OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Trimmers and Brushcutters Ryobi Trimmers & Brushcutters Ryobi/Homelite piston ring replacement

I have stripped down a Ryobi/Homelite RY70133 (aka PLT 3043) 30cc trimmer.

Coming to reassembly, I don't know which way around to orient the piston with regard to the exhaust port. Conventionally, a piston would have an arrow mark on the head face, which points to the exhaust port. On this model however, there is no arrow - only a part number and 'B' stamped into the piston head face.

The other clue I hoped for was by looking at the location of the retaining pins in the piston ring grooves. These are offset by 28mm circumferentially, but both are on one side, i.e. the pins must straddle either the intake port or the exhaust port. The exhaust port is much wider than the intake, and measures 21.7mm wide (22mm circumferentially). That means there is clearance on either side of both the exhaust and the intake port for the piston ring gaps to sit without snagging. So, I have no clue which port the pins would face, because on this basis they could straddle either port.

Anyone know the proper answer?

Thanks
Ian from Australia

PS: this is my first post, so hi everyone, and special thanks to the dedicated efforts of the moderators.

I assembled the trimmer with the two piston ring pins straddling the intake port. I reasoned that there was more clearance between the pins and the port on that side than on the exhaust side, so a greater margin of safety, and so more likely to have been the original engineering design decision.

I have used the trimmer to cut grass for about 20 minutes. Engine is running fine. So I probably guessed right.

I think it would run for a lot longer than that without noticeable troubles even if you guessed wrong Ian, but nevertheless what you did makes sense and is probably right. Sometimes the piston skirt is cut away on the intake port side and it is all easy, but if it isn't, it can be difficult to tell. The reason it matters is that the piston pin (gudgeon pin) is usually offset because there is much more pressure on the piston crown when it is on the way down than when it is on the way up. Consequently you offset the pin, or sometimes the cylinder itself, to reduce the connecting rod angle for the downward stroke. This alleviates the bearing pressure on the side of the piston which is being pressed against the cylinder wall on the power stroke. Of course the offset increases the pressure on the opposite side of the piston during the compression stroke, but this is a much smaller load and hence is not a real issue. If you put the piston in backwards, you will be magnifying the load on the piston thrust face during the power stroke, rather than reducing it.

Well, now that you mention it, looking more closely now at the spare piston I have (I had bought two junked RY70113 trimmers from the tip shop), one skirt is indeed about 3mm shorter than the other. I hadn't noticed that before. Happily for me, it is the side that has the location pins for the piston rings, which I had put on the intake side.

Interesting that, measuring rather crudely with my micrometer, the gudgeon pin does appear to be offset toward the intake (short skirt) side by around 1 or 2mm. So as you say the rod would tend to be more vertical on the downstroke. So the reason the skirt is longer on the exhaust side is that (despite the slight gudgeon offset) there is still some lateral pressure on the lower part of the piston against the cylinder wall on the exhaust side, i.e. there is a tendency for the piston to want to tilt over crown toward the intake and tail toward the exhaust, on the downstroke, due to the angle that still remains on the piston rod, especially at mid-stroke. Right?

Mighty interesting. It certainly helps to have a cylinder and piston in hand while thinking about this. I'm happy that I started my learning with these junk trimmers, I am amazed at what I'm learning. Thanks heaps.

There is pressure on both sides of the piston at different stages of the engine's cycle. During the power stroke, high gas pressure on the piston crown combines with the angle of the connecting rod to the cylinder, to produce a high sideways force on the piston. The side of the piston that is exposed to this force is called the "major thrust face", and the opposite side, which sees only the much lower sideways force caused by compression pressure, is called the "minor thrust face". If the pressure in the cylinder were constant through the power stroke, as it is with a steam engine, the thrust force, both minor and major, would be greatest when the angle between the connecting rod and the cylinder axis was greatest. This is the point where the centerline of the connecting rod is exactly at right angles to the line from the center of the crankshaft to the center of the crankpin. However in internal combustion engines the pressure varies widely during both power stroke and compression stroke. Thus the maximum thrust is at a point somewhere between when the pressure is highest, and when the connecting rod angle is greatest.