Top end rebuild question

[BlackRaptor]

Hey Guys I still new to the whole bike rebuild thing so i had a couple questions i was hoping someone could answer.

My 91 CR250R is 100% stock right down to the top end.
I got it from a guy last june and i asked him about it and he said they took it apart and the piston and rings looked fine and weren't in need a being replaced. The bike has great compression and always starts on the first kick and for a stocker bike keeps up pretty good with my newer modded CR so this makes me think that the top end it still fine.

But i've heard people say with older top ends the rings can fall apart and mess up the inside of the cylinder. This bike isn't riden too hard i mostly ditch ride and gravel pit ride it so it's not pinned all the time like in racing.

So i said all that to ask if this was your bike would you freshin up the top end this winter just to be safe?
Thanks.

 

[bedell99]

Oh ya, This is just a good preventive maintence thing. It cost 135 bucks for a piston and gaskets. Rather than be safe than sorry.

Erik

 

[Speeed]

in a word : yes
its cheap insurance

 

[BlackRaptor]

Thanks guys.
Thats what i'll do.

 

[uts]

If your compression is good then don't stress. Too many people stress about the're top end. If you replace it as the manufactures(25 hours is too much) say you are a luny. If you keep the air filter clean and use a good 2stroke oil then you're engine will last a lot longer than most people think.For us down under a top end is upwards of $300 which is O>K for a rich dude but for the regular dude just ride on until performance drops. I hjave never had a bike blow up.

 

[Rcannon]

I would say replacing the top end is a great idea.

For a long time I used my compression tester to evaluate top ends. 95% of the time you could detect a drop in compression. Changing the piston and rings would restore the lost psi.

Then I crossed paths with a 2002 CR 80. The owner is one of my best friends. His son rides the bike HARD.

Anyway, we tested the compression when the bike was new. I then tested it three month later. Nothing much changed. We presumed the top end to be ok and sent the boy out.

Three minutes later we hear "that" sound. The 80 stopped instantly.

We ran up to him . I climbed on the bike and kicked it. The bike started on the first kick. There was no comprression loss. The noise coming from the engine sounded bad.

Turned out that several chunks of the piston skirt had broken off. The piston was still attached to the con rod. It took us several hours to get the aluminum out of the lower end. We lucked out and got out all the small pieces. There is a chunk mossing from the lower case, but it has not caused any problems since.

I put the compression tester away and have never used it again. We now replace the top end every 4 weeks.

In my opinion, there is not much to be gained by checking compression. If you run a top end long enough to lose compression, you may be risking having the piston gernade. For the small dollar savings, it is not worth it. Change it out. That is the cool part abotu our "outdated" two strokes. A top end job is easy and cheap.

 

[uts]

Nice point about the old 2strokes being easy to fix rcannon. I sooo agree and this is why I WON"T get a gay assed four stroke.But I have done 100 hours on a 2stroke before, pulled it down. had the specs done and had hardly any wear. I guess it's different for each bike,

 

[Rcannon]

UTS, I could not agree more. I tore into my YZ 250 after 1 year of riding. I had been using a weisco piston and was curious as to how it was holding up. The piston looked brand new.

I doubt I ride hard enough to wear anything out. The only time my bike went fast is when studboy was riding it.

Still, I bought peace of mind by checking. I would be scared to run a stock Honda piston for 14 years as is possible in a 1991 CR 250!

 

[BlackRaptor]

I know i don't ride hard enough were mine needs a top end all the often but like Rcannon said. It's been in that bike for 14 years and must have a good 1000 hours on it. Because the people who had it before me did a ton of trail riding with it. I think the reason it lasted so long was they only used Synthetic oil in it and they only rode trails which aren't all that hard on hte motor. He also was always talking about how he hated running it at high RPM's. But just because of the age i think it's smart to freshin it up so it's good to go for another couple years.

Thanks for all the good input guys

 

[marcusgunby]

Had the same happen to a ktm 125 sx-had great compression-5 minutes later it needed a piston, cylinder, head and a few more bits.

 

[darringer]

At the very least, tear it down and measure the piston, bore,ring end gap, and check for play in the crank. Compression numbers are good for reference, but a terrible indicator of actual wear of the top end. With a bike that old, I would check EVERYTHING carefully.

 

[BlackRaptor]

You guys have me almost scared to ride the thing anymore untill replacing the topend. last thing i want is to be digging piston parts out of the bottom end.
Side question. whats a good synthetic oil for the bottom end? it's used honda synthetic purple it's whole life up till june. I don't have a honda dealer close to me so i've been using this Link to oil type
I'm not much of a oil expert is this good enough for the bottom end? or should i be running a full synthetic because of the age of the machine?
Top end i always run the AMSOIL DOMINATOR synthetic 2-Cycle Racing Oil

 

[Rcannon]

I have been using the Mobil One synthetic that I get from Wal Mart. The 15/50 weight works very well in my YZ 250. Shifting is much smoother than anything else I have used. I dont think we are supposed to use the other weights of the mobil one. The 15/50 weight, with a red cap, is ok for our clutches. The others may make them slip.

About the bike, what is the worst thing that could happen? Sure, it could blow up, but it is not as if you have been using the piston for 14 years or anything.....uh, well,


Seriously, lots of times the bikes do give warning signs. Most of the time a person can hear odd noises or feel a power loss. It is not like a woman where one minute things are wonderful, then BAM!!!!!

I would rebuild it as soon as you can.