The procedures that follow are generic in nature and will be valid for most
sport engines.

 If your engine is different, please contact the manufacturer
for specific instructions.
 

To dismantle the Engine

 Remove the cylinder head by extracting the retaining cap screws, mark which is the front.

Remove the cap screws holding the backplate in position.

Remove the carburettor and prop driver washers and collars (you may need a small gear puller for tapered collar types).

Usually, the cylinder sleeve can be withdrawn from the crankcase using fingers or prising the bottom of the sleeve upwards with a wooden dowel. 

 If it is stuck, try heating the crankcase in an oven at 200 deg. F for 10-15 minutes or soaking it in kerosene (paraffin) overnight.

 Mark which is the front to make sure that the ports are correctly aligned when re-assembled.

 Some crankcases have a very small timing pin to facilitate this.

Once the cylinder sleeve has been withdrawn, the piston becomes loose in the crankcase allowing the con rod to be slid off the crank pin.

 The crankshaft may need to be rotated to a particular position for this.

 Mark which is front of piston for re-assembly.

Slide out the crankshaft or gently tap the Prop end of the crankshaft with a wooden block if stubborn.

You are now left with the crankcase and bearings. 

 

To remove the bearings,

Either heat the crankcase in an oven at 200-250 deg F for 10-20 minutes or carefully hold the crankcase with pliers and evenly heat up over a gas jet, not too fierce. 

The bearings will usually fall out, but if they don't, tap the end of the crankcase on a block of wood to loosen the main bearing or on the side to loosen the front bearing

Small rods or wire hooks can also be used to pull out stubborn bearings. 

 Be careful not to damage the casting.

 Thoroughly clean the crankcase and crankshaft in meths or similar.

 Installing the new bearings

The rear bearing has no shields.

 The front bearing is usually supplied with shields on both sides.

 One of these shields needs to be removed, which can be done by picking out of the bearing with a sharp scriber or similar.  

This is important so as to allow lubrication to reach inside the bearing.

 Slide the new rear bearing on the crankshaft.

Heat the crankcase as before.

 Holding the crankcase with an oven glove, fit the front bearing into the crankcase and then slide the crankshaft into the crankcase and through the front bearing.

It may be necessary to restrain the front bearing to ensure that the crank does not un-seat it.

 With the crankcase at the correct temperature, the bearings should easily re-fit.

Do not force the bearings into place and do not use the crankshaft nut to pull the bearing into a cold crankcase.

This will damage the balls and races of the bearing and shorten its life considerably as well as cause increased drag and noise. 

Install the prop driver, washers and collars, the crankshaft should rotate completely freely with almost no drag.

 If there is any binding, re-heat the crankcase to allow the bearings to reset.

 

Re-assembly of the engine

Replace the piston into the crankcase and slide the connecting rod lower end on the crankpin. 

Check that the piston is facing the correct direction (remember your mark?)

Slide the clean, dry cylinder sleeve back into the crankcase. 

Do not lubricate as this will only bake on during operation and make it harder to remove next time

Make sure that the exhaust port in the sleeve lines up exactly with the crankcase opening.

Look for the timing pin or your mark.

Lubricate all internal parts with an oil compatible with the fuel you use.

Pure castor oil is good for this as is commercial glow oils such as Klotz Super Techniplate. 

Other lightweight motor oils can be used as long as they are compatible with the other parts of the engine.

Re-install the backplate (make sure that any gasket or O-ring is in good condition).

Re-install the head (make sure that head gasket(s) are properly installed and in good condition) and incrementally tighten the screws in a crossing pattern.

Reinstall the carburettor.

Your engine is now ready to run again. 

 

Remember that it will require a short break-in period for all the parts to reseat to their normal running
positions. 

 One small tank run at a rich setting should be all that is
needed.

Printable copy for the workshop on a plain background