Not my day :(

[no sub for cubes]

having recently put my kx together,and being told by the guy i bought it off the engine had been fully rebuilt i got it going and going and was pleased as punch....(for all of 2 weeks anyways)i had only run it up and down the road a couple of times as still finishing off bits to it..anyways last weekend started it up which it did fine ran it a couple of mins to then find it smoking bad  ......couple of questions on kxriders later and the symptons confirmed crank seals gone!!!grrrrrrr

so to cut a long story short started pulling it apart today,engines out now but on close inspection my very very low hours use piston has a gouge down the front exhaust side  ....now as far as i can see the cylinder looks ok no marks to correspond with the piston marks..i have included some pics and must admit it looks worse on the pics than it does in real but is it at all poss to lightly sand out this mark or is it time for another new piston 


                                         all advice much appreciated

[BDI]

The seizure was most likly caused by the main seal. When I was younger cleaned up pitons a few times and road the hell out those bikes. I would say yes you can clean it up I wouldn't recomend it but if you were to do it, I would but a nice criss cross on it like a freshly honed cylinder. I would also put fresh rings.

[cutting torch]

If most of that scuffing disappeared with a scotch-bright pad rub-down, and the ring grooves were good, I would reuse it.

I have a question, though.... What is the hole above the wrist pin bore for? Is it for pin oiling?

torch

[kaw rider]

Looks like a lite four corner seizure.

[martinfan30]

If most of that scuffing disappeared with a scotch-bright pad rub-down, and the ring grooves were good, I would reuse it.

I have a question, though.... What is the hole above the wrist pin bore for? Is it for pin oiling?

torch

Thats a good question. its not really a hole, just a small cavity that may be used to hold oil for the gudgeon pin as you suggest. Just my thought.

[no sub for cubes]

thanks for the replys guys,i did mean to ask what you guys thought caused the damage,the piston is only gouged on that one point.not really other signs of it seizing on the piston,that would make sense that it was caused by the main seals going..id prob be as well as to get the barrel honed? ,measured and buy another piston...just greaves me cos this one still looks brand new!except for the gauge and hotspot on the one exhaust side,the rings still look like new honestly only had the thing running for prob less than 30mins.. ,as i have no acurate gauges to measure the bore myself, can i get an idea from the old piston size? which reads std575ps TD in the centre and on the inlet side reads j4oc2 

[BDI]

If most of that scuffing disappeared with a scotch-bright pad rub-down, and the ring grooves were good, I would reuse it.

I have a question, though.... What is the hole above the wrist pin bore for? Is it for pin oiling?

torch

Thats a good question. its not really a hole, just a small cavity that may be used to hold oil for the gudgeon pin as you suggest. Just my thought.

It's just a hole, that's how wiseco machined the piston to get rid of some weight nothing more. They no longer do that to the pistons for the kx500.

[cbxracer30]

A great way I have found to prevent those ugly exhaust bridge scuff are 2 or 3 small holes through the piston right at the bridge. By small I meen like 3/32 shamfured on the outside with a large bit. Put the new piston on the rod {no rings} side cylinder back on and mark the exhaust bridges on the piston from the exhaust hole . I usually start the holes 1/4inch down from the rings and space them 1/4 inch apart. I'll try and post some pictures tomm.

[KXcam22]

I would suggest that you polish that one up and replace the rings.  I have run much worse.  I was rebuilding a friends YZ125 which had a center bridge and those holes stock (and the bucket of old pistons that came with the bike were all seized mildly over the holes anyway) -I got him some better oil.  I never required the holes on my K5. Cam.

[DoldGuy]

Did you let the motor "warm up" before putting a load on it? All forged pistons expand much quicker than the bore does and will seize ( cold seize ) generally on the exhaust side first / it kills me to hear someone start the bike & then rev the H... out of it.

[no sub for cubes]

your right cam I should try and get some use out of it...but ive spent time and money trying to put the thing together properly that id feel gutted inside knowing it wasnt 100%,doldGuy i hate seeing someone abusing a bike when its been run in!,so no when i got it running i diddnt wring its neck  ,just a little throttle let it cool down,bit more throttle and so forth,like i said its only been running at the most 45 mins on and off over 2 weeks,and rode up down the road 3 4 times....i`ll finish the rebuild and then decide bout the topend.....



                                      thanx again guys


                                               simon

[tharden]

I'd say someone all ready put sand paper on the piston in the effort to rebuild the engine.  If it was my bike, I would not be satisfied with that and buy a new piston.  Sure the one you have if in specs still will work, but knowing that it is damaged just would bug me.

[BDI]

Anytime I rebuild a two stroke I pressure test it. I test it before I take it appart and after I put it back together. This helps to diagnose why it blew up and helps to insure You didn't miss anything putting it back together. This is the reason I put case sealer on kx500 main seals now when I put them in.

[kaw rider]

Like Brian said, case sealer around your seal will help you out on pressue testing. You will be shocked sometimes were you find air leaks at.