Who's done a top end rebuild?

[Orange Rider]

ive done it on my 85 and it was easy. i cant imagine it being too much harder on a bigger bike, canit?

 

[GhostRider32]

I have heard that there are different size pistons for some bikes, I.E. an "A" piston might be 5/1000 smaller than a "B" piston for the same bike. If I remember correctly it had to do with the origional cylinder machined size. Can someone explain this to me? Thx.

 

[GhostRider32]

No one can explain this??? In a round about way, my question is how do you know if you need an "A" "B" or "C" piston size?

 

[ellandoh]

No one can explain this??? In a round about way, my question is how do you know if you need an "A" "B" or "C" piston size?

this thread is a poll , an old one at that, most knowledgable types have probably already been in here.
with that i'll say you should probably start a new thread or start using search function.

i can tell you this though, on my cr250 there is a letter A stamped on the left front corner of the jug

 

[kartstar]

I think the "A" "B" "C" pistons were in Hondas for a very limited year range, but seeing as most of the time you're not even offered the option when ordering, I wouldn't sweat it.

I have to replace the top end in my racing kart (CR125) after every race weekend - and as has been said, piston skirt pieces in your bottom end (and pipe and muffler) es no bueno!!

I recently bought a '96 CR250, test rode it, brought it home, and pulled the jug off to put new rings in. Half of its intake-side skirt was missing, and had been for awhile. There were pieces wedged in the oil slots of the con rod big end, but no other sign of shrapnel having gone thru - I can only imagine that (luckily for me) the skirt exploded and shot out the exhaust and the guy didn't even notice. I replaced piston, rings, small-end bearing and wrist pin, put it back together, now waiting for bottom end to go south or the end of the riding season (whichever comes first)! :coocoo:

I'm not sure I'd recommend this approach for less stupid folk...

 

[adam728]

I've only replaced the top end on my own stuff a few times, but I've probably done 100+ overall while working as a mechanic on sleds and quads (and tractors and cars and the boss's boat and whatever else).

 

[railer]

I got my 98 RM125 in Nov05 (so, about 7 mos. ago), and haven't done a top-end in it yet. I ride at the track on the weekends, and during the week, I'll maybe go out for an hour or two and ride in my backyard.

The guy I bought it from said it just had a rebuild, and he must've been right, becuase it has run great ever since I got it. starts on the first kick still. Can't complain. I'm worried about my topend a little after reading this thread though, but I'm about to deploy and don't have time or money to do it right now. So the bike's gotta sit a year before I can even touch it. Hope it runs good this wed. (goin out to practice on the track), and I'll have to do a rebuild when I get back to the states.

Brian.

 

[kshackleton]

Yes, hone the cylinder with a ball hone. 240 grit aluminum oxide ball hone that's about 10% smaller than the cylinder. That's what Eric recommends, that's what I use. If you don't the rings don't seal well and you'll be doing another top end very soon. That's the mistake most make. Can't just slap the piston in and go. Buy one of Eric's books it has great info. Hey Bike_Rider250 the wear limit is .004 time to replace it. That's the maximum wear. But I'd replace it.

I have a couple of questions here:

1. Kawasaki states that my 2001 KX250 has a cylinder that cannot be honed or bored....comments?

2. You state that the ball hone must be 10% smaller? than the bore....should that not be larger?

3. my compression [cold] is 175 psi, the bike starts first kick, and runs excellent. I have put perhaps 10 hours on it since I bought it [15 max], but I have no idea when the last top-end was done. Should I do it to be safe, or wait for the compression to drop a little?

Thanks

Ken

 

[shepherdblake]

I have a couple of questions here:

1. Kawasaki states that my 2001 KX250 has a cylinder that cannot be honed or bored....comments?

2. You state that the ball hone must be 10% smaller? than the bore....should that not be larger?

3. my compression [cold] is 175 psi, the bike starts first kick, and runs excellent. I have put perhaps 10 hours on it since I bought it [15 max], but I have no idea when the last top-end was done. Should I do it to be safe, or wait for the compression to drop a little?

Thanks

Ken

I think that if you have 175psi, then you are in great shape. Just keep an eye on it....

 

[crrod]

I replaced my first top end a short while back on my 99 CR125. I have about 5 hours riding time on it since and it is running great!

 

[timskay]

Is there perhaps a good rule of thumb psi wise when a top-end should require attention? say 125psi?
thanks

 

[2 stroke king]

im getting ready to replace my top end and one thing that told me my top end was messed up was my friend brought over a compression test and it only reach about 60 PSI and it should deffinitly be over 100, more like around 130.

 

[shane.ballard.nf]

compression is always a good indicator of an engines condition.
without good compression your motor is gunna run like ****.

 

[tracetrimble]

Even 130 is kinda low. I'm surprised it even ran at 60.

 

[SoFlo]

Well, you have to make sure that the engine is warmed up, and that you hold the throttle open, if not, you'll get a false low reading.