34 replies to this topic

[Wellyaabuddyy]

Hi guys, I had a piston burn down on my 2007 MXZ500ss with 1700miles about a month and a half ago due to a cracked boot. There was a piece missing out of the one piston as it was showing 35psi and 138psi in the other.. My plugs weren't white but I assume it had a lean burn down (I have a sno-stuff stinger and never jetted it).

ANYWAYS, I took the motor out and brought my jugs to Dick Edworthy in Northbay Ontario to be bored 0.020 and get two 0.020over spx pistons and put 2 size bigger main jets and turned down my pilots a quarter turn and put stock pipe back on; I finally finished putting it all back together today and it started 2nd crank with a 40:1 gas to oil in tank ratio. It idled fine and all but it didn't sound right even if this is my first rebuild.. So I decided to check the compression, 165psi in both!! Called Dick Edworty and he says 15psi over shouldn't matter. But once my pistons warm up, won't they just seize up?

HELP! What do I do..? Run higher octane gas from now on? Help!

[Wellyaabuddyy]

I did not do a squish test or a ring gap test I dont even know how....... Im assuming you need a special tool to do it?

[rblscoot]

We really need more than it doesn't sound right. What does it sound like. Did DE mic the cylinders and pistons while at his shop, check ring end gap while cylinders were there? Pistons won't seize up if DE mic'd everything like he should have. Still curious about the "Doesn't sound right"....

[djm]

I don't know where to start.

Checking ring end gap is pretty much mandatory... You just push the ring down in the cylinder using a piston to keep it square, and check the gap with a feeler gauge. If it is too small, then you fill one end, always in one direct across the ring face front to back. Using the uncut side to keep the gap and ring ends parallel.

Assume you also know that the rings have a top and bottom, often SPI/SPX rings are not marked with a dimple or an X as an indicator and you have to put a straight edge across the top...the top side will be shaped like a bowl.. High on the outside edge low on the inside edge. The bottom is flat.

Next, what base gasket was in the engine. How many holes in the middle? The number of holes indicate the thickness.. although you could measure it. 6 holes indicate 0.6 of a mm thick.
5 hole equal 0.5 of a mm. Usually when building a stock engine, you start with the same gasket.
And there is no standard gasket as Rotax uses the gasket to accomodate the stack up of parts tolerance.

That said, you can drain the coolant, pull the head off, put a single length of solder across the piston over the top of the wrist pin and in line with it (across the midde of the piston in line with the crankshaft. The solder must run to the edge of the cylinder wall on both sides. Then put the head back on with the plugs out.... slowly roll the engine over to compress the ends of the solder. then you pull the head back off and check the thickness of the end with a micrometer or a caliper.

That said, are you telling us you have the oil injection hooked up and you also put 40 to 1 oil in the gas? If so, you could have just too much uncompressible oil in the combustion chamber causing the increased compression. Just a thought! And it could be fouling plugs...

Why too does one up the main jet size from 360's to 380's when the sled has run 1700 miles.. with no issue and burned down because it had bad or leaky carb boots? I could see maybe going up one size.. or better yet, why not just put an extra 0.010 or 0.015 thousandths shim under the needles. These sleds if anything are a bit lean in the mid range. I also might have opened my fuel screws about a quarter turn from stock, but not closed them down.

We will assume piston to wall clearance is right because DE had the new pistons to mike when he bored and honed the cylinders.

[Jimbob SOS]

Did u ever find out where that chunk of piston went? Might wanna make sure it's not in the cases, split em! Anytime pistons fly apart the cases must come apart, unless you are a gambling man and then those odds are horrible. This has nothing to do with your compression





yet!

[djm]

Did u ever find out where that chunk of piston went? Might wanna make sure it's not in the cases, split em! Anytime pistons fly apart the cases must come apart, unless you are a gambling man and then those odds are horrible. This has nothing to do with your compression





yet!

Come on Jim.. he had the motor out of the sled, and took the cylinders off, you got to believe he cleaned the case out.. I usually just squirt some oil down there and wad up some paper towel and push it thru there until I am getting no chips out etc.. I am assuming that the hole in the piston edge is a melted one.. and all of the molten aluminum when out the exhaust port. But without pictures.. assumption is not a good word.

Now if I had a ring turn and catch a port and it busted parts all over.. I would pull the crankcase apart. But that doesn't sound like what happened here.

[mbret2004]

Checking ring end gap is pretty much mandatory... You just push the ring down in the cylinder using a piston to keep it square, and check the gap with a feeler gauge. If it is too small, then you fill one end, always in one direct across the ring face front to back. Using the uncut side to keep the gap and ring ends parallel.

Can you post a pic of the ring ready to be filed?

Tanks!

[DOO_TINT]

whether the ring end gap is .012in or .020in,,it will not make the compression test #s 15psi higher than the normal 150

[Jimbob SOS]

Come on Jim.. he had the motor out of the sled, and took the cylinders off, you got to believe he cleaned the case out.. I usually just squirt some oil down there and wad up some paper towel and push it thru there until I am getting no chips out etc.. I am assuming that the hole in the piston edge is a melted one.. and all of the molten aluminum when out the exhaust port. But without pictures.. assumption is not a good word.

Now if I had a ring turn and catch a port and it busted parts all over.. I would pull the crankcase apart. But that doesn't sound like what happened here.

Yeah I was assuming it was a broken piston and that makes me very nervous as to how many pieces and if I've found them all. I guess I am a little over the top on my maintainence.

[snowfreak56]

Are you shure it is not just your compression gauge reading high ?

[DOO_TINT]

its possible that there is too much oil,if he flooded it with assembly lube? and 40/1 with a working oil pump

[dynoused1]

its possible that there is too much oil,if he flooded it with assembly lube? and 40/1 with a working oil pump

First decent reply to this guys question.

[DOO_TINT]

First decent reply to this guys question.

djm already commented about the excess oil and plug fouling,,and if the rings were installed upside down/engine wouldnt sound too good/but the tapered rings are very hard to compress to locating pin when upside down and i dont think that would raise the compression but who knows?

[djm]

djm already commented about the excess oil and plug fouling,,and if the rings were installed upside down/engine wouldnt sound too good/but the tapered rings are very hard to compress to locating pin when upside down and i dont think that would raise the compression but who knows?

Yep, I would guess excess oil and base gasket too thin... but hard to really tell... without being there and knowing the details. But if it had 40 to 1 oil in the gas... and oil pump was running at 35 to 1.. more then 20 to 1 actual oil in the engine. Might foul and fart, would probably test high on compression.

The other items are just durability items.. do you have enough ring end gap... Usually compression is down around 90 to 110 lbs I believe if the rings are installed upside down.

Also assume that the gauge is the same gauge that previous pistons/rings checked low compression so probably moderately accurate.

[ZXMODV]

djm already commented about the excess oil and plug fouling,,and if the rings were installed upside down/engine wouldnt sound too good/but the tapered rings are very hard to compress to locating pin when upside down and i dont think that would raise the compression but who knows?

Couple of things. If the correct size base gasket and same barrels and head were installed,there is no reason for the compression to be to high.The readings he had from before were from an engine that burned down, and even the cylinder that read 138psi could could be low from that or being worn. Also, if you consider the amount of extra oil in the rings it could raise it to 165 psi,like djm said. The reading of an engine at operating temp is the only viable reading, unless your trying to diagnose warn rings, and then its just a cold to hot comparison, no extra oil.Didn't those engines crank around 145psi? djm? If he isn't the original owner someone may have changed the base gasket or milled parts.He should get a different, known good compression tester as well.And its common when a y piped 2 stroke burns down or a big end rod bearing goes that particles going out the exhaust get blown back into the other cylinder during the stuffer phase of the expansion chamber.One should always examine the chamber for pieces before putting it back on. Those hot particles can get stuck to the carbonized wall of the diversion cone and stuffed back into your fresh motor.