Ski-Doo Snowmobiles Forum banner
1 - 10 of 10 Posts

·
Registered
Joined
·
5 Posts
So I went to fire up my sled one last time before putting it away for the summer. Started fine but I noticed it smelled funny and I have read about antifreeze leaking into the motor. I took off the pipe and sure enough a little puddle came running out. The sled has 3800ish miles on it. I believe the antifreeze is from leaking seals around the piston, easy enough to change! I started checking out the pistons which have a nice dark brown color to them and have also read about the piston rock you get with the single ring pistons. I was able to get quite a bit of movement front to back out of the pistons by moving them with my had. Just wanted to find out what the proper tolerance should be on this. It doesn't seam very good for it but it still runs like a champ would hate to sink the money into a piston kit or worse have to go get the cylinders honed back to a true circle if the skirts grooved the wall at all. I have a feeler gauge to slide in and check it with if that will work?

Thanks
 

·
Registered
Joined
·
252 Posts
So I went to fire up my sled one last time before putting it away for the summer. Started fine but I noticed it smelled funny and I have read about antifreeze leaking into the motor. I took off the pipe and sure enough a little puddle came running out. The sled has 3800ish miles on it. I believe the antifreeze is from leaking seals around the piston, easy enough to change! I started checking out the pistons which have a nice dark brown color to them and have also read about the piston rock you get with the single ring pistons. I was able to get quite a bit of movement front to back out of the pistons by moving them with my had. Just wanted to find out what the proper tolerance should be on this. It doesn't seam very good for it but it still runs like a champ would hate to sink the money into a piston kit or worse have to go get the cylinders honed back to a true circle if the skirts grooved the wall at all. I have a feeler gauge to slide in and check it with if that will work?

Thanks
Piston to wall clearance should be .005" all the way around.
Remember that the piston swells as it is heated up. If it was tight in the bore when cold it would swell and then seize in the cylinder.
 

·
Registered
Joined
·
9,622 Posts
the top of piston is made smaller diameter than the lower section of the skirt were you measure the piston size
 

·
Registered
Joined
·
14,289 Posts
A couple of things. What has been stated is pretty much spot on. Guys that tell you that twin or dual rings stabilize a piston don't know what they are talking about. Doesn't matter if the piston has one ring or two, it will still rock in the bore when cold before the piston expands. The piston has clearance to the ring/s.

The only way you can tell if you have excess clearance is to pull the cylinders, mic the pistons and use a dial bore gauge on the cylinders and compare the difference. If you take the rings off, you could do a crude piston to wall clearance check, but there is a specific point on the piston that should be measured and compared to the cylinders.

Pistons do have a habit on a two cycle of having the skirts collapse, which will allow excess rocking in the bore and eventual failure of the skirt from rattling in the bore. That can be told by someone micing just the piston and comparing it to the spec.

Most 2 cycle high performance guys start thinking about replacing pistons sometime after 3500 miles due to skirt collapse or not knowing how much detonation the piston has seen. Detonation is what pounds the ring anti rotation pins out of the ring groove .. which allows the rings to rotate and catch a port.

Did the engine rattle when it was running at low speeds? This is a good indication of skirts being collapsed.
If not, you probably have normal clearance and the rocking is normal for a cold piston in the bore.

Another thing, the bores are notoriously hard, the nicasil plated cylinders do not normaly wear. Usually you just scrub them with scotch brite with soap and water and rinse and dry and they are good to put new pistons and rings in. New Pistons and rings are cheap insurance of 2 cycle engine life. They are considered a wear item and should be replaced periodically, much more often then a 4 cycle piston which has only one power stroke for every two revolutions where as the 2 cycle fires every rotation of the crank. No time for parts to cool off. Two the 2 cycle runs at much higher engine speed... probably lives at an average rpm of 6000 rpm. Where most 4 cycles only occasionally touch that kind of rpm.

Did the moisture you got out of the pipe or port smell like antifreeze or taste like it? If not, it was probably water. For every gallon of gas that is burnt in an engine, the by products of combustion is either a quart or a pint of water. I don't remember which number. And you typically see it coming out of the exhaust when you fire the engine up and don't run it long enough to get the exhast system hot enough to vaporize the moisture. Not saying you don't have a o ring that is leaking but that is very unusual. Unless it was pinched when installing the head.
 

·
Registered
Joined
·
5 Posts
Discussion Starter · #6 ·
I've worked on my fair share of dirt bikes and sleds, 2 and 4 stroke, I've just never had this much play forward and back, and it doesn't do it side to side. As far as the coolant in the exhaust goes, I guessed it was the round seal as suggested that separates the coolant from the combustion chamber. I removed the head to check for a wrecked seal but no luck both were in good shape. might try a slightly larger seal I'll grab from work tomorrow. Is their a way for the base gasket on the bottom of the jugs to leak coolant into the engine and some how make it out the exhaust? That's a pretty wild stretch but I have seen weirder things.
 

·
Registered
Joined
·
9,622 Posts
the o-ring could look ok but the head surface could be warped and draw coolant in combustion chamber,were it can/will damage the piston
 

·
Registered
Joined
·
14,289 Posts
I've worked on my fair share of dirt bikes and sleds, 2 and 4 stroke, I've just never had this much play forward and back, and it doesn't do it side to side. As far as the coolant in the exhaust goes, I guessed it was the round seal as suggested that separates the coolant from the combustion chamber. I removed the head to check for a wrecked seal but no luck both were in good shape. might try a slightly larger seal I'll grab from work tomorrow. Is their a way for the base gasket on the bottom of the jugs to leak coolant into the engine and some how make it out the exhaust? That's a pretty wild stretch but I have seen weirder things.
The pistons can't rock side to side, since the wrist pin and rod resists the rocking motion going side to side. Where as going fore aft, the piston pivots on the wrist pin so it is free to rock.

I would check the head with a straight edge to see if it is flat, if it is, I would put new pistons in and new o rings and put it back together. Then you don't have to worry about the clearances, you can have the new pistons mic'ed and the cylinders dial bore gauged and know what you have. Won't have to worry about how much detonation the engine experienced prior to your buying it, how much bad gas was run thru it... and how collapsed the skirts are. The other approach is the pull the cylinders and dial bore gauge them, mic the old pistons and see what the skirts look like. But if it were me, while you have it apart, it is cheap enough to put two pistons and rings in and not worry about the anti rotation pins or the skirts.

I have had friends who lost 600HO engines due to broken skirts, with as few as 1800 miles on one of them. Think the other went about 2400. That said, the 600HO is probably doo's second most reliable engine to the 500ss engine.

Once I got the engine back together, I would do a pressure decay/internal leak test on it. Must hold 5 psig for 3.5 to 5 minutes. I would also pressure test the coolant system to. To see if it hold's glycol.

Theoretically the base gasket should not leak any coolant, just air and lean the engine out.
 

·
Premium Member
Joined
·
11,990 Posts
Theoretically the base gasket should not leak any coolant, just air and lean the engine out.
But the base gasket CAN leak coolant, its just more common that they leak air. I too would replace the pistons, I was a victim of a collaped skirt that broke myslef. Put the PTO rod thru the block and ended my winter. It was still under warranty and engines were back ordered.
 
1 - 10 of 10 Posts
Top