Finding a Pro X piston kit

[ATV_Chick08]

If the bore looks good and measures within spec it should not need to be honed.  Anytime a cylinder is honed some material is removed from the bore, making the piston to cylinder wall clearance a little looser.  If your air filter does it's job and you run a lot of oil in the fuel it is not uncommon to be able to replace the piston many times without boring the cylinder.  If you hone the cylinder each time you replace the piston you will have to bore it more frequently.

Pistons need to be replaced frequently to keep the clearances tight.  Keeping the clearances tight makes the bore last much longer and the engine will produce more power.  Some of the seasoned racers understand this but the average guys lets the clearances get too loose and damages the bore requiring boring the cylinder to the next oversize each time the piston is replaced.

Thanks for the reply, I appreciate the information.  I bought this cylinder new from Duncan in 1996, and then they rebulit the whole motor a few years ago. I need to do a better job of keeping track of the hours on this rebuild. I didn't realize that it was down on power until I had my 86 rebuilt and had something to compare it to. I don't know enough about it to decide whether or not to replace the piston without it needing honed or bored. I should have just sent it back to Loren to begin with.

I am also curious as to how often it is recommended to change the piston. It's not like I get to go out and ride for hours and hours. I only race, and even then I don't put a lot of hours on my motors.

[ATV_Chick08]

How often do you recommend changing the piston?  I had Eric Gorr port my cylinder and bore it.  He sent the piston with it it is a 66.25 wiesco. It is on an 85 atc.

If your using a 66.25 piston and plan on using that size again you might want to try to pick one up on Ebay because no one makes them any more. It's annoying that these web sites show that they have them, then you call and find out they aren't available. Wiseco shows it being available  to order, but when I called the guy acted like I should have know they don't make them anymore.

[The_Steve_Man]

Thanks. I had it done yr and half ago and haven't put a lot time on it. I only rode it some last year. I will pick up another one.

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[Jerry Hall]

I am also curious as to how often it is recommended to change the piston. It's not like I get to go out and ride for hours and hours. I only race, and even then I don't put a lot of hours on my motors.
/QUOTE]

Pistons and rings will run much longer than the times I have listed below before your piston will be fatigued enough to fail.  My recommended times below are for the ones racing or want their engines to always be running within 98 to 100% of their peak potential.  

Most 250 sized single cylinder engines using OEM cast pistons that are producing around 45 RWHP will need the ring/rings replaced about every 3 to 6 hours.  OEM cast pistons should be changed every other ring/rings set change to keep the clearances tight and piston fatigue in the safe zone.  I see some 45 hp engines that need their rings replaced every hour or less because the engine builder did not know how to properly shape the exhaust port and or put the correct port chamfers on all of the ports.

On 250 size single cylinder engines producing 50 to 60 RWHP the pistons and rings need to be replaced more frequently than the times mentioned above because of the wider ports and higher RPMs that are associated with engines in this power range.  

For 250 size engines using forged pistons the above time on rings is about the same as when using cast pistons.  The forged pistons tend to collapse and get loose much more quickly than cast pistons and may need the piston and ring/rings changed at each scheduled ring change.  

The 310 and up big bore kits usually use forged pistons and will typically collapse even more quickly than the 250 sized forged pistons.  Forged pistons are much less prone to catastrophic piston failures than cast pistons when they get real loose.  Cast pistons require the piston clearances to be kept tighter than forged pistons to avoid high fatigue levels which eventually result in cracked pistons and eventually catastrophic piston failure.

My experience has been that cast pistons will usually produce a little higher power average than the forged pistons and can be subject to longer periods of full throttle without the piston crowns overheating.  

The above recommended piston and ring changes are based on .....no dirt is getting past the air filter and a lot of oil is being used in the fuel.  

The above results were patterns we observed over the years of frequently dynoed engines from many racers as their engines accumulated time from day to day and week to week racing.  If you look at the owners manual of any of the CR, KX, RM, or YZ 250s built during the last 15 to 20 years they were produced,  you will see that the factory engineers found similar recommended piston and ring changes due to ware/power losses for their 40 to 45 HP motocross engines.

[udontknowme]

cast pistons work fine until they go tits up. then you have a big mess  http://www.thumpertalk.com/topic/1072142-seized-engine-in-my-01-cr250-today-where-to-begin-pics-coming/