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Discussion Starter · #22 ·
I am dealing with a rebuild that has only 90 psi and I dont know why? It also stalls and dies when I stop after riding it somewhat hard and then try to go again. I didnt get any insight on that and I wish I would have.
 

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That's the fact. Engines barely run with 90 psi of compression. That's why an engine needs tight pistons and rings. That requires measuring by somebody, either the owner or the machine shop. I used to blow a lot of dirt bike pistons in my youth and got to be very fussy about piston clearance. I could break them before they got loose enough to not build compression.

It would be nice if you could just bore out a 550 and put new pistons in like the past but I'm sure it's probably designed so you have to buy new cylinders.
 

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No you don't have to buy new cylinders either re-plate them or you can bore them out. US Chrome and Indy Specialties do those all the time. As does any company around my area that does engine work like that. Main thing is you send the pistons/rings in or buy them from them directly and they will match it all up perfectly so when you put it back together there is no guess work. Both companies have a great rep for that stuff for a reason.
 

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On a Nikasil coated cylinder, if you bore the plating off, you have to put it back on. That's the part most machine shops cannot do. If you bore off the (very thin!) plating, the aluminum underneath won't stand up to the force of combustion for very long and the rings will tear up the cylinder really quickly.
 

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I'm not sure what year but Polaris went with Nikasil in the 550 and had a TON of problems after... I know it was in the Edge model... If it's sleeved, you should be able to see a fairly thick steel sleeve when the head is off. If it's coated, the coating is a LOT thinner.
 

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Silly question, but did you put the rings on right-side-up? They are beveled and upside down will exhibit exactly what you are seeing.
 

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SummitSpeedFly
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Hi. I need some help. I have a 2002 Polaris trail RMK 550. The manual says spec compression is 120 psi. Its fan cooled. I did a rebuild, new crank, new pistons, new rings. I gapped the rings and the pistons were the right size. The cylinders were honed. I reassembled used oil and broke it in but I am only getting 90 psi compression. I have used 3 different testers and tried them on another older sled that read 110 psi. The sled seems to run ok, start ok, except when it is warm and I stop for a few minutes and just let the sled idle and then when I push the throttle to go it bogs down and dies. I have to let it sit a few minutes and then pull it several times before it will start. Is this because I have added oil to the fuel for the break in or is it something else? Also should I have a good shop machine down the head cover or should I have them machine down the cylinder housing? Is it better to do the head cover or the cylinder housing? Am I stupid to do either one? Thanks for any help.
Well, my 1st question is what altitude are you at? I live above 9,000' and after a new top end, I only get 90 PSI cold and 100 PSI hot. You do have to compensate for the altitude you're testing compression at. Machine runs fine at 90.
 

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Discussion Starter · #40 ·
I did not know this. I ride mostly at about 6000+ ft though. Machine seems to run fine but when I stop for a few minutes and then try to go again it stalls and dies and I have to wait awhile before it starts. That might be due to the extra oil I added to the gas I dont know. What do you think? Would 5500 ft be high enough? None of the shop guys around have said anything about this and other sleds seem to be at 110 and 120. I put in new plugs and they are in pretty tight due to a leak down test I did. Should I take the washer out?
 
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