply inside the engine. Firstly, remove the water pump casing and the impeller and simply replace the ceramic seal, its rubber boot, and thrust washer, the brass washer and the dome nut. You will still have to buy a complete mechanical seal etc but you can change these parts easily without any major work, and without dropping the engine. Secondly, when you reassemble the water pump, replace the O-rings in the water pump casing and torque the impeller dome nut and the casing bolts to a little more than the lowest setting. However if these ideas don't work, you will have to do a full mechanical seal replacement.

If you have engine oil weeping out, there is only one solution - drop the engine and change the oil and mechanical seals.

Removing the old mechanical seal is straightforward enough even without dropping the engine. I managed to get the old seal out, but there was some minor damage to the lip of the casing where the seal fits. The problem was simply that the seal, especially the Yamaha one, is such a tight fit in the aperture that I'd say that it's next to impossible to replace it successfully without dropping the engine.

I know from correspondence with other owners that it has been done. I tried it, and wrecked one seal, after that I didn't feel inclined to try again, and did it the proper way. Sorry-and-all-that, but with this job I strongly advise you to bite the bullet and drop the engine.

The main disadvantage with not removing the engine and rear casing is that you can't get to the camshaft rear oil seal. which should be replaced at the same time as the mechanical seal.

Jason Wright says "I installed two Honda mechanical seals in an '81 GL500, screwed them both up, and to my delight, the local shops had the Yamaha seal in for half the price. I have included a few pix of how i installed the last seal, without pulling the engine, it worked excellently, no leaks. The first two seals, I had the engine out for, but by leaving the case on the motor, the shaft on the end of the cam works as a guide. I would recommend using a spare impeller cover if one can in case it cracks. It needs to be torqued until slightly snugged."


However ... I still don't recommend attempting this job with the rear cover in place.



Have at least one new mechanical seal, one oil seal, and a rear-end gasket set with O-rings, ready. You will also need access to a domestic oven and a 28mm socket spanner, or comparably sized metal drift.

This page will work on the basis that you are working with the engine out. This webpage show you how to do that; note that you don't need to completely disconnect the carburettors, just swing them on top of the main spar, out of the way. Beware petrol leaks at this stage.

The photos here show the water pump area with the engine in the frame, because that was how I tried to do it first. Your engine will be out, but the method of getting at the water pump is the same.

Drain the coolant and engine oil.

If you didn't do this during the engine removal, remove the two clamps which hold the chrome water transfer pipe to the bottom left hand face of the engine (left). There are 4 Allen screws which hold the clamps in place. Pull the pipe away from the body of the water pump and drain off any coolant remaining. There will probably be a spillage at this point. The transfer pipe is a push fit into the water pump casing, and has a sealing O ring.

Pull off the thin rubber pipe from the top of the water pump body.




The water pump casing and body is held onto the rear of the crankcase by 2 x 14mm head bolts and 3 x 8mm head bolts (right), one is quite well hidden under the leftmost bottom edge of the body. Always use an 8mm socket, as these little bolts are quite soft, and are easily rounded off if you try an open ended spanner.





The pump body can be a stubborn fit. Give it a series of taps all round with a soft rubber mallet to loosen it, and using a broad bladed screwdriver, gently prise it away (left and right) from the rear engine casing.








Be very careful not to warp the aluminium casings, and as the body starts to come clear, don't force the blade into the gap and twist, or you'll distort something. Patience is the key here.








Once the body is free, store it with its five bolts.

Remove the 10mm dome nut and the brass washer from the end of the camshaft, and pull off the impeller rotor. This also can be a stubborn fit, and again the key is patient gentle teasing with a broad bladed screwdriver. If you break off the end of the camshaft, you are in serious trouble!





Don't lose the copper washer and the dome nut (right) and once you've wire-brushed the impeller clean, store all these pieces safely. I often store related components in clear plastic sandwich bags, label them and tie the necks. It's best to replace the copper washer. Just inside the flat face of the impeller is a thicker, steel thrust washer, don't lose this as it's important. It sits right inside the rubber boot which holds the ceramic seal.





Water pump area, showing (ringed, left) the overflow hole. This is where the weep comes from. As the mechanical seal fails, coolant leaks past it and drains down the little overflow passage, onto the left hand side of the rear crankcase cover, your left boot, and the H-box. As you can easily see from the photo, this is what happened here.








Once all the bolts are removed from the rear crankcase cover, tease the cover away from the engine main section. Try not to disturb the gearshift mechanism, as it's a little troublesome to re-engage it properly and it's best left alone unless you want to dismantle it for some reason.

Pull the oil seal out with a pair of pliers. If you haven't yet removed the remains of the old mechanical seal, insert something like a socket extension from the inside of the crankcase, locate it on the rim of the mechanical seal, and tap it smartly with a hammer, taking care not to score or damage the aperture.

The rear casing needs to be completely stripped of all components. The page on replacing the stator tell you how to do this.

Once the rear casing is stripped, offer up the new mechanical seal to the aperture and see if it is obviously too big. (See the note at the top of the page about there being two different aperture sizes). If it looks as if it will go in, wash and degrease the casing thoroughly (I put mine in the dishwasher, with no ill effects) and then heat your domestic oven up to 250 degrees C and place the casing inside. Most of us wait until the wife is out before we do this.

If you don't completely clean and degrease the cover before heating it, the smell of burning oil is vile ... and the wife will know what you've been up to!

Leave it for an hour or so, and with your 28 mm socket and new mechanical seal ready, remove the casing and place it open or engine end down on a firm, flat surface. Smear some gasket sealant round the circumference body of the mechanical seal and position it squarely into the aperture from the outside inwards.

Place your socket over the seal so that the socket's edge bears on the outer lip of the seal, and with careful taps of a rubber hammer, knock the seal into its housing. Be especially careful to keep the seal square on to the crankcase so that it goes in dead straight. Once it's fully home, allow the casing to completely cool, and as the metal shrinks, the seal is held in the housing. Don't use any pressure or force on the central part of the mechanical seal as this is particularly fragile. When fully home, the seal's outer lip engages with the aperture in the casing, and it's very obviously fully in.

Check that the drain is clear - I used a short length of coat hanger wire. Check that no gasket sealant is obstructing the drain hole at the water pump end.

Insert the new oil seal with its closed face towards the mechanical seal.

Before refitting the gearbox output shaft bearing, spin it round close to your ear and listen for any crackling or scraping. Likewise feel for any unevenness. If it shows any sign of stiffness or grating, replace it. These bearings are very robust and a failure is unlikely.

Reassemble the rear crankcase components, using a new camshaft oil seal, which goes in with its closed face inwards, facing the mechanical seal (diagram at the top of this page); and a new shaft drive oil seal and gearshift oil seal. The latter both drift in from the outside. Refit the rear cover, as per the stator change page. Put the gearbox into first gear, to hold everything still as you replace the cover.

Clean the impeller rear flat face (left) and the rear crankcase water pump area, and fit the thrust washer over the end of the camshaft. In the photo (right) the new thrust washer and brass washer are shown.

The ribbed thrust washer is VITAL and the most important washer on the whole bike! So don't think you can get away with omitting it, if you try this, or forget to fit it, the mechanical seal will leak.




Seal parts left to right : mechanical seal, steel splined washer, polo mint, rubber boot, impeller, copper washer, dome nut.

(Thanks to Jim Nelson for the photo.)




Impeller with the new rubber boot and ceramic seal face, what I call the Polo Mint seal (right).







Soap the rear face of the ceramic seal (left) before fitting the impeller, to lubricate the seal until the pump is delivering coolant to the components.

It's absolutely vital, for the psychological welfare of your bike, to do this with a cake of Imperial Leather!

Fit the thrust washer, impeller with its flat face towards the rear casing, the brass washer, and then the dome nut. Tighten the dome nut to 6½ ft lbs (0.8 - 1.2 kg/m). Again NEVER try this without a torque wrench as the stem of the thread is thin and easily overtightened. If you snap it off you will have to strip the engine and replace the camshaft - and CX camshafts are rare.

Refit the engine as per the engine removal page. Don't run the engine without coolant, and check carefully for leaks. When refilling the radiator, use a 50/50 mixture of distilled or deionised water (battery water) and silicate-free antifreeze, with its distinctive pink colour. Silicate-free antifreeze is much better for the ceramic seal and cooling system generally. UK readers note that if you buy antifreeze from Halfords, it should be the more expensive "Advanced" formula, which does actually say silicate-free on the rear of the bottle.

The O-ring at the water pump end of the chrome pipe has a compatible replacement which is easily obtained from any UK Homebase DIY store's plumbing section. The part is Homebase 6 pack Assorted Small 'O' Rings, 5013669158680, Article 623904, £1.09.

I found that the water transfer pipe needs rather careful handling during refitting. It locates quite easily at the radiator end, but the water pump end has a double ridge, between which sits the O-ring. As the pipe is inserted fully home into the aperture of the water pump casing, it's quite a tight fit and the O-ring tends to deform. Then, it either gets trapped between the ridges and the casing, usually cutting it, or gets pushed right off along the pipe. Either problem will cause a water leak.

The O-ring sits between the two ridges on the chrome pipe's end. Smear a little grease all round the O-ring as a lubricant during reassembly. Also, tease the pipe's end into the casing very cautiously, checking with a lamp that the O-ring hasn't deformed. I used a very narrow screwdriver to gently push the O-ring along all round its shape, with the pipe as I inserted it.

When the water transfer pipe is fully home, its sits correctly in the two mounting points on the left hand lower crankcase. If it isn't locating fully at these two places, it's displaced, probably because it's not fully inserted into the water pump casing.

If it does leak afterwards, just slip it out and examine / change the O-ring. A leak here looks like a bad mechanical seal, but it isn't. Don't panic.

Incidentally I have sourced the water transfer pipe and junction-to-cylinder-head o-rings at 20 pence each and I bought a dozen each of these for a fiver all inclusive. If you need these o-rings, contact Seals and Components Ltd, Village Road, Norton, Shifnal, TF11 9ED, Shropshire. Tel: 01952 730685. Fax: 01952 730665. The larger o-rings which seal between the 90-degree water junctions to the cylinder heads are BS-119 and the smaller and slighltly thinner ones which seal each end of the watertransfer pipes to the thermostat and junctions are BS-118.

Additional Photographs

Inside the rear casing.


Valiant's old ceramic seal being prised out of its rubber boot. Note the score marks - the seal is knackered!








The rubber boot being lifted away. The previous spannerman hadn't bothered to refit the thrust washer, which is probably why the seal was knadgered at only 42,000 miles.








Knackered rear camshaft seal, seen prior to removal. This is the inside face of the rear casing, showing the seal correctly fitted with its closed face towards the mechanical seal. The best way to get this seal out is to insert a socket from the other side, and simply tap it free.








When I poked my gloved finger through from the water pump side, the oil seal disintegrated. Imagine the fun if this spring ring had come off and wrapped round the camshaft ...










The stained rear crankcase after stripping. Notice the most important accessory of all, which is seen here on Valiant's rear carrier!












Ian Shearer makes some useful comments : "I just put in a mechanical seal and decided to try it without heating the casing, as the Honda manual says to just press it in with the special tool, and I couldn't honestly see why heating would be required. Here are some photos of how I pressed the new one in. I used the same method to pull the old one out, and then I put the old one back in again for practice. It seemed OK, so I went ahead and pressed in the new one. The casing diameter was 28mm. Bike is a 650 Eurosport."

Diagram of the home-made insertion tool. The thin bolt is slim enough to pass through the mechanical seal (where the camshaft tail pokes through to drive the water pump) and as the nut is tightened, the socket bears against the mechanical seal lip, and pulls it into the crankcase aperture. The flange just spreads the load over the inner surfaces of the crankcase.



Removing the old seal.










Close-up of the seal aperture, showing where a previous seal change has damaged the rim.

The drain hole is clearly seen in this photo.








Pressing in the new seal, using the home made tool, and without heating the rear crankcase cover.







The new seal fully inserted into the aperture.








Job done, at (says Ian) "an ambient temperature of 8C".


When refitting the rear cover, don't insert or tighten the bolts until the action of the gear lever is found to be correct. It's extremely easy to dislodge the gear level actuating teeth whilst easing the rear cover into place.


MANY THANKS to Ian for taking the trouble to photograph as he went along.






More useful stuff from the late Rick Hoad of Derby

"If you have the smaller 27mm aperture, use a 30mm flap wheel and power tool to gently ream out the orifice to accept the Yamaha seal. Flap wheels don't grind the metal away like an abrasive wheel would."

BE VERY CAREFUL not to run the flap wheel in deeper than the drain hole!

If you open the aperture all the way through, you are also widening the hole for the oil seal - and it won't seal. Don't ask me how I know this ... aaarrrgggggggghhhhhhhhhhh !!







"Here you can see the 30mm flap wheel inserted into the orifice and it very gently eases out the diameter of the aperture."










"View from the inside face of the rear crankcase, showing the mechanical seal correctly inserted."

Thanks to crack spannerman (the late Rick Hoad) for these invaluable tips.







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