The infamous top end...

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Apr 26, 2006
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#1
So I'm riding along the track, and my bike bogs, right? So i ride it to the inlet of the field to the road. Shut it off, and it rolls in 1st gear. No more compression. If my piston isn't making any noise, does this sound like a typical ring job to some of you? Id be more than willing to keep my bike going for the summer if i can pop rings into it (given it really is as simple as people say) and not have to spend hundreds. Any comments or advice are great, even if they're short, or if they express how annoyed you are to read yet another question about this. Ive searched, and found that this can very well be my problem, but I have also found it can be a many other.

Thanks
 
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#2
The only way to know for sure is to pull it apart and inspect everything. If the cylinder and piston are in good condition and wear is within limits, then you might get away with rings and gaskets. But if you have that much of a loss of compression, unless the plug or head are loose, you're probably looking at a complete top. You may not hear a noise when a problem exists. My top and bottom were trashed yet the bike was quiet and still had good compression. Therefore, inspection is the only way.

Marc -
 
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#3
Thanks for the reply, but i have a question after that. Is inspection with no manual a good idea, or do you just need it for timing, torque limits, and basic reassembly?
 
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#4
It's a good idea to have the manual so you know what the wear limits and tolerances are. You can get a general idea of condition just by a visual inspection of the parts. Going further, you can mic out the piston and the cylinder at a number of points that will give you a much clearer picture of how much wear is present and where it is. In addition, you also need to know what the specs for rod side clearance, axial play etc. are to determine the condition of the crank for instance. For the $20-30 for the manual, it just doesn't make sense not to have one. If you're an experienced mechanic, the factory manuals are good, otherwise Clymer's manuals have a lot more pictures and basic info and therefore better suited for the lay person.
 
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#5
yes sir, i have a chipped piston on the top lip, and I am thinking it was caused by worn rings. There seems to only be Vertex Pistons and rings available, but there are different "kit sizes"????? Kit size A-B-C-D. Standard cylinder bore is 70 mm for all, but Piston size for C and B are 69.96 mm, A is 69.95 mm, and D is 69.97 mm. Can anyone translate for me?

I also wonder about reusing gaskets? Any thoughts?
 
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#6
I don't mean to insult you, but did your spark plug fall out? It would make a very hollow sound when you kick it over.. I only ask because it sounds like you went from a running motor to zero compression all at once.
 
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#7
I only ask because it sounds like you went from a running motor to zero compression all at once.

I take no offense. No my plug didnt fall out. It did exactly that, went from a good running motor, to no compression all at once.

Back to business, about the different sized pistons, I will do a search but I am open to anyones knowledge.
 
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#8
You carefully measure your cylinder then select the piston that gives you the correct clearance, typically .0025" or aprox. .06mm. Since the smallest piston is .05mm under the standard bore, I'd say they run it a little tighter. Best to check the manual for the correct clearance. I'm confused by B & C being the same size???
 
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#9
The cylinder may be marked as A-B-C or D. Honda does that and I think Yamaha does also. Honda marks theirs at the base near one of the mounting holes.