Should I replace my pistons?

herre

herre

New Member
Hi,
I'm working on my cb200 and had another question. Last week I opened up the entire engine. It ran ok but most of the gaskets were leaking so it seemed like a great time for the complete check up. Without thinking too much I just ordered a set of piston rings (but not pistons). After removing the pistons however there are some light scratches on them, and in the cilinders aswell. I can feel them a tiny tiny bit with my finger but they do not catch my nail when running over them and don't seem deep at all. My question is should I buy oversized pistons or is this normal and can i just replace the piston rings and use those pistons?

Haven't had the time to take a picture but it looks a bit like this:

http://farm6.static.flickr.com/5066/5612025477_0a20f2ac45_z.jpg
http://t2.gstatic.com/images?q=tbn:ANd9GcTOeNhPXUNEGPWwFlumbaPqDqJujGq14O52C4VyYlh9NIwpIqh8iQ1-ZucVgw

I have to say there is no real depth to the scratches. And the bike ran pretty good before taking it apart...
 
If there are any visible scratches, that you can feel with your finger, you should at least hone the cylinders before rebuilding. Clean the pistons well, and polish the tops as best you can. After honing, make sure you have removed the imperfections, file the new rings to properly fit, and you should be ok. Buuuuutttt... If you can afford an overbore with new internals, why not go for it? The 200 isnt exactly a screamer in stock trim anyhow. Any upgrade is good.
 
Always hone the cylinders with new rings - sounds like that's all you need to do. You can do it yourself with the right tool or just take the cylinders into your local small engine shop. Auto places usually can't hone smaller cylinders. Should be like $20-$30 to have them honed (a full bore/hone is usually $40-$50 per hole, so I'm guessing $10 / hole or so for just a hone).
 
This is general info: a cylinder has to be round for the rings to seal. What makes a naturally aspirated engine work the most is the vacuum that each cylinder can pull. The only real accurate way to measure a cylinder is with a dial bore gauge. If the cylinder is round and without taper, it's good. If it has some scratches but is round and straight, you can use a berry hone and put new rings in. If your piston is too loose, your overhaul will be short lived. It is possible to knurl a piston to close up some clearance but it has to be fitted for the right operating clearance. If you have good enough clearances to reuse the original pistons, you have to clean the piston ring grooves for the rings to have proper clearance. The side gap in a piston is just as important as the end gap in the cylinder. The cylinder has to be clean before assembly. If you've honed it, clean it until a white cloth shows no residue. If for some reason the ring groove in the piston is worn too much, the power level will be way done. You will have to decide whether the expense of the improvements is justified for the bike. Everything has a point of cost rational.
 
Allright much appreciated guys! I think I'll take it to an engine shop to get it honed. Reading this I'm thinking getting the cilinder honed and cleaning up the pistons + new rings might do the trick. Thanks!
 
<<getting the cilinder honed and cleaning up the pistons + new rings might do the trick.>>

I agree. I see no ridge at the top of the cylinder, so it is not all that worn. Have it honed as little as possible. Don't feel compelled to hone out every last trace of surface imperfections. Honing more make the bore larger and increases piston clearance. Very minor scratch imperfections will have little or no effect on the engine's performance and/or oil consumption. The rings will seat in just fine.

Was this engine ever run without a good air filter? That could be what caused what you see. Rule of thumb is that you can feel about half a thousandth with your finger nail. (Does that make it rule of finger?)

Have the piston clearance measured and compare with FSM specs for maximum clearance. Check ring end gap, and if it is beyond FSM maximum, then you should consider a re-bore. Alternatively, it is possible to get 5 thousandth over rings, and file the end gap to fit. (Requires a bit of skill.)
 
Actually it was ran with real filthy stock filters and then with some cheap sport filters. Once I bought the bike i replaced those with quality foam filters. Thanks for the tips! I'll make sure I don't get it honed too much and always use good filters.
 
Good filters are a must. I know it's popular to build a cafe racer with velocity stacks and a wire screen over them. That is OK for "bike night" or show purposes, but I would never ride a bike without good filters like K&N. I got caught in a sandstorm in Arizona once. You would not believe how much gritty dirt was in my K&N's.

In that vein, most people service K&N filters improperly. K&N says to clean them every 50,000 miles.

"But, but, but...they look dirty long before that!" you say.

K&N says that the dirt that accumulates on the filter media actually enhances their ability to filter. Also, some people ruin them by using compressed air. Using air to blow out the filters tears the delicate cotton fibers. They should be throw away if compressed air was ever used on them. K&N sells a cleaner, but I usually use some mineral spirits or kerosene followed by dishwashing detergent and hot water.
 
I'm using twin air foam filters. They should be about the same quality as K&N. Any tips on cleaning foam filters? I guess it's about the same but compressed air probably can't do as much damage on them?
Might look into getting some K&N's though!
 
Don't use compressed air on foam filter. Use the same method I described. Be sure to use filter oil.

I don't think that foam filters remove fine dust as well as K&N cotton filters.
 
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