How to clean a model engine.

I really despise castor oil….. As soon as I stopped using fuel that contained castor, my engines suddenly became reliable and stayed clean both inside and out. While ‘back-in-the-day’ castor might have been the best oil available, synthetics have now greatly surpassed it. However the same old arguments still crop up from those who swear by the oil:

 “It’s more tolerant of a lean run”

 “It doesn’t lose viscosity if the engine becomes too hot”

 “It smells nicer coming from the exhaust”

Ok, I admit the last point could be true, but as for the first two I reply stating why should your engine ever be allowed to run too lean? If correctly set up that state of affairs shouldn’t ever happen. I suppose it could be argued that “if you use castor you then need castor”. As soon as your engine’s exterior varnishes over with sticky burnt on oil you will lose air cooling efficiency and rely upon castor oil’s high temp properties to keep your now overheating engine running (It will also need constant re-tuning as a result; an easy way to recognise poor castor oil users as they are forever adjusting the needles!).

Consequently if I buy a second hand engine that has clearly been subjected to castor torture it is immediately stripped down and given a comprehensive spring clean.   Over the years I have tried multiple methods for cleaning model engines and have summarised my findings here:

Cleaning agent	Cost	Disadvantages	Result
Wire wool	Negligible	Evidence of a complete lack of mechanical sympathy? Hard work to do and removes not just the burnt on oil but also the protective anodising and/or sand blasted appearance.	The engine is clean but left vulnerable to exterior corrosion and scratches. It will most likely require polishing to get any kind of aesthetically pleasing finish. Re-sale value is nil.
Ultra-sound sonication	High	Specialist equipment.	Depends very much upon your equipment. I tried high-spec equipment in the laboratory at work. Does not harm the metal or finish but I found it did not remove all traces of exterior varnishing.
Dishwasher tablet in boiling water on a hob	Negligible	Due to the ionic properties of detergent the exterior anodising could darken – this is especially true if a steel pan is used or bolts/fixatives are washed at the same time. Bearings will be destroyed if washed.	Requires no additional scrubbing with a brush and can be immediately re-assembled. The engine is cleaned very thoroughly but left darker than it went in. This won’t affect the running of the engine, but it might affect re-sale value.
Antifreeze in a slow cooker	Nominal	Require a slow cooker that is then kept solely for hobby use.	The dirt, if not removed in the cooker, is left very loose and comes off easily with a (soft plastic) scrubbing brush under running water.   Once rinsed, oiled and re-assembled the engine is left looking immaculate.
Power/Oven Cleaner (e.g. Dawn or Cillit Bang)	Minimal	Messy and caustic!	Can be performed with the engine still in one piece. As per detergents above, the engine will be very clean but the anodising can darken.

Wire wool aside, I’m sure the detergent and ultra-sonication methods could be optimised in someone else’s hands, but I’ve settled on the antifreeze method and all my engines run perfectly following reassembly.

Antifreeze Model Engine Cleaning Method:

Step 1: Heat up the antifreeze.

NB: Before beginning, a word of caution regarding the coloured anodizing on Irvine engines (mainly their cylinder heads and black carburettors).  The colour is invariably stripped off in the cleaning process.  Anodizing on all other manufacturers I’ve cleaned is unaffected.  True anodizing is effectively an attractive and hard-wearing anodic applied layer which being aluminium oxide in nature shouldn’t be affected by anti-freeze (which of course is designed to work in aluminium engines all day long).  I can only summise that Irvine must use something else e.g. paint, or haven’t sealed correctly allowing the dye to leach and/or use a soluable dye.  This won’t impact on engine performance of course but might the resale value if it’s an old classic.

I use either Halfords red concentrate or Prestones antifreeze/coolant concentrate but anything will do the job providing – and this is important – it has corrosion inhibitors. This will ensure the aluminium anodising will not be harmed. If you have any concerns about food contamination, particularly if using a ceramic type, then I would urge on the side of caution and mark the cooker for hobby use only. A suitable slow cooker can be bought for less than £20 new from the likes of Argos so this need not be a particularly expensive sacrifice.

Step 2: Dismantle the engine.

Getting those bearings and cylinder liner out need not be difficult. I’ve seen videos online of blow torches being used here and cringe! A blow torch flame will work, but keep in mind that at 1500 °C it is both dangerous and you are well above the 660 °C melting point of aluminium. Consequently it won’t take much to go too far and spoil the finish. A much simpler method is to turn on the oven to 240 °C (gas mark 9), place the parts on a baking tray and ‘cook’ for a couple of minutes. More often than not, if face down, you will hear the bearing/cylinder liner drop out on its own accord. If not then (wearing thick oven gloves!) a quick tap on a block of wood will do it. Don’t resort to hammers, etc. If things won’t budge try increasing the oven temp first or leave it in for a bit longer. If a pair of pliers really is required then use a rag between the blades to ensure nothing is scratched. If the engine is hot enough the liner should slide out easily. Always make a note of any marks/guides present prior to dismantling so things go back together in the right direction. It is also worth pointing out here that cylinder head bolts should always be loosened/tightened in opposite pairs to even out the stress and prevent any distortion.

You will need a puller tool to remove the prop driver on most engines (I use this one from Machine Mart).

If heat doesn’t assist in removing those small bearings supporting the cam shaft deep inside 4 stroke engines then they are best removed using the pressed candle wax method (plenty of youtube videos on this).

Step 3: Soak the engine parts.

Add the parts to the slow cooker. Ensure all parts are completely submerged or you may get a tide line mark. Add a little water if needed to raise the level if required. Place the lid on to prevent evaporation and leave covered on a LOW setting overnight. I’ve often found the grime is loosened after only a few hours but everything is quite safe left for several days if required (in a well ventilated area). It is quite safe when using antifreeze to soak the steel and aluminium parts together. Plastic parts are also quite safe, as are ball bearings. I have even soaked an engine intact (which is useful for four strokes) – just ensure when rinsing afterwards that you liberally squirt WD40 into the engine to expel any water and then re-lubricate with afterrun/machine oil (Water Displacement 40 is NOT a lubricant in itself!).

Step 4: Final cleaning and reassembly.

Once ready, take out the parts (wear rubber gloves at all times so as not to scald yourself) and scrub with a stiff brush and soap under hot water. The dirt, if not already gone will come away very easily. The hot water just ensures everything dries quickly and I also place the parts on a radiator or oven shelf at 50 °C to dry. Coat any steel parts and bolts in WD40 immediately after washing or they will develop a surface rust.  Bearings if not to be replaced should be rinsed in WD40 then soaked in after-run oil. Check needle jets are clear of obstructions.

Reassembly is simply a case of reserve order to dismantling, using the oven to heat things up again. Use the exploded diagram in the instruction booklet as a reference – particularly with regard to the direction of the cylinder liner and piston. Once assembled, lubricate with afterrun/machine oil and then sit back and admire your handiwork.