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Discussion Starter · #1 ·
Getting ready for the upcoming sledding season. 4200 Northern Wisconsin trail miles. She's slowly developed knocking noise - sounds like piston slap. Starts and runs flawlessly. Compression unknown at this point.

Take a listen. Would you do a top end at this time or wait another season? Or might this be something else?

https://photos.app.goo.gl/9CpooSq2Rp8tThdr7

Ron
 

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That sounds like a big end connecting rod bearing. Take the clutch off and see what it sounds like then. In my experience you can’t really hear piston slap or detonation on a 2 stroke....they make a lot of noise. You might hear a piston with a chunk out of it, but by the time you have enough piston clearance to make that sound, it has grenaded.
 

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Thanks for the video. It normally really helps, but in this case I'm not sure what I'm hearing other than being something that isn't right.
 

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It's a 16 year old 800 with 4000 miles. You have used up most of this engine's life. I don't think I would just do a top end if the compression was low. I don't think many went much past 5-6k miles without being opened up for repairs.
Do you have a dial indicator? Check crank run out, see how wobbly it is.

Remember your core is worthless if the crank pokes thru the side.
 

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It's a 16 year old 800 with 4000 miles. You have used up most of this engine's life. I don't think I would just do a top end if the compression was low. I don't think many went much past 5-6k miles without being opened up for repairs.
Do you have a dial indicator? Check crank run out, see how wobbly it is.

Remember your core is worthless if the crank pokes thru the side.
6420kms I don't think most of its life is gone. Just need to check everything out that's all. Mine had 3000 more kms and was Mint. Engine was perfect. All depends on how people treat and maintain them.

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6420kms I don't think most of its life is gone. Just need to check everything out that's all. Mine had 3000 more kms and was Mint. Engine was perfect. All depends on how people treat and maintain them.
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You are correct, a few may run past that mileage untouched. But if I got to 10000km on an 800HO I would consider myself lucky. I have several friends that owned them. Only one went past 5000 miles, he was lucky, did the same maintenance as the rest of us.
My neighbor got real good at replacing cranks.
They are the reason I stuck with the 600. Until now.
 

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You can get the 800 to last as long as you keep iso in the pto and mag bearings my 800 has 13000 granted it’s been rebuilt but since I re grease every other year I got way more than 5000 miles. McB pistons and crank still going strong it’s now my back up sled since I have a 850. My run out sucks too????????.but with 4000 miles and you never greased the crank I wouldn’t just do the top end. Honestly if it’s me and I have the engine out I always split the case. If the sled is making noise it’s a no brainer to pull it all apart
 

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Compression is 113 and 115 psi, primary and mag side, respectively.

Ron
Yep time to pull it apart. That's low for an 800. And you will start to have issues very soon.

While it's out check the bottom end regrease with iso. Be a new ride soon.

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Do you have another 800HO to compare with, or even a 600HO? A hot vs cold test will show obvious problems.
 

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I have an 07 800x P-TEC with 7000 miles and compression is down to 132 per side.It used to be 150 per.

I have greased the PTO and MAG with ISO. Its slightly down on power compared to what it was but still

more than enough for me. Im pretty mechanical but I have a shop if needed that does tons of these nd they

told me not to bother if doing just the top end because if u tighten up the top the bottom will surely go rite after.

They said just drive it and do the whole motor eventually.The rings that are currently in the sled will have molded to the current wear

of the cylinders, changing rings and or pistons may be pointless but a dial bore gage measurement on the cylinders

is a definite doo to access before spending money on new top end .U may end up needing cylinders as they could be

egg shaped.Pretty typical on any 2 smoke engine especially an 800 (lots of heat and stress) Ive rebuild my share of

Doo,s 600 and 800 and this is what I have found. The good new is the compression is fairly close per side.Did u hold the throttle wide open

when cranking it over. Start it from cold, run for 30 seconds and test.Then run it for 15 minutes and test again.let us know the readings.

This will give us more to work with for a better assessment.
 

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Discussion Starter · #15 ·
All - ended having Big John at Sled Head Racing do complete rebuild. This guy is the best and knows his shi*t. Broke it in per his instructions and the old girl runs like a raped ape. Love the old Rev!
 
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