Hole in my piston!

[hoonda]

Hi guys
little bit of background info;
i have a old early 70s Honda cb125 which has been a slow project for about two years.
http://s20.postimage.org/utredtfkd/bike.jpg

It was a bucket of bits when i got it and have purchased good coil, good rectifier, and a bare "running" motor (un inspected) off trademe.
I put it all together with a new battery and only the bare minimum of wiring (stator to rectifier battery coil and points) and got a good spark, but compression seemed a bit on the low side so reluctantly i pulled the motor down and found the cause

THIS: http://s20.postimage.org/5wo0wwr2l/photo_2.jpg
Down side of buying a un inspected motor other than that the rest of the engine looks very clean inside and out

the questions i have are.... what could have caused this so i can make sure it wont happen to a new piston, annnd anyone know where i can get a reasonably priced standard size piston kit appart from honda??

thanks in advance guys this bikes been doing my head in hopefully this is the last hickup!!

[ducatilover]

You can run a modified CB750 piston in 'em can't you? Takes it to around 145 or 148cc
I looked in to it years ago for a mate, but can't remember off the top of my head.

They're a 56mm bore, and you can take them out quite far, but the mouth on the case needs to be modified if you do so

I suppose that's not much help

[hoonda]

yea have heard that just today actually would rather not bore it out if at all possible hoping just to put a new standard in there? any cons to this?

[ducatilover]

If the bore isn't munted and worn, no real issue. You can still buy OEM pistons, you can buy aftermarket, and you can buy Chinese made ones
I'll be buying some bits in soon-ish and could source one for you

[hoonda]

yea bore is still nice and smooth, cool mate thatd be great? any idea on cost? not after anything to special but will need rings and pin aswell

[ducatilover]

It's a single cyl isn't it?

An OEM NOS one should be around the $50USD mark, or a little less. Chinese ones (probably Loncin) are about $15US...
Full gasket kit should be less than $30 US, so a head gasket by itself should be mere pennies

[SS90]

the questions i have are.... what could have caused this so i can make sure it wont happen to a new piston, annnd anyone know where i can get a reasonably priced standard size piston kit appart from honda??

thanks in advance guys this bikes been doing my head in hopefully this is the last hickup!!

What sprk plug where your using when this happened?, by this I mean, what are the numbers on the side of the insulator (white bit), it looks to me like the spark plug used was too hot a heat range.

[The Reibz]

looks to me like the spark plug used was too hot a heat range

Either that or the engine was running super lean

[sinfull]

Either that or the engine was running super lean

I thought that too, but that fucker aint melted it looks like someone's whacked it with a big center punch !
Bet the plug has marks on zee tip of him !

Someone put a longreach plug in, where it not spose to be maybe and when they reallised, perhaps replaced the plug and sold it off quick !

[sinfull]

Hi guys
little bit of background info;
i have a old early 70s Honda cb125 which has been a slow project for about two years.
http://s20.postimage.org/utredtfkd/bike.jpg

It was a bucket of bits when i got it and have purchased good coil, good rectifier, and a bare "running" motor (un inspected) off trademe.
I put it all together with a new battery and only the bare minimum of wiring (stator to rectifier battery coil and points) and got a good spark, but compression seemed a bit on the low side so reluctantly i pulled the motor down and found the cause

THIS: http://s20.postimage.org/5wo0wwr2l/photo_2.jpg
Down side of buying a un inspected motor other than that the rest of the engine looks very clean inside and out

the questions i have are.... what could have caused this so i can make sure it wont happen to a new piston, annnd anyone know where i can get a reasonably priced standard size piston kit appart from honda??

thanks in advance guys this bikes been doing my head in hopefully this is the last hickup!!

Nead it !!!! hehe or liquid metal and up the compression a tad lol Wonder how that shit handles heat haha fuck it would be a quick fix if it could !
But i see there is a hunk of alloy missing from the centre there so best ya find that too !

Bugga !

[Eddieb]

Try the Honda shop in Te Aroha. They are well known for having/getting parts for old Honda's.

[hoonda]

It's a single cyl isn't it?

An OEM NOS one should be around the $50USD mark, or a little less. Chinese ones (probably Loncin) are about $15US...
Full gasket kit should be less than $30 US, so a head gasket by itself should be mere pennies

cool man sounds good not fussed on which one, loncin sounds good if your getting some in chuck one in there for me and itd be worth doing a full gasket kit i reckon and yea single cyl, manual says piston O.D 56.46mm or something will check.

yea never had this engine running or apart until yesterday, definitely looks (and feels) like its been exsposed to some excessive heat,
bought it of an old bucket racer who had like 20 of these old Hondas lying around

[spanner spinner]

this is classic detonation damage due to either to lean a mixture or to much spark advance, both cause the piston crown to heat up making it go soft but the clean hole in the centre is the give away. The detonation waves cause the cracking as the pressure wave hits the top of the piston with the same force as a large hammer, and as the piston is already hot it's easy for the pressure wave to punch it in. Check the ignition timing and also the advance retard unit as these can jam at full advance if the springs have gone soft or if the shaft has wear. having seen lots of motors that have digested foreign objects the holes in the pistons aren't as clean as this one and over length plugs leave a prefect impression of the end of the spark plug on the piston crown.