Destoying Pistons - Snowmobile World : Your #1 Snowmobile Forum
 
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post #1 of 10 (permalink) Old 01-06-2007, 11:47 PM Thread Starter
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I've got a 1998 670 HO that I bought in pieces. I bought a Skidoo shop manual and put it back together. I put 30 miles on it then it started to knock. A piece of the top of the PTO side piston ring land had broken off and so had the ring for an inch on the intake side of the piston. I took it to a rebuilder and he honed the cylinders and supplied me two new SPI pistons. I put these in and after another 30 miles the same thing had happened to the PTO side piston again. This time, however, the piston was damaged on the mag side of it - identical damage. Top ring land broken for an inch or so and the top ring is ripped off. Both times there has been no damage to speak of to the pot.

Any ideas on this one?

As I was writing this I started wondering if the pot alignment is off will the piston bang into the head or might a bad lower rod bearing cause this too?

Thanks.

Woodler
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post #2 of 10 (permalink) Old 01-07-2007, 10:04 AM
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Any chance you could post a couple pics of the pistons?

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post #3 of 10 (permalink) Old 01-07-2007, 01:46 PM Thread Starter
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[attachment=32254:OEM.JPG]

This pic is of the OEM piston from the PTO side that the top crown ripped off of the intake side. No idea what caused it and it did no damage at all to the pot. Honed both pots and installed new SPI T-Moly pistons on both sides.

[attachment=32255:SPI.JPG]

This is of the SPI on the PTO side that shows signs of seizing on the exhaust side - in the light of day. I guess it's time for better lighting in the garage. Looking at the pot - there is scoring in it above the exhaust port and a ring imprint across the mag side of the liner.

The scoring on the piston and pot leads me to believe that the base gasket on the PTO side pot leaked and leaned it out. According to the instructions from the SPI piston box this is what they describe - is it right? All through this the mag side Pot and piston have worked perfectly both with the OEM and SPI pistons.

I'm going to order a new pair of wiseco +0.020 pistons (because apparently you can't get 670 HO +0.020 SPI T-Moly Pistons) and have the pots bored, buy a new medium thick base gasket and give it another go. Any suggestions to go with this plan?

Thanks.

Woodler
Attached Images
File Type: jpg OEM.JPG (46.9 KB, 44 views)
File Type: jpg SPI.JPG (39.0 KB, 37 views)
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post #4 of 10 (permalink) Old 01-08-2007, 04:21 PM
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Quote:
[attachment=32254:OEM.JPG]

This pic is of the OEM piston from the PTO side that the top crown ripped off of the intake side. No idea what caused it and it did no damage at all to the pot. Honed both pots and installed new SPI T-Moly pistons on both sides.

[attachment=32255:SPI.JPG]

This is of the SPI on the PTO side that shows signs of seizing on the exhaust side - in the light of day. I guess it's time for better lighting in the garage. Looking at the pot - there is scoring in it above the exhaust port and a ring imprint across the mag side of the liner.

The scoring on the piston and pot leads me to believe that the base gasket on the PTO side pot leaked and leaned it out. According to the instructions from the SPI piston box this is what they describe - is it right? All through this the mag side Pot and piston have worked perfectly both with the OEM and SPI pistons.

I'm going to order a new pair of wiseco +0.020 pistons (because apparently you can't get 670 HO +0.020 SPI T-Moly Pistons) and have the pots bored, buy a new medium thick base gasket and give it another go. Any suggestions to go with this plan?

Thanks.

Woodler[/b]
I would have shop check the cylinders to see if they have a bad port job on them and the ring is bulging into the exhast port and taking out the ring

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post #5 of 10 (permalink) Old 01-09-2007, 03:10 PM
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Any damage on the RAVE valves?
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post #6 of 10 (permalink) Old 01-09-2007, 07:53 PM
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wow...thats so heinous i could bearly look... summit 800 might be on to something. what was your squish and compression like?? any possibillity of suckin the tank dry and pickin up water? i can see why your focusing on the x haust side but maybe it wasnt necesarily a x side problem. could of it been running lean... that would at this point be a possible only contributing factor. as far as bearings go..you shold be able to tell if theres up and down play in the rod assembly, i dont think thats a likely cause. hmmnnn....a scenario that would fit would be a exauhst restriction..it would even lead to detonation first blowing the top of the piston apart possibly at the exhaust ring land and then grenading the rest of the piston.... that could be tied into rave valves not opening..though a little unlikely... look at the pipe. damn. i m gonna have nightmares tonight.
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post #7 of 10 (permalink) Old 01-11-2007, 07:08 PM Thread Starter
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The piston to head clearance was just over 0.020". The compression was good but I never measured it - I was at the inlaws and he's not overly mechanical. The tank never got lower than 1/4 either. I checked the rod bearings and there ins't any up and down play so I don't think that was it. As for the exhaust restriction - the ports and RAVE's are clean and move and the y-pipe is in good condition.

I'll have to pull the RAVE valves and clean them once I get the pots back from the machine shop. Thanks for the advice and ideas.

Woodler
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post #8 of 10 (permalink) Old 01-11-2007, 07:19 PM
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.020??? thats real, real tight. are you sure? normally a 670 motor thats set up tight should be no more than .060. 670`s in general are touchy in that area...most folks like to set them up around .065 for trail riding and .050 to .055 for drag racing 500` at a time with good fuel.
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post #9 of 10 (permalink) Old 01-11-2007, 08:27 PM
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Since you said that the engine or sled was a 1998 670 HO which never existed am I to assume that the motor is from a 98 - 99 Summit X 670 or a 1999 MXZ 670 HO. In both cases the head design is a "Pent roof" chamber (ring shaped protrusion around the spark plug). Check the outside of the cylinder head for an "X" mark in between the spark plus just ahead of the thermostat housing. No "X" means you probably have the wrong head for the HO pistons (the squish band angle is wrong). The regular 670 has pop up domed pistons and the HO has near flat tops Mix them up and what you have happens.

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post #10 of 10 (permalink) Old 01-12-2007, 08:07 PM Thread Starter
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Thanks, Thumb Doctor for the info. I'll check to make sure I've got the right head. I'm pretty sure it's an MXZ HO engine out of a 99 put into the 98 body. The guy I got it from had it for playing in the hills in Southern Alberta. It had been converted to pre-mix with no t-stat or expansion tank but I've replaced all the original parts - this is the last problem I've got.

The non-dealer affiliated shop I was at assured me that the domed pistons would work in this engine and gave me the thick base gasket to offset the higher dome of the piston. I've ordered the right pistons this time and will check the squish and make sure it's according to specs as per the manual, this time.
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