when do you replace pistons?

[smithll]

Hey, I am new at posting so please be patient.

So last time I rode my xs650 the engine was making a clacking noise, I figured it was the timing chain and tore down the engine today to look and see what is wrong?

I took the head off and the timing chain and adjuster look OK to me, so I and looked at the head.

The left side looks oily with lots of carbon build-up and the cylinder bore and piston look pretty good.

The right side valves look better, not oily and not much carbon build-up but the cylinder bore and the piston look scuffed to me, no big gouges, but there is some marks in the cylinder bore and the piston skirt does have some wear. I do expect some wear after all it's 30 years old.

OK I"m trying to get to the point! What I really want to know is should I replace the pistons and rings? or do I just need new rings and a cylinder hone? How to access how much wear requires new rings and a hone job or new pistons and rings? Thanks to all of you who respond!
LARS

 

[shotgunjoe]

To determine if pistons need replaced, you will have to measure the cylinder with a dial bore gauge. However, the fact you are asking this question leads me to believe you may not possess the skill to properly use one... better left to a professional. (not being a smartass, or a jerk, just being honest. These measurements are critical)

If the rings are broken or damaged they need replaced, if they are removed from the piston, might as well replace. Do you need oversized rings? Depends on what the bore measurements are, if they are within spec stock rings and pistons can be used. If they are out of spec, the cylinders will need boring and oversized rings/pistons are necessary.

 

[retiredgentleman]

Welcome to the group;

When you take an engine apart, you need to measure the components to see if they are in spec or not. That's the only way to know what needs replacement.

The Yamaha Service Manual is where you find the specs:

www.biker.net is one site but there are others.

 

[grizld1]

Re. dial bore gauges, using them takes skill, as shotgunjoe says, and it also takes money. The cost only starts with the gauge itself; to be sure of accuracy, the gauge needs to be zeroed with a setting circle that's close to the range you're inspecting for. If you're buying decent new tools you can maybe get away with spending $350 for what you'd need to mike out the stock bore with reasonable accuracy. Someone who's really, really good at it (and that would not be me!) can measure to a tolerance of half a thou' with a cheap telescoping gauge and a micrometer. That's too wide for my taste.

 

[jammerj]

Like the previous posters said, you need to take measurements. If the piston & bore measure out within specs, then usually a light hone won't remove too much, & you'll still be within the recommended spec. However, if the piston is scuffed (the bore can be repaired, the piston can't) it will probably need to be replaced. I'm saying this generally, without seeing it it's impossible to say for sure.

 

[Punkskalar]

Don't jump the gun, you said the tensioner looked good, but what about the front chain guide? Does it have the plastic on it still, or is it all aluminum?

 

[smithll]

Thanks to all who replied.

OK, I think I'll take the pistons cylinder and valves to a machine shop and have them measure the parts and see just how worn they are.

The cam chain tensioner still has it's plastic, but it probably is 30 years old, can just the plastic be replaced?

I don't want to split the cases as the bottom end seems OK, But I do want to freshen up the top, so I may have a few more questions a I progress.
THANKS GUYS

Lars

 

[hotdog]

Lars,

Both the cam chain tensioners can be changed out without splitting the bottom end.

Cheers.