Tough little Honda

[sidwyz]

So on the weekend I was riding Shona's bike,
A cb100, after starting it up it was smoking.

I went out anyway and rode it hard for a couple of practice sessions and 4 races and it didn't miss a beat, in fact it has never gone better, apart from smoking a lot.

I pulled it apart today and this is what I found.



so after all that it didnt stop or seize or even run bad, the top ring is still intact and free so compression was still good.

Was fun to come home and tell wifey i blew up her bike.

[TZ350]

Good pictures, and plenty of shrapnel there.

Attachment 246996

Does your damaged piston look like picture 1.1.2 in the PDF, it does to me.

This is my take on it, the damaged piston is starting to develop the classic melted piston look in the piston pin area from debris being shaken up and down. Eroding the piston and marking the cylinder at the end of the pin travel as they slam to a stop at each end of the stroke. The clip and debris were not able to escape the piston cutout and were held there and shaken up like they were in a cocktail shaker.


Attachment 246999
This all started with the piston pin clip coming out. The clip was trapped in the piston cutaway area and couldn’t escape or become trapped in a transfer port like it might in a 2-Stroke. As it was shaken violently up and down it eventually broke up the oil control land and ring releasing debris that marked the bore, the marks are both sides as the debris can travel to the other side of the piston through the hollow piston pin. The piston pin did not leave the classic tramlines, maybe there was not enough time or its ends just did not dig into the bore.

Attachment 247002 TDC BDAttachment 247003C

The shrapnel damage evidence, is in the symmetry of the markings on the cylinder wall matching the piston cut outs, on both sides and at TDC and BDC. Curved at TDC matching the pin cutout in the piston and straight at BDC matching the straight bottom of the piston cutout. And finishing at the exact extremes of the piston cutouts travel. I expect if you put the piston in the bore the damage will exactly match the cutouts at TDC and BDC.

The clip may not have been fitted properly or was being hammered by the piston pin due to a miss alignment problem and ejected, further investigation of the damaged piston can reveal which.

Attachment 247000 Attachment 247001

If there is a miss alignment problem it will show in a diagonal wear pattern on the damaged piston. Typically the top on one side and the bottom on the other side touches the bore and there is diagonal wear lines between them on the thrust sides of the piston skirt. With such a low mileage it may be hard to see but if its there, there should be some indication.

Someone else had the same problem with their Vincint and was curious about the cause, Bucket racer posted quite a well researched analysis of the failure, his posts were complete with references to proper piston failure manuals for anyone interested in such things. There was quite a thread going about it at the time.

[bucketracer]

Attachment 246087

Gudgeon pin damage seen when a circlip has been left out or come right out.


Attachment 246086 Attachment 246085

For what its worth, my guess is that a tang has broken off a circlip possibly by hammering from the pin due to a bent rod.

See page 10 http://www.mlcmotorfactors.co.uk/Tro...s/PistonTT.pdf

The damage is caused by the tang rattling around like a jack hammer at the ends of the stroke. The damage is both sides as the tang can rattle its way back and forth through the hollow little end pin. And the damage is wider than the gudgeon because the piston is relieved on the sides. Debris rattling around like this typically leave the piston pin hole looking melted.

Links to PDF's on Piston Damage.
http://www.boosttown.com/engine/piston_damage.pdf
http://www.ms-motor-service.es/ximag..._leseprobe.pdf

The piston typically looking melted is the giveaway clue to whats happened. Here is a post with some links to real Industry references on piston failure.

[sidwyz]

Thanks for all that great info, will be some good reading.

After a quick look it looks like circlip related damage (according to your links), but my circlips were still in place and there is no witness marks on the pin, the pin was hard to get out but that may be due to the damage around the circlip groove.

Who knows?
Will look further and try to learn from it.

This motor does look like it has never been apart though.

[bucketracer]

I would be very interested in seeing a photo of the cir-clips, although it does not have to be a cir-clip just some sort of debris. I have a morbid fascination with engine death.

[SHELRACING]

............. Was fun to come home and tell wifey i blew up her bike.

Been there too had too tell gma I'd blown her motor and back tyre doing burnouts

[sidwyz]

I would be very interested in seeing a photo of the cir-clips, although it does not have to be a cir-clip just some sort of debris. I have a morbid fascination with engine death.

I must of been asleep when I pulled the clip out because



Yeah I like to look at broken engines too not sure why, it interests me a lot, just not when I have to pay the bill!!

[SHELRACING]

I must of been asleep when I pulled the clip out because



Yeah I like to look at broken engines too not sure why, it interests me a lot, just not when I have to pay the bill!!

Me too, there's some fascination in ' When things go bad ' perhaps a new thread is in order. I bet the FXR riders have a few pics

[speedpro]

I bet the FXR riders have a few pics

Not just them