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Bore Gauge Needed to Install New Piston?

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I have been trying to educate myself so that I can do the best job possible installing a Wiseco piston into my 04 KX250. It will be the first time I have done this, but I am not too worried about it. Have you all taken cylinder diameter measurements when doing the top end work? I was looking at cylinder bore gauges and it looks like a decent new one is ~$120 minimum. I am thinking it may not be all that necessary though because I will be able to see if the coating is looking good, and that should tell me something I think - that is if the coating is uniform and not pealing off anywhere it is probably reasonable to think the cylinder diameter is close to standard. Also, I will be able to take my new Wiseco rings, place them at different points in the cylinder and measure the ring end gap. My manual says standard ring end gap is 0.0098 - 0.0177" - and Service Limit is 0.030". If I can assume my new rings are the right size and don't need to be filed then I can get a sort of backdoor cylinder diameter measurement, thought only known within 0.007" or so. Fitting the piston and checking clearance may not be an option, but if it is feasible, then I can get another measurement that is related to cylinder diameter. I do have a good caliper, so I can measure outside diameter no problem. Have those of you that have done new top ends several times taken measurement of the cylinder diameter to see if it is within spec using a bore gauge? Do you think the above ideas will be helpful in getting a decent idea of cylinder diameter or not? Any experience you could share about this would be great. Thanks.

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I have been trying to educate myself so that I can do the best job possible installing a Wiseco piston into my 04 KX250. It will be the first time I have done this, but I am not too worried about it. Have you all taken cylinder diameter measurements when doing the top end work? I was looking at cylinder bore gauges and it looks like a decent new one is ~$120 minimum. I am thinking it may not be all that necessary though because I will be able to see if the coating is looking good, and that should tell me something I think - that is if the coating is uniform and not pealing off anywhere it is probably reasonable to think the cylinder diameter is close to standard. Also, I will be able to take my new Wiseco rings, place them at different points in the cylinder and measure the ring end gap. My manual says standard ring end gap is 0.0098 - 0.0177" - and Service Limit is 0.030". If I can assume my new rings are the right size and don't need to be filed then I can get a sort of backdoor cylinder diameter measurement, thought only known within 0.007" or so. Fitting the piston and checking clearance may not be an option, but if it is feasible, then I can get another measurement that is related to cylinder diameter. I do have a good caliper, so I can measure outside diameter no problem. Have those of you that have done new top ends several times taken measurement of the cylinder diameter to see if it is within spec using a bore gauge? Do you think the above ideas will be helpful in getting a decent idea of cylinder diameter or not? Any experience you could share about this would be great. Thanks.

Well, its always a good idea to check the work of a sleeve installed, or a new liner coated.

This way you can avoid future top end failures.

I recommend a good "Snap" gauge set. Bore mics. If you ride/race alot, and are doing top end jobs... It will pay for itself.

Plus a good set of digital vernier calipers to measure the inside mic's.

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Using a dial caliper to measure the inside edge of the cylinder won't be of much use. It is not in the wear area.

Without buying an inside micrometer, you might want to clean up the cylinder (search green Scotch-Brite pad) and take it to a machine shop. Look in the manual for the place inside the cylinder where the measurements are to be taken. Hopefully, you own a service manual.

You should be able to check the plating for uniformity. Beyond just looking at it, feel for any chipped edges, scratches, gouges, etc.

Since the ring is not the most accurate measuring device, I would not use it as such.

Congratulations on wanting to do it right, instead of half-assing it.

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DO NOT ASSUME your new rings will come with the spec end gap that your service manual specifies. Read the instructions that came with your piston/rings. They will say to measure the end gap and make sure there is at least .004" of end gap per inch of bore. So if you have a 3.0" bore, you need a minimum of .012" end gap. Because of manufacturing tolerances, it is not always possible for the rings to have the correct end gap right out of the package. Wiseco does this specifically because some people like tight gaps, and others need more. It is up to the engine builder to gap the rings, just like gapping a spark plug.:p

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