OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Victa Two Stroke Engines Fitting piston rings

Hi all, this may sound like a stupid question but what is the best method to fit the piston rings onto a new piston?

it would seem that need to be crimped by some tool to get them to lay down in the groove correctly before i put them into the cylindar??

any help would be great

First, putting the rings on the piston can be tricky, since the compression rings are cast iron and if you just pull them apart to expand them, they may break. There is a tool called a ring expander to eliminate this risk, but while it is probably essential for deep, high-tension rings like those on many diesels, it isn't really needed for a lawnmower engine. Just remember to pull them gently wider at the gap, not halfway across the ring, so the bending deflection will be somewhat distributed around the ring's circumference. Expand them barely enough to get them over the piston, and treat the cast iron as glass: it isn't much more flexible than that.

Second, having put the rings on the piston, you have to compress the rings to get them into the bore. With anything larger than a lawnmower engine this calls for a ring compressor, but for the low-tension rings lawnmowers use, some people just compress the rings one at a time with their fingers. Most of us regard that as hard work, and prefer to use an ordinary ring compressor you can get from a speed-shop. Remember though, if you are working on an engine that has big end bearings that are not easily dismantled (that is, a one-piece connecting rod) you have to put the piston in from the bottom of the bore. That means the ring compressor ends up looped around the connecting rod after the piston is in the bore. That is OK if the ring compressor can be opened up to make a gap in its circumference, but most of them can't. For one-piece-rod engines, you usually use a piece of sheet aluminium with a twitch of tie wire around its outside, as a ring compressor. When the piston is in the bore successfully, you cut the tie wire and open up a gap by partially unwrapping the sheet of aluminium from its circular shape.

g'day, i use my fingers to compress the rings as grumpy suggested. just compress the ring until the gap closes up and give the piston a gentle bump or two with the handle of a hammer. also give the cylinder end a run over with a file to give a slight chamfer to make it easier for the rings to slide in.

hope this helps
matt

P.S with the old rings i use a thickish "feeler" gauge to get the rings off. Its hard to explain the technique, it involves getting the gauge under one end of the ring and working it around the piston and at the same time gently pushing up on the ring. Confused? me too.

cheers
matt

When removing rings, you usually don't want to break them, just in case you have further use for them. This means lifting them out of the groove gently, starting at one end. You lift one of the free ends so it is barely out of the groove, and push it just slightly toward the crown of the piston, as you try to continue to lift further around the ring's circumference. You need to get the ring into a helical shape, and slide it up the piston while keeping it helical. If you don't, it will drop into each ring groove in turn and you'll have to lift it out over and over again, which as a way to spend your time, gets old pretty quickly (especially if its a V8 rather than a single cylinder). So, you lift the end, slip a thin steel shim under the lifted end, and work the shim progressively around the ring as you push more of it toward the piston crown in that helical shape. Be sure the "lead" of the helix is as small as possible, or you may permanently twist the ring and ruin its sealing performance.

Thanks everyone. I slid the piston and conrod in from the head side and gently clamped the rings one by ony with my fingers until the locked into the chamber all done and working fine now!

Power torque Warning


When putting new rings in a power torque its a good idea to just put the lower ring on the piston and get it in the cylinder then slip the top ring on and get it in the cylinder.


With both rings on the piston, while holding the lower ring with your fingers and tapping the piston itno the bore the upper ring being out of its groove and really really sharp can take bleeding big chunks out of the fingers you are using to hold the lower ring into the piston with.

I not so much of an issue with a full crank as it has more of a shallow taper on the bore to lead the rings into the bore.

I managed to put them in however i know have another problem

once the new piston and rings were in, the motor started after about 10 pulls on the cord and ran for a while until the head gasket blew (new gasket).. I replaced the gasket again tried to start it but it really takes about 10 pulls to start, although once it does it runs really well and it obviously has good compression.

Hot/warm start is really good, but if i leave overnight it takes agaes to start again.. could it be fueling issues?

First, it sounds as if you didn't get the head gasket installed correctly the first time. Typical problems are uneven head bolt torques, or a bit of crud under the new gasket when you installed it. However if it hasn't blown again it sounds like it is fine this time.

Hard cold starting but easy hot starting is usually due to the choke or primer not operating correctly. If I recall correctly in this case you are talking about a Powertorque with an evil plastic carburetor. I don't do those. However, did you prime it correctly before you tried to start it?

first what model engine and carby set up. Second try this give it one pull and pull the spark plug out and check if it is drawing fuel through. Have you set the armature gap to the flywheel. Have you checked the spark plug for good spark. If you are pulling and it gets fuel and spark and wont start try removing the exhaust and see if it starts. If it does it is not breathing properly. Have you checked and set the valve clearance.
leshby

COrrect it is a VSX160 PowerTorque. I have fitted new primer cap and O ring also, does not seem to help much.. I primed it about 5 times although does not seem to help much..

It sounds as if the primer may not be working. You could check by putting a spoonful of 2 stroke fuel directly into the cylinder through the spark plug hole, then don't prime it, and see if it fires from cold. If it does, your primer is not working as it should.

thanks Grumpy! I am going to check with another primer and will also try and adjust the throttle cable to see if that makes any difference.

Has the primer face got the right needle in it.

Variations in manufacutring tollerences mean theres a few different length needles to get the fuel height right.
With the primer face in your hand assempled tip it so that the needle closes and check that the float is sitting right, generally the relief in the float should be concentric to the outer circle in the face for the post the main jet screws into. If its too long the lower edge of the float will barely be in past the outer edge of the face. If its too short the float will come in as far as the hole for the jet.
Dont roll the o-ring on. Pinch it with your thumb at 6 oclock and fore finger at 1 oclock and then stretch it on the rest of the way around.

The head could be warped. Try it on the block with no gasket and see if it sits flat, if its warped it will rock.


I assume the motor is not over revving? Are the o-rings on the inlet manifold OK? I have seen split mainfolds as well.

I fited the old primer cap back on and it worked a treat! this is the second time that i have purchased a brand new primer cap and needle and they DO NOT work properly..

many thanks to everyone for their input!

Thanks for reporting back, luvpsi. I'll close this thread: if anyone wants to add another post to it, just PM a moderator.