black spot under piston and other top end ?'s

Mar 1, 2005
231
0
the piston i recently pulled from my cr60 has a black spot under the crown.i thought it might be burnt on oil but if i scrape at it with my fingernail it will come right off and underneath it looks fine.could it just be carbon under there?im hoping so since the piston looks great otherwise.i imagine since the crown hasn't even become completely covered in carbon and i can sill read the oversize number stamped onto it, that this top end has not been in the motor for very long.but i bought the motor used off that auction site so i dont really know how long its been in there.i would really like to reuse the piston if possible since i cant get this size for it anymore.also the cylinder walls have some rust colored stuff on them,my dad said it could be burnt on oil but im not sure. other than that it also looks great with no scuff marks at all.also when i put it back together do i have to use a torque wrench?reason being is at this point im kinda broke and we dont own one i remember year ago i rebuilt a motor with my dad not using one and cant remember any problems,just thought i'd ask.any input will be appreciated
 

njrealtor55

Member
Sep 22, 2005
76
0
burnt oil under the piston means the wrist pin is getting good lubrication. don't worry about it - I'd only worry if there was none.

you have to mic a piston to see the size to tell if it is in need of replacement. Personally, I'd just replace it - as you have it out now. Better to start with new parts on your newly acquired machine than guessing.

Always use new rings and gaskets when doing a top end.

the cylinder.. run a ball hone thru the cyl about 3 times (3 passes) and see if it helps. most likely it should clean it up just fine.

torque wrentch.... well, yes you should use one, but if you make sure to tighten everything - and then double check it being extra cautious... i'd say you could get by without a torque wrentch. I wouldn't reccomend it - but it can be done.

make sure to measure the ring end gap before installing the piston. a lot of people i know who rebuild 2 strokes always forget this step.

hope it works out for you. :)
 

cmack

Member
Sep 26, 2005
6
0
Burnt oil under the piston is typically a sign that you're running too lean and that the crown of your piston is getting too hot. The pre-mix should be burning off clean and you should NOT have a burnt spot under your piston!

Check out Eric Gorr's site... He has a piston guide under two-stroke technology for reference!
 
Mar 1, 2005
231
0
thanks for your replys,if the piston measures within spec and is otherwise ok can i just replace the rings and reuse it?reason being is i can no longer get a 1st over piston from honda which is the same size as this one.also,once i bore it to 2nd over and use that piston it will be like waiting till the end since the 2nd over i got from honda was the last one in the u.s. according to their inventory list.other than the spot the piston looks great with very low hr's i would think.also is it ok to run synthetic oil in the motor?i have been using the same oil i use in my 125: bel-ray H1R and gear saver.i just remember reading somewhere that the older motors may be better off with regular oil. just thought i'd ask
 
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