OS 50 Hyper Max engine internals

Written by Rob Turnbull Sunday, 06 April 2008 00:00

OS50 Internals

In this article we will take the OS 50 hyper engine apart and go through the steps needed to put it back together again.

This should give you an insight into how you can service the engine internals.

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Removing the piston
Removing the piston ring
Removing the con-rod
Refitting the con-rod
Refitting the piston ring
Refitting the piston assembly
Refitting the piston sleeve
Refitting the cooling head
Refitting the baseplate

Removing the piston assembly

The piston assembly is removed through the head end of the engine so the first thing that needs to be removed is the head. Remove the six M3 bolts that hold the head in place and pull the head off the engine body.

The next step is to remove the baseplate to allow the con rod to be removed from the journal which is what connects it the the crank. Be careful with the gasket that sits between the baseplate and the angine body as it is fairly fragile.

Make sure the gasket is in good condition and replace it if it is broken or badly crushed flat.

The sleeve needs to be pushed out of the engine body now. Positioning the con-rod will allow you to use your finger to push on the back edge of the sleeve so that it emerges from the head end of the engine body enough to then be removed.

Next the con-rod needs to be detatched from the journal. Position the crank so it sits at top dead centre (TDC) and then hold the engine upright and use a finger to ease the con-rod off the journal. It will pretty much fall off so only a little gentle persuasion is needed.

Removing the piston ring

The ring sits in a groove near the end of the piston. It is sprung metal and is held in place by a locating pin. This will be covered in more detail in the section on re-fitting the piston ring further down this page.

To remove it, it must be carefully and gently prised off the end of the piston. Get a fingernail behind one end of the piston ring and pull it outwards a little, enough to lift it over the end of the piston using a winding motion as you remove it.

Do not stress the piston ring or you may damage it. That said, it will take a little bending (required to remove it) but be careful!

Removing the con-rod

The con-rod is connected to the piston by a rod which is pinned into the piston by spring clips on either side.

Removing one of the two clips will allow this rod to be pushed out of the piston and then the con-rod is free to be removed.

Be careful not to lose the spring clips when you remove them, they're very springy! I find using needle nose pliers and a twisting motion gets them out quite easily.

Refitting the con-rod

The con-rod is not symmetrical on the end that connects to the journal (on the crank), it is stepped out more to one side than the other and this is so that it sits flush on the baseplate when everything is fitted together, while keeping the other end centred in the middle of the piston retaining bar.

Also note that the brass bushing in the journal (larger) end of the con-rod is chamfered on one side to make fitting it onto the journal possible, while the other side is flat flush.

The cut-away at the base of the piston is there to make room for the base of the crank when it turns to bottom dead centre (BDC).

Get the piston and con-rod positioned so that the parts will be fitted together correctly and then slide the con-rod retaining pin a little way into the pistion.Slot the con-rod onto the end of the retaining pin and the push the pin all the way through the piston. Slot the retaining pin clip back into the piston and make sure it sits inside the groove which makes sure it stays in place.

Refitting the piston ring

The piston has a pin in the groove where the piston ring sits which prevents it from rotating around the piston. The ring must be lined up so that this pin will sit in the cutaway on the inside of the piston ring where it is split. When the sleeve is fitted over it, the piston ring is compressed into the groove of the piston so the pin must be in the right position or it simply will not fit together.

Fit one end of the ring into the groove near the retainer pin and using a gentle winding motion, fit the ring back onto the piston. Be careful not to overstretch the ring or you may damage it.

With the ring fitted, slide it into position so the pin sits in the gap where the ring splits.

Refitting the piston assembly

Line up the cutaway side of the piston so that it will sit nearest the base of the crank (stand the engine upright and the cutaway points to the top).

Slide the piston assembly into the engine and with the crank positioned at TDC, slot the con rod onto the journal. It will probably need a little working into position but should not need to be forced.

Holding the con rod over the journal, turn the crank a little bit each way to work the con rod onto journal, it will slide on fairly easily with a little wiggling.

Refitting the piston sleeve

The sleeve has a notch in it which must be lined up with the retaining pin on the front of the engine. This ensures that the vents line up with the exhaust port too.

The opposite end of the sleeve has a shamfer to help with fitting it into the engine.

With the sleeve lined up correctly, slot it over the end of the piston. Compress the piston ring into the piston ring groove, making sure the piston ring retaining pin sits in the gap where the ring is split. The ring should sit perfectly flush all the way around when fitted correctly.

Work the sleeve over the pistin ring area and once the piston ring is inside the sleeve, slot it into the end of the engine.

You can make sure it is going in straight by working the chamfer on the engine. When it lines up correctly it will start pushing into the engine.

Push the sleeve all the way into the engine until it stops. You will need to hold the con rod in place through the base of the engine to make sure the sleeve slides over the piston. Make sure the retaining pin is in the middle of the cutaway in the sleeve.

Once the sleeve is fully fitted, check the piston is sliding freely inside it.

Refitting the cooling head

The cooling head contains a thin metal shim inside the combustion chamber. it should be in good condition and replaced if not.

The head is fitted to the engine using six M3 bolts. They should be tightened down in opposite pairs to ensure a flush fitting to the engine. Position the head so that the narrower sides are fitted to the side of the engine

Refitting the baseplate

The baseplate has a cutaway which should sit facing the head of the engine to allow the piston to move through its full range of travel.

It has a plastic shim that sits between the engine body and the lip of the baseplate. The shim should be in good condition (not split or overly compressed) and should be replaced if not. The baseplate is fitted using four M3 bolts which should be tightened down on opposite corners to ensure it is flat and level until fully tight.