Routine tear down

[theforeman]

had a bit of time today and the bikes done 15hours since i did the top end so thought nows a good time to see how it all looks popped the head off to see this:no::facepalm:
264944264943
any clues as to why? is the head salvageable?

[Asher]

Is that a small piece of piston missing from the just below the ring? looks to be the same size as all those marks

[slofox]

Is that a small piece of piston missing from the just below the ring? looks to be the same size as all those marks

Either that or it's got worms...:sick:

I'd be dubious about fixing it but then I'm an obsessive-compulsive bastard at the best of times.

[carburator]

Im probably going to get flammed, but ive put worse looking heads back on a bike before.
any reasonable engineering firm could machine it for you as long as the orginal CC is retained.

there is a interesting mark on the transfer port of your piston, so something has been sucked in
and done a bit of a dance around the chamber before being spat out.

id be having a fish around in the crankcase with a magnet see what else comes out..

[CRF119]

Ive got two guesses.

Guess one: Last time it was rebuilt somebody left something in the crank case that was not meant to be there. It would have to be steel. The reason i say this and not a bearing failure is because the marks have been there a dam long time and if it was a bearing that long ago the bottom end would be rooted by now.

Guess two is a screw has come out of the reed block? There are normally screws holding the reeds down.

Take the sharp edges off those marks on the head and run it again. Sharp edges cause detonation. Throw piston away and inspect bottom end for play. If there is play replace anything worn. If there is no play id risk flushing the case with a kerro gun which clears 99% of shit out then instal new piston and cross fingers you never have a problem again. Also inspect reeds and intake for any chance of stuff coming from there.

Good luck. Ive seen worse. Mates KX125 piston destroyed its self. We just split the cases and flushed with kero gun. (Petrol 2 stroke oil mix 10:1) New piston in and it runs better then ever.

[Akzle]

you f*ing what.?!

is this the bike listed in your stats?

F*ing jeebus.

mos def has the looks of a screw.. grubscrew maybe. although the top of the piston looks like more raised than indented?

fuckit. file it all as flat as you can, rub it over down to 800grit. slap it back together. yeehaw.

[Stylo]

Ive got two guesses.

Guess one: Last time it was rebuilt somebody left something in the crank case that was not meant to be there. It would have to be steel. The reason i say this and not a bearing failure is because the marks have been there a dam long time and if it was a bearing that long ago the bottom end would be rooted by now.

Guess two is a screw has come out of the reed block? There are normally screws holding the reeds down.

Take the sharp edges off those marks on the head and run it again. Sharp edges cause detonation. Throw piston away and inspect bottom end for play. If there is play replace anything worn. If there is no play id risk flushing the case with a kerro gun which clears 99% of shit out then instal new piston and cross fingers you never have a problem again. Also inspect reeds and intake for any chance of stuff coming from there.

Good luck. Ive seen worse. Mates KX125 piston destroyed its self. We just split the cases and flushed with kero gun. (Petrol 2 stroke oil mix 10:1) New piston in and it runs better then ever.

Agree.

You need a smaller version of this mate for a start :-)



264956

[schrodingers cat]

New piston and ring
Weld head, machine.
Carry on

[Pornstar]

what kinda piston did ya put in? could it be the ring locate pin let go? anyway, new piston def(original honda), new head if you want, but......na. no wurrys. just make sure the little bit is gone, cant be a screw, no T shape.

[mossy1200]

spark plug tip?

[theforeman]

the top end was rebuilt with all Honda bits, i will get in tomorrow and give the crank case a good clean out see if anything comes out of it, the bottom end was rebuilt just before i bought it probably done about 40 hours

[cheese]

I'd get a head off ebay. Should be cheap as. Def a new piston.

Looks kind of like a screw to me. Check the reed block.

[F5 Dave]

Mossy is onto it, its the sparkplug tip. Look at the indentations, rectangular like the end of a tip. This can happen & some bikes specify an expensive plug to lessen chance mid 90s RMs, not sure of later.

Hmm, but you rebuilt it? You'd have noticed the bike stoppnig & plug missing the bit. . .

Either way I'd give it to an engineer to skim off the worst of it. that will lower the compression & increase the squish so skim a bit off the flat surface to bring it back, but say a bit more than 1/2 the distance, so if you'd skimed 1mm from the squish area take .7 off the sealing surface, this should give you a similar squish & compression without going crazy.

Welding isn't a good option, casting metal will burn all its zinc & go porous unless you are super good & lucky.


. . .If you wanted to be sillly, what I'd do is skim it almost all out, then measure the squish & skim it till its 0.6mm from piston at TDC & then measure the volume, work out what type of gas you will religiously use & the max com you can run with that & then machine enough out to match that, but not remove the squish area on an MX bike, but instead into the top of the head (leaving the spark plug where it is of course, you could even toroid it a bit. Either way this would take several cuts & a heap of measurements, so the first way is an easy compromise that shouldn't give you any trouble.

[cheese]

http://www.ebay.com/itm/2004-Honda-Cr125-Top-End-Head-Engine-Motor-Cr125-04-/160820144554?pt=Motorcycles_Parts_Accessories&hash=item2571a0a9aa&vxp=mtr#ht_2798wt_1037

[F5 Dave]

you make a compelling case, but what is it with USPS shipping calculator on ebay presently? Seems shipping costs of something small that used to be $12 is now, well like $42.

So $65 buy now, $42 shipping, convert to NZ, 3 weeks wait.

Hmm, no I'd machine it.

[CRF119]

Mossy is onto it, its the sparkplug tip. Look at the indentations, rectangular like the end of a tip. This can happen & some bikes specify an expensive plug to lessen chance mid 90s RMs, not sure of later.

This was my first thought but because the bike never had a problem and this was just an inspection i did not mention it. Also bike would have stopped plugged removed and problem found. You might have gotten a few dents not as many as seen here.

[ktm84mxc]

Yep I'd go for a spark plug tip as it's square , you can file down the high points and just fit a new top end with out an noticeable power loss, it all depends on your budget.
My 84 KTM has a similar pattern in its head from a roller bearing pin coming loose never been an issue , a mates PE175x tossed a piston circlip did the same thing still ran sweet.

[pete-blen]

Looks like the ring lowcating dow has come out of the piston..
Yer do realise it could have migrated though the transfur ports
to the bottom end... whats it like in the crank case...?

[barty5]

seen this a number of times and it has always been the big end its small bits of the needle rollers doing the damage until it completely fails. this is is as long as others stated the end of your s/plug hasn't fallen off.

[theforeman]

Was back in the shed today checked over the bottom end , no play in bearings or any debris came out when I flushed it, tidied up the head looks good, checked over the Carb and reeds nothing missing, I did replace a plug a few months ago on a ride didn't take much notice of it but that's what I'm guessing it was, new piston here tomorrow assemble and away we go, thanks guys for your input:niceone:

[F5 Dave]

if you got paranoid one could use a good torch & rotate the thrust washers on the big end with a thin object. You would see some damage to the washers if anything had passed between them & the rod from the BE bearing.