Top End time- a few questions...

[Eberlyjp]

Alright guys, 2003 yz 125 top end rebuild in time for spring. Ive never done it, but I've read just about every how-to I could get my hands on and it seems very simple. A couple questions-

Is it dumb to put an oem piston in again? I see they're cheaper, but should I pay the little extra for a wiseco? (yes, money is an issue). What are the break-in/longevity pros and cons for each? Or, any other pistons to consider? Obviously the bike has been rebuilt before, perhaps wait to make that decision till I pull the top end off? This bike will only see recreational track days and a lottt of woods riding.

Wheres the best place to order the kits and gaskets? I would be inclined to use ebay unless anyone had a particularly great experience with any other online stores.

I do not have a shop manual. I plan on getting one, but does anyone happen to know the torque specs for the base and head? I'm probably just as well off just finding it out on my own- get familiar with the manual....

Anyone in Lancaster, PA wanna come help me out? Just kidding, well not really. I'll provide the beer.

Thanks in advance.

[MXtras]

Originally Posted by Eberlyjp

Is it dumb to put an oem piston in again?

I prefer OEM pistons. It's all I use anymore.

Originally Posted by Eberlyjp

.....should I pay the little extra for a wiseco?

Personally - I would not. I have had numerous issues with Wisco pistons over the years including fracturing and complete skirt failure. I have never had the problems with OEM pistons that I have had with Wisco. Ever.

Originally Posted by Eberlyjp

..any other pistons to consider?

Vertex. I have heard good and bad, but never experienced any bad. My experiences have been good. I am talking about the black coated pistons.

Originally Posted by Eberlyjp

Wheres the best place to order the kits and gaskets? I would be inclined to use ebay unless anyone had a particularly great experience with any other online stores?

Yamaha of Troy or cheapcycleparts. No affiliation.

Originally Posted by Eberlyjp

... does anyone happen to know the torque specs for the base and head?

I do not have them memorized, but I can look through my notes and report back later this evening.


A few notes:

Make sure you check the piston clearance before you install. Do this on the bench from the top and bottom of the cylinder.

Check the ring end gap before installing. Do this on the bench, also.

Stuff a rag in the crankcase when installing the circlips in the piston. Plan ahead and install one of them before fitting it on the rod.

Make sure everything is spotless before assembling.

Make sure you get the circlips in correctly. Triple check.

Make sure you get the ring end gap in the correct location on the piston, triple check.

Look at the exhaust bridge in the cylinder carefully. Check for plating failure and/or scuffing.

Scott

[MXtras]

And this thread might be of interest to you one of these days:

05 YZ 250 Top end A cylinder/B piston

Scott

[MXtras]

Oh - a helpful hint when breaking the top end off the case:

Drain the cooling system and flush with clear water a few times, drain completely. Pull the carb, pipe and coolant hose from the head. Pull the powervalve cover and disconnect the PV. Loosen the base nuts 1-2 turns and loosen the spark plug but leave it seated - just break it free a bit. With the the head and spark plug still installed, kick the engine over sharply. This will pop the cylinder off the base gasket. Now loosen the plug and take it out and then remove the cylinder. You do not have to remove the head if you don't want to, but new head gaskets might be a good idea. I usually do two to three rings before I actually remove the head and replace the head gaskets.

Scott

[Eberlyjp]

Wow. Great advice all the way around. Many things there I would not have considered. This forum is great.

[Eberlyjp]

That technique of kicking it over to pop off the jug....can you risk messing up the cylinder wall? I imagine that could be potentially harmful if the piston turns over once or twice and the cylinder isnt completely secured....just a thought. You've had good experience with this method before? If so, good enough for me......

[Eberlyjp]

I guess if you only loosen the bolts a little bit, it won't be loose enough to be off centered or anything.

[MXtras]

The cylinder will remain aligned. The cylinder can be way out of whack without damaging anything anyway - the piston pivots on the wrist pin, there is lateral slop in the rod on both ends - not to mention you are popping it over once, you're not running it. The cylinder is still being held in place laterally - just not vertically. I have done one or two this way...

Another thing - clean the crap out of the motor before you tear into it. There's nothing worse than tearing into it to replace a piston and ring and knocking a clump of dirt into the crank.

Scott

[kxyzf]

i would go over the entire bike with a toothbrush basically its what i did on my 03 its way easier to do it that way and have it clean....but i pulled the whole motor so i had to take off the swingarm and regrease those and put a top end in it. if you plan on doing it with the motor in the frame....get it as clean as you can i must have spent 3 hours cleaning my bike before my complete teardown...yeh get a manual for the torque specs install the right piston and have the cylinder replated if needed other than that its a couple bolts.

[MXtras]

Originally Posted by Eberlyjp

....does anyone happen to know the torque specs for the base and head?

Cylinder head nuts = 20 FtLbs
Cylinder base nuts = 22 FtLbs
Reed Block Screws = 7.2 FtLbs (snug)

Let us know how it goes.

There is one other thing that I suggest doing. It's a bit tricky but it reduces the risk of hanging a ring on a port. It goes like this - after the ring is on and you slide the piston into the bore but before the cylinder is seated, slowly and gingerly rotate the cylinder just above the studs to identify the ends of the ring - it's a very gentle "feely" thing. What you are feeling for is the ring ends to hit the location pin. Find the limits, and cut it in half. I do this twice before finally seating the jug. This is harder to do that describe. It's not a huge issue if you do not do this, but if assembled with one ring end hard against the location pin there is a higher probability of the ring snagging a port the first time things start to heat up and migrate. The ring should expand and just slip in the groove, so it's not a big deal. Just thought I'd mention it while I had time.

Scott

[Eberlyjp]

Thanks for all the help. That ring trick makes sense....

I'll probably leave the engine in the frame, unless I'm feeling ambitious, but you could eat off my bike right now either way. Spent about 4 hours cleaning the whole bike last weekend because it's not running, I was bored, and anticipated this project.

If I do pull the engine, what other things would be worthwhile to do while the bikes apart? The bike is in overall great shape, but I just got it a few months ago so I wouldn't mind switching things out while I have the chance such as reeds and whatnot so I can monitor how long they've been on the bike, even if they don't necessarily need replaced right now. I've seen those hour monitors on 4strokes, do guys put them on 2strokes too? I would love to be able to keep track of my hours after the fresh build.

Thanks again, I'll hop on here when its all done.

[Eberlyjp]

Another thing- If I pull it apart and the piston itself is in good shape, its just a matter of compression because the ring is worn, can I just replace the ring?? Or would you recommend just replacing everything while I've got it apart? Bearing, wrist pin, circlips, etc? Is there a specific grease I should use on the bearing?

Sorry for the million questions, but its a learning experience.

[MXtras]

I have no issues with all the questions, dude. This is why this place is my favorite hangout. Lots of helpful, kind and generous folks here - without all the typical BS.

Originally Posted by Eberlyjp

..what other things would be worthwhile to do while the bikes apart?

I would strongly suggest rebuilding the rear suspension. All of the pivot points require frequent attention. I will re-grease the rear suspension three or four times a season - depending on the amount of use and how many times the bike is in wet conditions.

This can be a daunting task the first time, but it's quite simple just to take things apart, keep them arranged, clean, inspect, replace the seals, lubricate and re-install. Re-doing the shock is also a good idea - the volume of oil in the shock is ridiculously small and can go south quickly. Re-doing the shock is something I can do for you or you can have a local suspension guy do it for you - I would not suggest trying yourself the first time, plus re-charging can be trouble unless you are equipped to do so. Just cleaning the valving, freshening up the oil and topping off the nitrogen charge makes a big difference in performance and life.

I would also suggest re-doing the rear wheel bearings and replacing the seals. The seals are the first line of defense and they are relatively inexpensive. All new seals for the suspension and wheels would set you back maybe $30. Replacing all the bearings will set you back $400. Cheap insurance.

Originally Posted by Eberlyjp

I've seen those hour monitors on 4strokes, do guys put them on 2strokes too?

I do not but I have seen a few guys that do. I keep logs of riding time, performance, jetting, weather conditions, etc. I go by my records. I do a lot of PM, usually and I do pressure wash the bikes every time they are ridden.

PM is a lot cheaper than breaking stuff and it's a lot more fun to ride than it is to push the bike back to the truck!

Originally Posted by Eberlyjp

can I just replace the ring??

Yes, you can. if you do not know how long it's been in there, it would be a good idea to replace - they do fatigue and wear, obviously. In addition to the skirt of the piston, the ring groove wears also. A worn ring groove allows the ring to walk and skew and will promote unnecessary wear on your bore at the ends of the stroke. I usually will replace two or three rings on a piston before replacing - depends on your budget, how you run the engine and your mix ratio. A fresh piston is probably a good idea.

Originally Posted by Eberlyjp

Bearing, wrist pin, circlips, etc? Is there a specific grease I should use on the bearing?

Yes - you should replace these items when the piston is replaced. If you are just replacing the ring, do not disturb these items but do carefully evaluate for wear. Any discernable play (other than horizontal) is unacceptable. There should be NO vertical slop or looseness. Replace all if there is play. Side to side play is normal.

Do not use grease - use premix. Coat the piston, bore and ring before final assembly. Don't go overboard.

Any other question - fire away.

Scott

[Eberlyjp]

Sounds good. I'm going to replace everything (piston, rings, wristpin, bearing) this time while I have it open so I have a point of reference for how long everything has been in the engine going forward. I can tell this bike has been well maintained by the guy I bought it from (air filter, bearings, etc) so I have a good base to start with and can keep up with it now.

I read some posts you had earlier I believe about changing the seals (gotta love the search feature) on the bearings as cheap insurance, makes sense. I ride with my girlfriend and her dad, who's bikes are meticulously maintained, so I definately dont want to be stranded anywhere haha.

I've got the fluids, pipe packing, filter, air pressure, spokes, all the basics outa the way and logged, so now its onto the more challenging stuff- top end, float, jetting, bearings, fork seals, etc. I'm slowly learning as much as I can and basically rebuilding (or at least double checking) everything on the bike. Saves a lot of heartache out on the ride days.......and its a lot of fun to work on.

Its amazing to me how easily this stuff can be learned just by asking questions and reading on here, as well as some youtube videos. Rocky Mountain Atv on youtube has some great tutorials that i've been checking out.....

[Eberlyjp]

Where at in Virginia?? I went to JMU.

[MXtras]

I am near Va Bch - down in the south-eastern corner.

Scott

[Eberlyjp]

Allright guys, todays the day...tore it down last night, me and a buddy and sam adams are gonna tear into it today and see what we can get accomplished. Thanks for all the help so far...

A few other questions:

Oem piston- what should my break in procedure be? SHould I basically run a full tank of gas through it with casual riding? Maybe run a richer premix than I normally would? Actually, I dont think that would make sense......let me know.

Anyone know what my ring gap should be?

And just to make sure- The arrow on the piston dictates exhaust side/front of bike, correct?

The bikes all clean, broken down, parts seperated and organized. I think i'm ready to go!

[Eberlyjp]

Wow. Best feeling ever. Spent all morning rebuilding it, got it put back together, fluids, etc, and it fired up FIRST KICK. It ran better than it has in a long time, amazing throttle response. Just sits there and idles beautifully. Might be a bit lean, but I can fiddle with that later.


Thanks to everyone on here, especially MXtras....huge help!!!

[bitshifter]

Eberlyjip, where do you ride? I live in Avondale, Pa and ride at Blue Diamond MX park in Delaware. I am always looking for a place in Pa.

[Eberlyjp]

Hey, we have memberships at Tower City (rausch creek) and i make it out to rocket raceway every now and then which is closer to pittsburgh. Only 25 bucks to ride the track which is real nice, on saturdays and sundays. Otherwise, its pretty much just farms and friend's cabins and that sort of thing. Whats blue diamond like? Never been....