Changing the impeller in a Tohatsu four stroke 3.5 hp outboard engine

You can see the video about changing the impeller, the gear oil and the shear pin below.

Below is a photographic summary of how to change the impeller etc. The photographs are not brilliant - they are saved 'frames' from the video.

Changing the impeller: replace every 12 months or every 50 hours

Step One: have the outboard in vertical position, gear lever in neutral and the kill cord out. Prise away the rubber bung on the lower shaft unit with a flat tipped screwdriver. 

Step two: you should see the gear rod linkage bolt. This is it in the neutral position and below in the forward position. Check by moving the gear lever that you know which position is which and return it to neutral. 

Step three: loosen this bolt with a 10mm socket. DO not remove the bolt  completely and DO NOT alter either of the the heights of the two rods it links

Step four: using a 10mm socket loosen the two base bolts. 

Make sure that you loosen then and then unscrew them using fingers. Make sure that you support the weight of the lower unit with your other hand. I had a blue tray on the ground beneath to catch anything I dropped; or that might accidentally fall out

Step five: wiggle gently the lower unit whilst pulling GENTLY! Ease the lower unit out and try to get it to the point where everything is still inserted in the correct places but only just. At this point take photos of the interior so that you can see where everything goes when you later re-insert the lower unit. 

When the lower gear unit is removed this is what you should see looking up the shaft. The left hand pipe is the water pipe leading up to the engine, part of the water cooling system. The right hand rod is the gear selector rod coming down from the gear lever.  This is what happens to the inside of the shaft if you forget to flush it out on occasion after an extended trip at sea! I have no idea how to treat this and didn't find anything on it on the internet either. 

Step six: take the lower unit to the work bench and clamp the foil using a piece of thin wood to help secure it and to avoid damaging the paint work. This shows the pump housing unit which holds the impeller.  The black plastic pump housing is held in place by four 10mm bolts. from right to left - standing upright are a) the gear selector rod - DO NOT adjust its height in any way. b) the drive shaft   c) the rubber boot into which goes the water pipe from the shaft. 

Step seven: carefully remove the bolts and inspect their threads. Gently wiggle the pump housing off its base and up the drive shaft. Try to save the gasket which lies below it but I recommend having a spare just in case

Step eight: the impeller will either be in the housing just lifted off or like mine, still resting on the base unit. It is held in place by a small woodruff key. DO NOT lose this - it holds the impeller securely in place and stops it spinning freely around the drive shaft. Lift the impeller up and off the drive shaft for inspection. REMEMBER that woodruff key. 

I use pointed pliers to grab and securely hold the woodruff key

Step nine: inspect the impeller - see how worn it is, whether there are any tears or creases. At this point, having got this far, just replace it immediately with a new one but inspect the old one to see if  it showing an unusual degree of wear and tear - this would indicate something is wrong and it might be worth taking the lower unit to an outboard engineer.

Step ten: now inspect that lower gasket and the water pump lower section. Again, mine was heavily scaled by corrosion and salt deposits but they were not blocking the water flow channels. 

Step eleven: grease the lower pump housing unit but make sure no dollops of grease block the water flow channels. use a dab of grease to secure the woodruff key in its correct flattened place on the base of the drive shaft. Slide the new impeller down and line up the notch on its inside hole with the wood ruff key. push gently the impeller over the woodruff key. It it has gone on correctly, spin the shaft clockwise and the impeller should spin with it. The impeller should not spin around the base of the drive shaft without the drive shaft being physically turned by you. It helps if you have greased the impeller base and top and individual blades lightly - or used some washing up liquid as an alternative. 

Step twelve: check the inside of the pump housing for any scratches or grit/sand. Clean it and then grease it. 

Slide the pump housing back down the drive shaft - it helps if you have pre-greased this -  and then ease it down over the impeller. At this point it is very important to get the blades on the impeller facing anticlockwise - so spin the shaft clockwise as you gently but firmly press the housing down over the impeller. Make sure it seats correctly back over the gasket and aligns with the base unit. 

Step thirteen: grease the four bolts and insert them back in the holes. Hand tighten then up first. I always do the bolts in diagonal opposite pairs. In this way the unit will sit correctly when you tighten up the bolts. Do it the same way when you apply the ratchet and socket and remember - this is a plastic housing - so tight enough that they don't work loose but not so tight that you crack the plastic housing!

Step fourteen: try not to make this mistake! I took photographs of taking off this linkage and then didn't refer back to them. Thus I put it back on wrong and it caused me endless problems trying to put the lower unit back into the shaft until I had realised my mistake. On this photo, the gear linkage is wrong. the hole at the front is the one which should be put over the gear rod so that there is a 'spare' hole behind it!!

Step fifteen: this is the frustrating part and it can go easy - or hard - and there is no rhyme or reason for it!  You have to juggle three things - getting the water pipe back securely into the rubber boot on the pump housing, getting the drive shaft spindle at the top into the hole at the the very top of the shaft and getting the shaft gear rod into the spare hole on the linkage bolt. This took me almost 45 minutes and it is a task that would make a Saint swear profusely! 

My tips - firstly locate the drive shaft in its hole at the very top - it seems to be an inverted cone shaped affair and so once the shaft is within the cone - it will guide itself correctly into the engine hole. Then, secondly, get the water pipe into the rubber boot, not all the way, but enough. Thirdly locate the gear rod in that spare hole - remember it is critical NOT to adjust the height of either gear rod. When all three are aligned - then gently push the unit home. there should be no resistance and the flanges of the lower unit and the shaft should fully meet. Any resistance and something isn't aligned - so withdraw the unit and start again!  Good luck!!

Finally - remember when the unit is back in to do the reverse order of things - support the unit whilst you re-insert the bolts and tighten them up. Then making sure that that gear linkage bolt is in the correct position - tighten that up. Move the gear lever into forward and the bolt should rise up and slightly disappear. Move it back to neutral and the bolt should reappear in the centre of the hole. Then replace the rubber bung and give everything a good wipe down. 

Changing the shear pin and inspecting the propeller

Step one: with kill cord out, engine in neutral, straighten up the ends of the split pin so that  you can pull the pin back through the hole and lift off the prop. 

Step two: inspect the prop shaft. Remove the shear pin by just pushing through the hole and remove any green weed or fishing line that may have been caught up and wound around it. 

Step three: regrease everything and replace the shear pin. Do not use the old one - just replace it with a new one. Always carry two or three spare split pins and shear pins and know how to replace the shear pin just in case you ever need to when out in shallow creeks or beaches!

Step four: check the prop for splits. Scuffs can be gently sandpapered back down. 

Step five: put the prop back on and use a NEW split pin. Make sure you bend those ends apart well - so it doesn't work loose - that would be a disaster!

Replacing the gear lube in the lower unit

Step one: have the engine in vertical position, kill cord out and in neutral gear. Have a jug below to catch the oil. I put it in a tray  just in case I knock it over!

Now a top tip! Before you drain the oil - check the upper drain hole - remove the screw and check that there is actually a hole there!  I didn't do this - I just assumed it would be! See previous posts and the video for the disaster that happened as a result of me making this assumption!

Step two: using the correct sized screwdriver, loosen the lower drain bolt and then unscrew it by hand. As the oil begins to dribble out, loosen and remove the top drain screw to relieve the vacuum. Watch out, it might suddenly splatter slightly as the tension is released. 

Step three: allow the oil to drain out completely. I was expecting around  180 ml to drain out. 

Now is the time to inspect the oil appearance - if it looks milky then it suggests water in-gress - which implies that the prop shaft seal may have failed and needs replacing. Run the oil through your fingers and check whether it feels gritty. If it does - it suggests gear wear - take it to a specialist outboard engineer service dealer. 

Step four: replace the oil - use the correct lube oil - for this Tohatsu - it is SAE 80W -90. I use the tube and all you have to do is to cut off the nozzle tip, insert it in the bottom hole and squeeze the oil in until it dribbles out of the top hole. At this point insert the top screw. I always replace the little grommet washers on these screws as a matter of course. When it is in tight, quickly remove the gear lube tube and insert the lower screw and tighten. A tiny amount of oil will be lost - any more than that and you will need to start again and put more in to replace the lost amount.  You can get a tube adaptor that screws onto the lube tube and the other end screws into the lower drain hole. As this is the first time I have done this - I got it. However, be warned - despite me asking my chandler to check that it would fit my engine - it didn't - it needed a separate 8mm adaptor which you buy separately. the actual screw thread on the end of the tube is 10mm!

I ended using the metal bit of the tube assembly screwed onto the lube tube. 

After putting back in 180 ml and ensuring the screws are tight, clean up the skeg area.