Piston Seized in Barrel

[parrbd]

Maybe if you block all the ports with plasticine then fill the cylinder with hydrochloric acid. You would need to keep your skin and eyes safe.

[Andy_C]

Reading through this topic, it looks like the piston will be shot, so little poiint in trying to salvage it - a new one is only around £30 max and you will most likely need a rebore and new piston anyway.

I would be inclined to get working with a drill and chain drill the piston crown, then join up the chain with a small chisel - aim to leave the centre of the piston intact so that the top of the con rod will not be damaged - aim to be able to pull the outer part of the piston away from the centre, hopefully you get the picture.

This will leave you with part of the piston still seized in the cylinder which you can then put onto your workbench and get to work on with a light chisel / release agents / heat etc, and you wil still have the centre of the piston attatched (snd possibly seized) into the small end.

If you do this I would be inclined to stuff the crankcase with rags to sop the swarf getting into the cases - and support the cylinder during the "operation".

Either way I would think you will need a new piston / rings and rebore.

Good luck.

[Puffs]

Yes, it will certainly benefit from a re-bore & oversize piston, and possibly (probably) a new big-end & main bearings; yet in general best avoid metal filings & other alien stuff to enter the engine.

But before you go drill holes in your piston (as now suggested twice), best first check how & at what location the piston pin attaches to the piston in this model. If it is like in the KTM 500 2T, with 2 wide bars coming from the piston crown down to the pin, then you could free the barrel by drilling holes like that. In principle, after sufficient time, and with risk of ruining the barrel. But in these smaller bore engines the pin usually attaches to the sides of the piston.

The key issue is: Why/where is it stuck?

[Andy_C]

Puffs, you are absolutley right - why is the piston seized so solidly?

I have had seized pistons in the past and usually the application of diesel or penetrating oil gets things moving.

At then end of the day the piston and rings are probably knackered so I would look at getting the piston apart in some fashion by drilling / chiselling - just try and avoid damaging the cylinder.

[Puffs]

I know of the following mechanisms (and there may be more):
1) Piston seized in running engine due to insufficient clearance. The piston became warm (possibly due to ignition and/or jetting, or just too little clearance), so that it expanded, leading to more friction, hence more heat & more expansion, and then: KA-CHUCK. Classic seizure. You can hear it coming. Normally when cooled down piston is free, but there is some damage (maybe you can get away with just sanding that location, which is the highest point, with course sandpaper).
2) Something similar can happen with the rings, if they have insufficient end-gap (rings get much hotter). This may give more damage, but in a 4-stroke usually the piston remains free afterwards.
3) Particularly in a 2-stroke, the hot rings can expand into any of the ports (except the inlet), brake off and become embedded in the piston (when that is pushed upwards / pulled downwards due to the running crankshaft). Then the piston can be a bit stuck, even after cooling down, but some mild persuasion usually suffices. Diesel or WD40 does not really help, but maybe a bit. Re-bore probably required, new piston & rings certainly.
4) Piston rusted in-place due to years of disuse in a humid place. Given enough time & humid conditions, a steel cylinder wall (either a sleeve, or a cast iron barrel) will rust, and thereby expand (a bit). Iron oxide has a bigger volume than just iron; it has absorbed oxygen, and the metal lattice has broken down & is blistered. Similarly the rings will rust. This I wouldn't call 'seized', but rather 'stuck'. Here is where WD40/diesel could really help, but frankly I have never had this myself. In this case, you're likely to need a re-bore, as the cylinder wall will be unevenly pitted due to corrosion. More importantly, if the wall/rings are corroded that way, why should the lot below not be? So expect the big end & main bearings to be similar, and likely a full overhaul is required, including a new conrod. And then you'll have a new engine.

[dirtsurfer]

In the post mortem examination of my 250 with a similar dissassembly issue, I concluded that the bike experienced a seizure similar to number 1 in Puffs post above , however in over a decade in custody of a luddite previous owner, water was allowed to infiltrate to the crankcase causing the crank and main bearings to rust. So a big part of the difficulty I had getting mine out was that the big end / crank were damaged as well . A frozen crank is not conductive to a freely moving piston. I was amazed by both : how solidly the piston was stuck in the cylinder and when eventually removed; how little damage there was to the bore and piston. Even the rings showed only a section of about 15 mm where they had fused to the piston.
Is it possible that your piston wont move because its got nowhere to go?

[Puffs]

Yes. You can first have it seize, and then put it out in the yard to rust for 20 years before you pass it on to an MZochist...

(but without kidding: I wouldn't mind having your SMZ!)

Pistons are tapered: the top part (near the crown) is slightly narrower than the bottom part (skirt = where you measure the clearance), this is because the top part is exposed to the compressing & exploding gasses, so it gets hotter and expands more (while by contrast the skirts get cooled by fuel droplets evaporating & depositing oil). But if the crown gets too hot it expands too much, and then it can seize there, above or in-between the rings. If that happens on a compression stroke, the piston can deform a little & seize the rings. Carbon deposits can help that too, and the other cause can be too much clearance, causing the piston to tilt leading to point-loading & local friction heating of the piston. If you put back a used piston, always clear the ring grooves with a piece of an old ring & check for sufficient up-down clearance around the new rings.

How much taper & how much clearance depends on how it operates in the engine (low power cruising / high power racing), but crucially also on the alloy used for the piston (both expansion, friction & wear properties). This is why some piston manufacturers can ask a much higher price than others.