Tohatsu 9.8hp 2 stroke thermostat removal and test

[nala]

Having recently replaced the water pump impeller, I thought I'd check the thermostat.

The reason for this is they say that the thermostat should be changed every couple of years with 2 strokes.

But, before removing anything make sure you've got the necessary parts to replace whats's removed.

The steel bolts which are 6mm dia with 10mm hex head can be reused.

Note the middle bolt which goes through the cylinder head into the water cooled exhaust will be a bit stiffer to remove than the other bolts.

As you can see there is muck on the threads of that particular bolt which will need a good clean before refitting.

The plugs look like new just a light wire brush and a re-gap, a tight 1mm before fitting.

I will use an old hacksaw blade ground to a chisel point, and the serrated side too.

This is great for scraping off stubborn and old gaskets.

Put cloths into the cylinder bores to avoid any debris from the gasket getting inside !

Initially, I slackened the cylinder head from the top right to left , going in an anti clockwise inward spiral with a 10mm combination spanner assisted with a 16mm combination spanner for extra leverage.

But, they didn't seem that tight to undo ?

The two lower bolts may have to be done by hand estimation depending if you have the torque wrench adapters to fit ?

So, to torque back up it's the reverse , starting from the centre outward in a clockwise spiral.

The bolts have 4 tightening stages 50% 5Nm 75% 7Nm 90% 9Nm 100% 10Nm

Then some say check torque after 5 to 10 hrs running to the 10Nm.

Another says after 1hr re-torque again to 10Nm.

But, regardless do both as the head gasket does get compressed .

[nala]

The spark plugs look as new but I do put redex into the fuel mix.

[nala]

Testing and checking the thermostat which had 52 °C stamped on it was put into a plastic cap off an aerosol oven cleaner spray.

I boiled a kettle and poured carefully into the orange cap with the thermostat placed into it.

Then, it's just a waiting game to see if the thermostat opens, which it does !

But, I've ordered a new one to put in and will test in the same way before fitting.

The biggest flaw by the manufacturer is that every time you want to check and or replace the thermostat the cylinder head has to be removed.

So, it's a head gasket and thermostat and maybe 2 x spark plugs ?

And you can do it yourself if your confident to do so ?

May not be a flaw at all because when the cylinder head along with the thermostat are refitted you'll be have a good idea of what the cooling water jackets/galleries are like.

Thus prolonging the engine life and piece of mind when your out there knowing there'll be no cooling issues.

[nala]

Removed the old head gasket and remnants by using the adapted hacksaw blade , carb cleaner and a drill with rotary wire brush.
Cleaned the threads on the middle bolt which is now clean .

Looking at the cylinder head the bolt holes are numbered so the tightening/torque sequence is straight forward.

[nala]

Re fitted the new thermostat and head gasket .
I was able to torque the lower 2 head bolts using a small 10mm combination spanner .
I put the ring end on the 10mm hex bolt head and the 3/8th drive of the torque wrench slots nicely into the open end of the spanner.Using the spanner as an extention to use the torque wrench.
Thus enabling me to torque them to the right sequence and pressures.
Gap checked on plugs, HT leads connected, fuel line connected and pressurised using the inline bulb.
Choke pulled out, throttle to start/run position and pull recoiled starter cord.
Started first pull ! ! !
Ran engine to warm up and thermostat opens allowing water to exit via upper exhaust to assist cooling.
Note you will only get exhaust alone coming from there untill it warms up then cooling water will come through too.
Will wait an hour for it to cool down then re-torque to 10Nm, then after 5 to 10hrs running will do the same again.