piston seized after doing top-end.........

XtremeAir125

Member
Jul 5, 2001
120
0
Recently, I did the top-end in my 99 CR125 w/wiseco pistons. I took my bike to the dealer afterwards and they jetted it perfectly to what FMF suggested. I have a FMF Fatty with a PowerCore II Shorty Silencer, and Boyesen CarbonFiber Reeds. I also used a BR8ES spark plug at the time. Once I got my bike back, I putted around my lawn for about an hour, taking my time to break it in. After that, I went inside for about a few hours and came back out to have some fun ;) So I took my bike up the road and revved the hell out of it. I was in 4th gear and all of a sudden i lost the power...nothing. When I got home, i tried kickstarting it and it was stuck. So i took apart the top end and noticed that the piston had scuff marks on all four sides. Did I forget to put more pre-mix oil in the lubricate the piston? Or was it cause I used a hotter spark plug? Any suggestions? Thank You!
 

MBFTY

Uhhh...
May 4, 2001
192
0
First of all...

First of all, you didn't break that engine in. 1 hour isnt nearly enough to break in a new piston and bore.

Did you do the ring gaps correctly? If you didnt, they will expand and destroy the top end about as quick as you said yours went out.

Let us know...
 

XtremeAir125

Member
Jul 5, 2001
120
0
I checked the ring gap and it was all good. I must've not broken them in long enough. I must've been going 35 when it seized (good thing i didnt flip it!!). Could the plug have been the problem? When i took it out, it was almost melted just about? Next time im gonna run 9's. Thanks for your post.

Ryan
 

MBFTY

Uhhh...
May 4, 2001
192
0
Melted...

If it was melted, that could mean an extreme heat problem...

Check your jetting! Check your jetting! Check your jetting!

There is a possibility that the dealer didnt do it right. You also need to break the engine in better next time...
 

XtremeAir125

Member
Jul 5, 2001
120
0
Thanks! I will give my dealer a call in the afternoon and tell them what's up? Geez, after I just spent $300 for them to fix it look what I get! Thanks bud!
 

EricGorr

Super Power AssClown
Aug 24, 2000
708
1
Although FMF makes a valiant effort to post baseline jetting specs on their web site, they don't post other important data like;
1) Fuel brand and specific gravity
2) Oil brand and pre-mix ratio
3) Air temperature or density altitude
4) Spark plug heat range
They also don't have any disclaimers like your riding ability, terrain where you ride, and bike condition have a bit more to do with jetting than your choice of exhaust system. Boyesen does the same thing, instructing people to drop their jets by 3-4 sizes. I can't tell you how many people seize their bikes from well meaning instructions then call me up asking what went wrong.

Don't blame your dealer for your bike seizure, most mechanics try their best and assume that if a big company makes a particular suggestion that its good as gold. Your bike seized because the piston expanded at a faster rate than the cylinder. So it either has too lean of a main jet, you loaded too hard just after start-up before the coolant temperature stabilized, or perhaps the left crank seal is leaking (very common problem on CR125s) and the extra air seeping by the seal leaned the mixture like too small of a main jet and caused the seizure.

Good luck, Eric
 

davearm

Member
Mar 29, 2000
30
0
Since you seemed to seize all the way around the piston, it sounds like it got way too hot. Being lean I would think would have seized it up on the exhaust side. Perhaps the coolant level was not adequate. Sometimes there are air pockets that do not get purged and those areas steam and obstruct water flow.
 

capt hookleg

Member
May 30, 2001
25
0
sneezed

If you would check eric gorr web site, it has a piston diagnostic area that shows the different types of seizures and the common causes. A four corner seizure like you discribed is usually cause by either a cold seizure or to small of main jet. FYi:(
 

Yamamoto

Uhhh...
Apr 3, 2001
349
0
OEM Rules!

I hate forged pistons.

So i took apart the top end and noticed that the piston had scuff marks on all four sides......This is 4 Point seizure ...caused by Inproper warm up or too much heat.....could also be an air leak.

My experience with Weisco is that you need to really let it warm up even after your have warmed it up early on.

example take aluminum foil and heat it up ...in seconds its ambient again.

The piston expands at a much faster rate then your Cylinder and if its not fully wamed up you will seize yor piston.


Did I forget to put more pre-mix oil in the lubricate the piston? If you forgot to Premix your gas then thats your problem.

You did refill the radiators? RIGHT?


I think you have an air leak or you were way to lean....did you hear a shearing noise when you got on the bubble.

Melted spark plug ......another lean symptom
 

Mudboy

Member
Dec 3, 1999
95
0
As for those saying "an hour isn't enough of a break in" that just isn't true on a 125 two stroke. I put a new top end in my 250, warmed it up for five minutes, then revved the crap out of it riding sand dunes. If you've installed everything right and jetted it right, there really is nothing more than a little warm up required before you twist away!
 

Yamamoto

Uhhh...
Apr 3, 2001
349
0
Originally posted by Mudboy
As for those saying "an hour isn't enough of a break in" that just isn't true on a 125 two stroke. I put a new top end in my 250, warmed it up for five minutes, then revved the crap out of it riding sand dunes. If you've installed everything right and jetted it right, there really is nothing more than a little warm up required before you twist away!

Thats the way i do Mine ....let it warm once shut it down...start and let warm .....then beat the Bajesus out of it.OEM

Weisco takes longer to warm up and brake in because its Forged not cast.

I still think he has an air leak....boot to the carb,seal,lose spark plug ...SOMETHING

that or just jetted WAY to lean.

Maybe we will see.
 

Anssi

Member
May 20, 2001
870
0
Even the jap manuals don't tell you to break in after a top end for more than an hour (although the tell you to only heat-cycle the engine for a few times (warm up and then let it cool down)). And these guys tell you to change your piston after 5 races on a 250, so they are a bit on the safe side.

My guess is on imsufficient warm-up after you took your little break.
 

Camstyn

Hoser
LIFETIME SPONSOR
Oct 3, 1999
2,247
2
Mudboy, what piston did you use?
Did you look in the motor after break-in?

You're lucky you didn't seize the engine.. I bet it's going to look pretty rough inside when you pull the cylinder.
 

Mudboy

Member
Dec 3, 1999
95
0
I used an OEM piston. My bike came with a "pro kit" that had parts for a top-end job. I let it warm up, ran it around slowly for about five minutes, then laced into it. FWI this is how most of my riding buddies do it as well and our bikes haven't had problems. As long as you're getting a good seal with the rings and have it jetted right, it's either going to work or it won't. Let's say you warm it for 5 minutes at 2000 rpm's: that's 10,000 piston strokes! If it ain't working then, it ain't gonna work at all!
 

spanky250

Mod Ban
Dec 10, 2000
1,490
1
Originally posted by Yamamoto
well that will not melt spark plugs
If the tip of the plug was melted awat, it was either very lean, or detonating badly for some reason, possibly both, the pinging could have been caused by lean jetting. The short wamr up time you described can certainly seize a piston, new or not, and it's much more likely with a new piston, especially a forged one, but that would not melt the plug. After repairing it, I would jet much richer before riding it again, and jet it a step at a time after break-in ( which needs to be more than one short warm up). Why did you rejet anyway? Did it run rich or lean before? How much did you change the jetting? If your engine is basically stock other than the exhaust, you shouldn't have to change it very much, and my experiece with aftermarket pipes is they require you to make the main one or two sizes richer, not leaner. I'm sure some bikes are the opposite, but none that I have owned.

Originally posted by Mudboy
As long as you're getting a good seal with the rings and have it jetted right, it's either going to work or it won't.
I have to disagree. the running time is not the issue, it is heating-cooling cycles that a new piston needs to relieve the trapped stresses in the metal from the manufacturing process. Without this, it is just a guess if it will run or blow. If it is a forged piston with tight tolerances, it will most certainly seize this way.
 
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motopuffs

Member
Mar 15, 2001
182
0
How well do we really understand the physics of breaking in a piston?

If you look at some of the older manuals, they say to run the engine easy for a few heat cycles, then take it apart and remove the high spots on the o.d. with emery paper. This is a pain in the pooper, but really the best method, particularly if you have tight clearance.

Let me ask this: what does break in do? I mean, what is actually going on in there?

For sake of argument, let's say that during break in, we are wearing the high spots off of the piston with the cross hatching on the cylinder wall. I might note that there won't be any cross hatching on many cylinders (well used and not honed). According to the accepted plan, the motor isn't revved or loaded like normal, so expansion of the piston will be less than full race conditions. In other words, the high spots won't have a reason to wear down any significant amount, because they aren't forced against the cylinder wall (also lubrication is present). So now your break in is done, you go out and rip on it, the piston expands way more than during break in, and you seize it...follow my logic?

Heat cycles: isn't or couldn't this be already done at the factory? Couldn't I do it in the oven before ever assembling? The crown of the piston gets pretty hot, but the o.d. shouldn't see more than a few hundred degrees.

Aluminum transferrs heat and expands so quickly that I don't think it's too likely that you can really wear down the high spots during break in, in a controlled manner.

There are many reasons for high spots/areas of tighter clearance:
1. relief of stresses after machining
2. cylinder profile doesn't match piston (note that they don't both have to be round)
3. nonuniform transfer of heat/nonuniform expansion of piston
4. overheating exhaust bridge area (this is a bummer)
5. improper maching or casting of piston (not likely, but could happen)

A mechanic for which I have much respect will heat-cycle a few times, up and down the trail for a few minutes, and say "it's ready to go".

As far as warm up time/cold seizure, this is always a point of debate. This also comes back to piston clearance (I'm ignoring the crank bearings for now).
If you have a generous clearance, and you are starting the bike on a hot day, the engine is 90 F, and the most the cylinder might reach (within say 10 minutes) is 160 F, I have doubts that there is enough difference in the cylinder bore size due to thermal expansion to make any difference. My opinion: if you cold seize an engine in the summer, you clearances were too tight already (on the verge of destruction). The bozos revving the bike on the starting line for 15 minutes are just building more heat in the motor which they don't need. Some time for fun I will do the calculations on this.

If you really want to get serious, take measurements before assembly to check piston to cylinder clearance. Or how about using some thin shim stock between the two? I admit I don't always do this. When I have, I found a wiseco piston to be a tighter fit (this was just on one bike, but...) approaching the point of being too tight, so between this factor and several others, I decided to stay with OEM.

Anyway, as you can tell, this has always bugged me, because many folks have all sorts of opinions on how to break in a bike, but very few can explain why and how.
 

spanky250

Mod Ban
Dec 10, 2000
1,490
1
Originally posted by motopuffs
Heat cycles: isn't or couldn't this be already done at the factory? Couldn't I do it in the oven before ever assembling?
The answer to this is no. The heat cycles release the frozen stresses in the metal that are left over from the manufacturing process. We performed an experiment in Tech school on this subject. We took two pistons, one brand new, and one used, and put both of them in a 500 degree oven for twenty minutes. The used piston came through this test unscathed, but the new piston was distorted beyond recognition. The heating cycles during break-in help to relieve the stresses, while the cylinder keeps the piston from distorting. Since the piston is trying to change shapes during this process, rushing the break-in and trying to push the piston too hard too soon will allow it to distort enough in the cylinder to seize. I also believe that the heat produced by the combustion is not the only factor at work, you also have the huge forces placed on the piston by the acceleration and decceleration that occurs at the end of each stroke, and by the extreme pressure rise that occurs at the moment of combustion.

I am no engineer, but I have always carefully broken in my pistons, and have never had a new piston fail during break-in, nor have I ever cold-seized an engine, because I carefully warm my engine before riding. I have many friends that ride, and most of them are less careful about these things, and all of them have had piston failures and seizures over the years. Even the bike that I currently ride had been cold-seized by the former owner, requiring a new top and bottom-end rebuild.
 

motopuffs

Member
Mar 15, 2001
182
0
Spanky: were both of the pistons in your test of the same type? Just curious. Although some areas of the piston certainly see 500 F, other areas, notably the skirt on the intake side, shouldn't get anywhere near that hot. I suppose this points to why my idea wouldn't work...the heat in the engine is not uniform over the whole piston.

Heat is transferred to the top of the piston from combustion, then the piston has to reject this heat, I think primarilly by conduction thru the rings and the skirt. The rings have constant contact, but the skirt just floats. I'm sure the rod pulls some heat too. Anyway...

I'm a bit skeptical that the cylinder "keeps the piston from distorting". If the clearance becomes zero, which is what you describe, the engine will seize. It doesn't matter if you are at idle or full romp...no clearance, no run.

Anyone else had a spontaneous water leak in a solder joint under the kitchen sink? I just did. Only took 15 mins to fix, but still a little disconcerting. Please don't ask who did the soldering in the first place.:scream:
 
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