Engine Rebuild Kit Recommendation - 1999 yz125

[bb3924]

Engine seized up as result of exhaust side of piston's skirt breaking apart and falling into lower end of engine. Yes there was engine oil and also coolant, both before and after the "seizure"... or that would have been a big lecture to the boy!

I read on another blog this is usually the result of running to lean, thus too hot and pop??? In hindsight, the bike was accumulating some oil/sand at the exhaust port/muffler connection on the cylinder. It was initiated by an over full oil condition, which was corrected, weeks before. But, maybe that popped the seal. The bike started, ran well, plenty of power, it was odd. Another blogger suggested it is caused by too much tolerance in the crank bearings, causing piston slap?

I have torn down the engine to lower end engine case splitting stage.

Cylinder has small gauge (about 3mm length, maybe 1mm depth and 1-2mm width) from top of port upward. Thus, top 50 - 60mm (about 2") of cylinder are perfect... which is combustion chamber.

Metal piston shards remain in lower end of engine. Although when I pulled the flywheel, crank broke free and permits about 200 - 270 degrees of rotation. And, rotation exposed many shards of piston skirt metal wedged in between crank, removed accessible metal shards, yet crank will still not complete full revolution.

So...
Questions are;

1. does the cylinder need rechroming or can I simply hone it to remove any high edges as the nick is so low in combustion chamber?

2. looking for guidance on how much to change out... lower end bearings and seals (about $60) or crank too (about $75)? top end kit (about $125) (depending on brand)
3. any experience/recommendation on engine rebuild kits; wiseco, nomura, hotrod cranks, moose racing - all offer a complete lower and top end rebuild kits from $260 to $400...?

4. any recommendation on cylinder rechroming companies? (us chrome)??

About 10 manhours to get top end, clutch cover, clutch housing, flywheel, flywheel cover and stator plate off, and remove engine lower end from bike frame... including all the requisite steps to begin those tasks. Likely a shop would do it faster. Only mention the manhours to give an idea to other weekenders considering the task.

Thanks in advance.

[blackhat250]

Is it a nakarsil bore ? these can be coated back to standard size, google is best friend here , find company to price it,,

[bb3924]

I broke the engine free when I removed the flywheel.  The flywheel puller threads into the center of the flywheel and is hollow so a ram bolt pulls flywheel off the crank shaft.  When the crankshaft rotated, chunks of piston were revealed lodged in between the two metal discs on the crank.  Removed all visible fragments  and could only achieve 270degrees of crank rotation without "force"  Then, attempted to flush out crank case with diesel fuel using an drill powered transfer pump spraying into an inverted engine.  That actually flushed out most all of the piston metal fragments.  Crank shaft would achieve 360 degree rotation.  It seemed like a cheap fix.  Let engine drain overnight.  Next day, the crank turned but there was a catch about every third of forth rotation.  I knew one of the crank bearings was impacted... and that may have been the cause of it all.

I then split the cases.  At that stage, it only makes sense to change all the bearings and seals.  It proved that the crank bearing on the flywheel side of the engine and one of the transmission bearings were both not spinning like they should.  The crankcase does not have much free space around the edge of the crankshaft, less than I thought.   Case is in excellent condition, no cracks or gauges.  Used automobile aluminum rim cleaner and then a water bath and the aluminum cases look like factory new.

So, after research and consultation, here's my recipe for a total engine rebuild on a YZ 125.

- hotrod lower end kit to include crank shaft, crank shaft main bearings and wrist pin needle bearing and seals
- hotrod transmission bearings & seals
- hotrod water pump rebuild kit
- hotrod top and bottom end gasket kit
- namura piston, wrist pin

The cylinder bore has a small nick, about 3mm length and 1/2mm depth extends from top of a port, left side of bore.  I was advised that a light hone and performance will be minimally impacted, if at all.  The reasoning provided was the imperfection is low in the chamber so excellent compression can still be achieved.   I plan to compression test after build.  If poor, then I'll go for a re-chromed cylinder exchange.  Cost is $290 gross with a $70 core charge after you send them your cylinder.  USChrome was recommended by more than one source.   Hidden costs of exchange are the cost and time to remove/reinstall the  head studs and  power valve from your core and install back into new cylinder.

Was advised that namura, though less cost than wiseco, is a solid if not better piston.  Learned that Vertex piston is OEM on the KTM bikes.  Hotrod has the best crank and bearings and the cost is slightly more as a result. 

All in for the recipe of parts - $350, with cost saving of almost $200 achieved by sourcing on-line versus local dealer for same parts.  Had I went with Wiseco, I could have done it for about $285.

Tools that I've had to purchase for this project;
- flywheel puller $20
- case splitter tool $60
- crankcase puller (tusk) $66
- blind bearing puller $50

So, $550 between for parts, tools.

2 cycle is a pretty simple engine and the 125cc motor when out of bike is quite small.

Cases are stripped of bearings, cleaned of oil, gaskits, and case sealant residue and prepped for the oven... Bearings are in the freezer.  My first engine rebuild is about to begin.

I'll track my hours and share the lessons learned.

 

[bb3924]

Installed crank, and transmission bearings in both cases.  Had bearings in bags in freezer overnight and then put cases in the over at 300degree temp.  When they reached the point when water vaporized, turned off heat in oven.  Then each side crank bearing.   I had to supplementally heat the case with a propane torch to get bearing to slide into case.  Wipe the bearing, that you just pulled out of the freezer, quick wipe and into the correct position.  Both crank seals went smoothly.  Both blind bearings required physical seating, which I did with a rubber mallet and impact socket where the outer edge of the bearing (race) matched with the socket size.  Again, heated the case with torch first.  Bearings seated well, with some persusion.  Then I placed the case halves on a wooden bench with cloth below and covering each case half and bearings, to keep dust or dirt from getting into bearings.  Bearing install 1.5 hrs, with bake time.   I warmed cases for about 15minutes in the oven @‌ 300 degrees. 

Suggestions 
- dry fit the bearings onto the case, noting location of each bearings with a tag/map - the tranny bearings.   
- put each bearing and them into individual ziplock bags in the freezer, which includes a part reference - so you know which individual bearing fits each slot... it will speed it up and keep the bearings cold... mine were altogether and they "warmed" as a group. 
- keep torch moving and heat both inside and outside of case at bearing location... quick on the bearing move from freezer to case..  I watched video and tech was using dryice, which might help by lowering the bearing temp further.

Next steps are bearing seals, crankshaft, transmission.

[Cynic]

Need pictures,

also, a warm air gun is much safer than a butane torch. The naked flame can hot spot the metal.

You cannot heat with a heat gun to a dangerous level (for the bearings and cases), nor turn yourself into the human torch in the garage.