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1999 KX250 Piston choice


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First off, I know that there are multiple posts on the cast/forged piston debate. I keep jumping between being firmly decided on one to the next.

Here's my situation. This is the first bike I've ever owned (1999 KX250) and have ridden it for 2 seasons now. I use it for mostly trail riding with a few visits to the local track to display how much of a noob I really am. I noticed piston scoring through the exhaust port and not knowing the history of the bike I opened up my can of worms.

I am at the point where my head was sent out and re-plated (due to small chips in the plating) and I found play in the crank bearing so the case is now split in half. I ordered the Hot Rods bottom end kit (CBK0055) and swapped out my input shaft and output shaft first gear with a '97 transmission as found in other threads here on TT. The crank was a wiseco, but rebuilding it was not an option anyways as some previous owner decied that the crankshaft threads on the stator side were too long and cut off all but a few threads 🤦.

So now that leaves me with the decision: What piston to use? Wiseco? Vertex? Pro-X? OEM?

I know that many people like the wiseco/forged pistons for longevity. People stated that the forged pistons didn't need as much clearance as in the past, etc but these posts were back from early 2010's. I am leaning towards Wiseco as I assume if anything forged has become even more versatile but my thoughts keep thinking "How likely is it to cold seize? Is it more likely to score my new cylinder than a cast?" and "If more clearance is needed for forged should I be running a cast in my recently plated (stock size) cylinder?"

I just tend to over think things sometimes but the last question is the one that is bugging me. Thanks for any opinions/help.

Also, is the case sealant just like a high quality silicone? I hear people mentioning HondaBond and YamaBond is good for this. Is this the same stuff you use on for grips?

Edited by troymac1ure
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You can go to auto parts store and get Permatex "Moto Seal" same as yamabond, hondabond. But has to be the moto seal. Don't use anything but those 3 or kawibond ofcourse 👍 lightly cover one half.  I always do left side.  put everything in right side. Goodluck 

Edited by Motox367
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Wiseco is still a good choice for pistons.  Just ride easy until the radiator is good and warm to the touch.  Same thing you should be doing with a cast piston.

Cast pistons will not last as long and can crack a skirt off it you run them too long.  In practice you would have to be ignoring a hell of a lot of noise to run it long enough to blow up.  They rattle and slap around for a while before they come apart.

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10 hours ago, troymac1ure said:

"If more clearance is needed for forged should I be running a cast in my recently plated (stock size) cylinder?"

You can run the piston in the stock bore. No modifications to clearances are needed.

You just replated the cylinder? If so, they usually size it to piston on hand. Did they provide a piston with your replate?

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3 hours ago, MatthewMCRepair said:

You can run the piston in the stock bore. No modifications to clearances are needed.

You just replated the cylinder? If so, they usually size it to piston on hand. Did they provide a piston with your replate?

They didn't. This was my first time dealing with any in-depth dirt bike related stuff. I asked if I should purchase and send a piston with my cylinder but they stated I should wait until they inspected it to see if it was already bored or needed to be bored first. They were great to deal with and had a quick turn around time, but just told me that the cylinder was plated to stock size. I didn't get any paperwork with measurements or the such. I'm an automotive technician so I know my way around engines, but I'm new to small engine repair. Our shop doesn't have bore gauges small enough but I think I can get my hands on some. First I think I'll see if they can email me the paperwork as I'm sure they must have done the measurements for the replate. 

BTW your videos are great! 

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So I received an email stating "So your finished bore size was 66.40mm. We do offer Wiseco & ProX pistons if you’re looking for after market." I would like to assume that this means the bore is perfectly spec'd. Being that they are 1 of 2 places in all of Canada that do replating I'm sure they're good at what they do. Do you think I should still try to get my hands on a bore gauge to double check? I see Wiseco has one set size for that bore, but ProX makes 66.35-66.37 pistons. I found a post stating to have 4.5-5/1000" which converts to 0.11-0.12mm correct? In this case if the bore is 66.4 and I subtract 0.11mm = 66.29mm that means that even a 66.35mm is too large? Sorry for all the questions, just trying to work it all out in my head. Thanks.

EDIT: The service manual states that clearance should be 0.059 .... 0.069 mm so that falls closer. Still 0.009mm over for the ProX 01.4322.B piston.

Edited by troymac1ure
Found correct specs.
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1 hour ago, troymac1ure said:

So I received an email stating "So your finished bore size was 66.40mm. We do offer Wiseco & ProX pistons if you’re looking for after market." I would like to assume that this means the bore is perfectly spec'd. Being that they are 1 of 2 places in all of Canada that do replating I'm sure they're good at what they do. Do you think I should still try to get my hands on a bore gauge to double check? I see Wiseco has one set size for that bore, but ProX makes 66.35-66.37 pistons. I found a post stating to have 4.5-5/1000" which converts to 0.11-0.12mm correct? In this case if the bore is 66.4 and I subtract 0.11mm = 66.29mm that means that even a 66.35mm is too large? Sorry for all the questions, just trying to work it all out in my head. Thanks.

EDIT: The service manual states that clearance should be 0.059 .... 0.069 mm so that falls closer. Still 0.009mm over for the ProX 01.4322.B piston.

You need to measure your bore. Just because they say "stock size", there are tolerances involved.

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On 6/22/2017 at 0:00 PM, turbo dan said:

I would buy a stock size wiseco and run it, like I've been doing for 15 years.

As far as I can find Wiseco only makes one piston for the 66.4 bore, which is the 617M06640 (excluding the single ring and racer's choice piston). So does that mean that the exact cylinder specs are useless anyways? If so, is this due to the fact that it is forged and not cast? 

I can get tools to get precise measurements of the cylinder, but it seems that if I go the wiseco route it really doesn't matter anyways. @turbo dan Were you suggesting to forget doing the precision measurements beforehand and just grab a wiseco? Like I said, I'm new to the small engine world so suggestions from those who have rebuilt their share of engines is very appreciated. 

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If you're into maximum precision you can measure the bore and order an A/B/C/D piston to try to maintain optimum clearances.  In practice a hundredth of a millimeter or two makes no significant difference, especially for a forged piston which will run slightly looser clearances anyway due to the nature of forged aluminum.

Plated bores can be effectively inspected visually.  If the plating is worn enough to allow excessive piston to cylinder clearance it will show it.  If it is worn it needs a replate, not an oversized piston.  If its not substantially worn it is serviceable with a standard size piston.

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So now I'm even more leery. I picked up an inexpensive set of telescoping gauges and borrowed a nice outside micrometer from work (Which reads the 50mm spacer properly). After many measurements about 30mm down in the bore (as stated in the manual) and at the top I get 66.37mm. So now my original concern that even a stock piston / ProX didn't seem to have enough clearance for a 66.4mm bore and now it's even smaller. I just double checked with a digital caliper as well and it reads 66.3mm (obviously the precision isn't there but it only hit 66.3mm with some pressure put on the caliper arms, otherwise 66.2mm. Am I missing something? Are 66.35mm pistons actually 66.35mm or meant for a 66.35mm bore? 0.02mm of clearance seems very questionable to me.

My co-worker and I both sent our cylinders to get replated at the same time to the same place but he's having to wait for finances. His is a '98 KX250 I think I may try to measure his cylinder and see what readings I get in comparison. I'm just concerned that with my new replate things may be too tight and seize more than too loose.

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It's been sitting unused in our shop for probably 10+ years (which is probably the last time I used one). It contains a 50mm @20C check/calibration(?) rod which measures @ 50.005mm. I checked the cylinder at least 6 times and came up with 66.37mm five of those and 66.38mm the other time. This is the model micrometer I am using and telescoping gauges from Princess Auto. The service manual states that the bore should be 66.40mm-66.415mm with service limit of 66.48mm. I am hoping to check the other same cylinder that was also replated to see if I get different readings from it (that fall within spec). If the other cylinder reads 66.40+ then I know it's not my measuring and I'll be contacting the company that did the replate as I assume I may have issues running a piston that tight. Any other ideas gladly welcomed. Also, thanks @MatthewMCRepairfor urging me to check my actual bore size.

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