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Hi everyone!

I own a 1981 Yamaha XJ 650 (4KO). I'm having trouble to find the exact measures of the piston rings to this bike. I found a list of 4 different sizes. I these were never changed before in my bike. So, could someone help me to learn wich ones do I need?
There's a link bellow, in this site, to the list of the rings I'm telling about.
Thank you

http://www.totalmotorcycle.com/YamahaSe ... Piston.gif

You need to measure the piston bore to know which ring set and pistons you will be using. See the different piston sizes, the different ring sets go with the pistons.

Thank you for your answer Jonhj. I still have a doubt about the bore. By bore you mean the hole/gap/space where the rings fit into, isn't it? Sorry to ask. I just want to be sure because english is not my mothertongue

As you can see in the example the piston travels up and down in the cylinder bore. So the cylinders were pulled off the engine to expose the pistons. You measure the cylinder bore to see if it is worn out of specification. You can have the cylinders bored out to the next larger size (+.25mm over) and fit new pistons and rings.

I hope this helps.

The bore is the diameter of the hole the piston fits into.

You want to measure it at the widest point in two directions. At three different heights: bottem, middle and top of the cylinder.

Ok Johnj, I think I understand. You mean the measure that I need to know is from the interior of the cylinders, in orther to know the amount of consumed wall ( of the cylinder ). So, to compensate this consuming, I should choose the right set of rings that fill this consumed gap, for example 0.25mm or the next one, isn´t it?

You said before each set of rings come with the right set of pistons. So, can´t I use the old pistons anymore? Or will the new ring set fit the old pistons anyway? Does the new piston set keep the same interior diameter like the original ones, or is it really necessary to buy new pistons?

Thank you. You've been very helpful

All right Sevulturus, I understand now about this "bore" word, what and how to measure it.

Thank you very much you too for the help, Sevulturus.

Yes you measure the interior of the cylinders. Like Sev said measure it at the widest point in two directions. At three different heights: bottom, middle and top of the cylinder. If the bore is worn beyond specification you will have to buy both oversized pistons and rings and have your cylinders bored out to fit the pistons.

Basically friction will over time wear down the sides of the cylinder wall (and the sides of the piston). This wear is not even as they are hotter at the top and colder at the bottom, and the pistons slap forwards and backwards, not side to side.

Anyways, this is less common in newer bikes but it's possible to buy pistons in multiple sizes. Usually they are shipped at a stock size (totally random number here), lets call it 25mm diameter. You could then get pistons that are 25.02, 25.04 and possibly even 25.06 mm (once again TOTALLY random numbers).

As the cylinder wears (cylinder is just an iron pipe cast or pressed into the aluminum body) it loses it's circular shape and starts to allow combustion gases, or oil to slip past. Meaning you lose power and start to burn oil, both bad conditions.

So you would get the cylinder bored out to the next bigger size and then buy new pistons to match the cylinder. All of this would be done at a machine shop.


All of that was a really long way of saying:
1) you need to measure first, and diameter of the piston and the bore must be accurate to within 1/10000th of an inch
2) from there we can tell you what to do

Probably just take it to a shop.