Hi Gang!

I felt it was time for a new thread as my previous no carb vacuum problem has been cured, but now I have a top end rebuild giving me trouble.

As from the previous thread I had a burned up top end--my fault completely as I forgot the oil line on the carb during a test ride.

Let me start by stating I have never attempted a top end rebuild before.

Upon initial tear down I found the rings worn and piston/cylinder damage. Bottom end looked good with no noticeable metal, no bearing play, etc...

I ordered a Niche cylinder kit off eBay. I was hoping for a quality kit...it may have been a China kit.

The kit showed up a few days later, the video on the eBay auction recommended but did not require chamfering of the ports. I watch a couple youtube videos and felt I could chamfer the ports. That seemed to go really well. Then I washed with dish soap the cylinder very thoroughly. I left it to dry completely overnight and the next day I reassembled the bike. All seemed to go well on reassembly except the assembled piston and rings seemed a bit tight, but never having done a top end I assumed it was OK. Then I started the bike up. I was sure the oil line was installed, also for good measure I mixed up a bit of pre-mix in the tank to supplement the OEM oiler during break in.

The bike started and ran great! Following advice given I used a gently break in procedure and let the bike run 1 minute at idle, shut down, cool completely and re-check head torque. I repeated this 3 more times using small throttle blips at each warm up cycle. At the second cycle the bike stalled upon return to idle. I assumed this was a carb/fuel issue. The bike restarted easily. While cooling I re-checked the compression it was 90psi. I thought it was a bit low, but thought maybe the rings were not seated completely. I went through the remaining two heat cycles.

Then I went for an easy lap around the yard using no more than 1/4 throttle. Within 5 minutes the bike stalled and would not restart. Compression check showed 40psi. Quite frustrated now I was thinking of taking it to a shop to fix. A few on the board here talked me out of that idea.

Today I tore it down and found all looked pretty good except the rings are very very tight on the piston. So tight I cannot remove them. Also the rings feel very smooth and I estimate them to be the same diameter as the piston. Likely this explains loss of compression.
















My thought is: Rings were either wrong size or I installed them incorrectly causing the tight fit in the cylinder. Initial compression was good and allowed easy start up of the bike. The rings wore quickly and thus...loss of compression.

I contacted the supplier of the kit and they immediately sent me and email link to contact tech support with pics and description to start a warranty claim. Hopefully they will send another complete kit and I can try again.

What are thoughts of the pro's here on the board? Any tips, suggestions, advice I can use for next time?

 

Hmmmm.
Good job man! NEVER GIVE UP

Confucius says "Must check ring gap before installing rings onto piston!"
File ring until gap is in spec if needed.
3ea china top end kits installed this summer I have not had to file a ring (all were in spec)


to check ring gap, put ring in cylinder bore, press it down with piston a half inch or so (this makes ring square to bore) and measure gap
use a flat file if needed and lightly file edge of ring until in spec, remove any burrs

Rings go on piston in no particular order with lt80 china kit (as far as I can tell there are NO marks on the rings and both rings are identical)
Meaning there is no top ring and bottom ring, they are both the same part.
But you MUST get the ring gap on the pin
There are two pins, one in each groove on the piston
This orientates the rings so the gaps cannot overlap and keeps the correct ring gap for good compression!

The rings must remain in the grooves and with the gap over the pin when you install into cylinder (lightly oil piston and cylinder) with arrow pointing towards exhaust
It should be tight fit but not so tight you cannot freely move piston up and down in bore
I hang bottom of piston skirt out of cylinder sleeve so you can get to circlip
install one circlip, hold the whole assembly over crank
oil connecting rod and wrist pin bearing
install piston/wrist pin and second cir clip
push cylinder down onto engine and onto base gasket


Also with the China kit good idea to CLEAN THE SNOT out of all the parts and pieces, they look clean but I assure you they are not

I am FAR from an pro or anything on these small engines so be kind

 

So...personally, I'd get that jug bored .5 over and install new oversized pistons/rings. Have it done by a reputable shop or have Jack (LT80) do it. Then you'll know you're starting off right. You didn't mention if you mic'd the jug/piston or gauged the rings...a good shop/Jack will do that to ensure a proper fit and a long life.


I just got my motor back from Jack...he does excellent work...you may not need a full bore, but he'd be the one to tell you after measurements & inspection...

 

OK so the parts supplier is making good on their warranty request and sending out a complete new kit(cylinder, head, piston, rings, etc...) at no cost and no need to return the previous kit. On some levels this is good as I am starting from a new kit. It should be here next week. When I get this thing back together and running I might send out my 'first replacement' cylinder to a machine shop to have it bored and mated to a Pro-X piston. The local shops here seem quite spendy. I see an eBay ad for a shop in Florida to do the work seemingly inexpensive. This way I will have a 'ready to go' replacement since I seem to be accumulating these LT80/KFX80 and likely will need to freshen up another bike.

What do you guys like for assembly lube? I used raw Lucas 2-stroke semi-synthetic oil (same that I run with the fuel) to assemble the thing last time. Now with reading and talking to some 'monday morning quarterbacks' I'm hearing motor oil, ATF, WD-40, 4-stroke assembly lube, build it dry. You name it...I've had it recommended.

Also what about start up and break in? I've heard build it and fire it and go like hell to seat the rings hot and fast; build it and let it sit overnight to soak, build it and fire it but take it easy for 30 minutes, build and fire it with heat cycles of only a couple of minutes each. I suppose everyone has their own method.

I'm leaning toward short heat cycles, followed by easy ride, followed by run it like I stole it. But we know how well my last top end held up!

 

For initial start-up use a high quality 2 stroke oil...just make sure everything has plenty of it.


LT80 provided guidance on initial break-in and is what has worked for me in the past...Fire it up for a minute then shut down/cool down...do it again...then next time "blip" the throttle...do that again...


First ride - make sure it warms up before taking off (cold seizure)...then take for a drive...opinion differ here, but I ride casually constantly moving through the RPM range...I ride casual for the first tank (I put a bit more oil in the tank for break-in).


I don't think you did anything wrong the last time...just make sure you measure/gauge this kit before you throw it back together. If not comfortable checking it out then bring it to a shop...the small $$$ spent will keep you from this constant rebuild cycle...

 

first ride, I like to heat soak, and any ride for that matter
I live at 9000' so its not as warm as other parts of the country

Do not just warm it up from a cold start and then go ride, shut it down for a few minutes. Let the heat you generated from warm up, let it soak into the bottom end case, crank, and bearings
This ensures all proper lubrication as you wont be running a hot top end with a frozen crank, give all the metals time to expand to their running clearances so to speak.
Then fire it back up and go run it

Snowmobiles that are treated this way far outlast sleds that are not

More important for cold starts (dead of summer right now), but this is something I learned from Indy Dan years ago. When Indy Dan speaks on these forums, we listen and take notes. Much like many of our residents here on Atvconnection.com (thanks guys)
I am sure many people know who Indy Dan is in the two stroke world

Heat soak your small engines, two strokes!!! Cheap insurance (polaris sleds are most important LOL but it applies to all machines. polaris sleds 800 cleanfire shear crankshaft at drive clutch taper....problems, sound familiar?)

 

Not sure I follow a "cold crank"...cold seizures are the result of the piston/rings expanding faster than the cylinder wall...actually, keeping the rest of the engine cool/cold is a good thing as heat is the #1 killer of everything!!!


Of course, in the sled world at least, too cold can lead to lubricant flow issues...not at all a problem with the normal operating temps of the average ATV...

 

I'm not sure I follow all of this, how come the bore is blued but there are no marks on the piston, or rings by the looks of it? I guess the top ring must have been well oversize, but the bore and piston must have been OK or the piston would have seized. Why are the rings stuck now?

 

cold crank is simply the metal has not warmed up yet, in winter (starting to feel like fall here its less then 50 degrees this am) you can have alot of heat in the top end from your initial warm up but the crank and bearings are still cold, that is all I was getting at.

Looks like the ring gap is not riding along the pin, it looks as if the rings are over the pins
that's not gonna work!
I do not see blueing in the cylinder?

 

"Rings are over the pins" - not sure how that happens...it'd be pretty tough to get the piston in the jug if done during install...almost looks like there are no pins??????


Had to have been something really wrong as there is no obvious signs of severe cylinder damage...


No matter...hopefully the new parts are good-to-go!