Topic: Broken piston after 50miles CB 750 K2

[dave500]

if an engine is pinging at those rpms at light throttle or cruise something is way wrong,maybe way too advanced ignition?,maybe way too high compression?or both?,you can expect a slight ping at those rpms if you say load(open the throttle) the motor uphill or too tall a gear selection?

further to pewes post if you cam is stock high comp pistons may be too much,with a hotter cam you lose some compression because of the valve overlap,thats why youll find higher compression pistons in hot motors.

how have you set your ignition timing?its best done with a strobe light,also check the advance mechanism is working,with high compression you may need to restrict the curve to limit is range but start with a higher initial setting.

[Ellz10]

Subscribed for the outcome.

[Stev-o]

the engine ran perfectly at slow RPM 900-1000
but I hear the ping in 3500-5000RPM and don't know what is this

Is there a reason you won’t post a picture of the spark plugs and cylinder head?

Thinking he is not currently in possession of them, may be at the shop?

Also keep in mind that English is his second language as he is in Poland.

My guess at this point is low octane fuel run with a high compression engine with timing off.

[PeWe]

Bores might be damaged.
Cruzinimage F1 pistons +1.0mm (62mm) might be possible if total piston-bore clearances are under 0.025mm.
https://www.cruzinimage.net/2017/08/17/77-78-honda-cb750k-k7-k8-1-0mm-over-size-pistons-set-62-00mm/

Safest can be to go 836.
Cheap cruzinimage for stock cam.  This with head gasket included.
https://www.cruzinimage.net/2017/08/25/69-76-honda-cb750k-836cc-65mm-bigbore-pistons-kit-w-head-gasket/

There is a good thread about these on the forum. http://forums.sohc4.net/index.php/topic,89779.0.html

[Tracksnblades1]

the engine ran perfectly at slow RPM 900-1000
but I hear the ping in 3500-5000RPM and don't know what is this

Is there a reason you won’t post a picture of the spark plugs and cylinder head?

Thinking he is not currently in possession of them, may be at the shop?

Also keep in mind that English is his second language as he is in Poland.

My guess at this point is low octane fuel run with a high compression engine with timing off.

Yes Sir,

I did my research and realized as I read the possible language struggles and possibly a barrier especially with all the American misuses of the English language.

And I know his machine shop is probably like our machine shops and has taken a defensive posture. You do read as 18 post all the spark plugs have the same color and the mixtures are good…?

Just wanting our friend in Poland to know #3 may not be fixed just with a flat top piston..and could possibly reoccur without finding the real cause.

Like some have posted 1&4 tolerated the same without damage as far as we know as pictured. Without the “rest of the story it’s like Dave500 inferences conclude it could be a combination, etc, etc.  looking at the head would indicate whether it has stock chambers or whether they’ve been opened chambered modified. 

Carry On..😇

[Geoff Hastings]

the engine ran perfectly at slow RPM 900-1000
but I hear the ping in 3500-5000RPM and don't know what is this

Low octane as others have mentioned?

While I agree that low octane will cause detonation but, this engine has the same octane for the other three cylinders so I would discount that theory.

https://www.quora.com/What-happens-if-you-use-low-octane-fuel-in-engines-with-a-high-compression-ratio

[Stev-o]

the engine ran perfectly at slow RPM 900-1000
but I hear the ping in 3500-5000RPM and don't know what is this

Is there a reason you won’t post a picture of the spark plugs and cylinder head?

Thinking he is not currently in possession of them, may be at the shop?

Also keep in mind that English is his second language as he is in Poland.

My guess at this point is low octane fuel run with a high compression engine with timing off.

Like some have posted 1&4 tolerated the same without damage as far as we know as pictured.

With #1&4 on the outside of the cylinder bank with more airflow cooling and slightly cooler temps, could this have been part of the difference?

[Little_Phil]

the engine ran perfectly at slow RPM 900-1000
but I hear the ping in 3500-5000RPM and don't know what is this

Low octane as others have mentioned?

While I agree that low octane will cause detonation but, this engine has the same octane for the other three cylinders so I would discount that theory.

https://www.quora.com/What-happens-if-you-use-low-octane-fuel-in-engines-with-a-high-compression-ratio

I had a 550K which I took apart with serious signs of detonation just on cylinder 4.

[madmtnmotors]

the engine ran perfectly at slow RPM 900-1000
but I hear the ping in 3500-5000RPM and don't know what is this

Low octane as others have mentioned?

https://www.quora.com/What-happens-if-you-use-low-octane-fuel-in-engines-with-a-high-compression-ratio

While I agree that low octane will cause detonation but, this engine has the same octane for the other three cylinders so I would discount that theory.

All cylinders could have very well been on the verge of detonation induced meltdown. This occurred after only 50 miles and if it could have been driven much further I'll wager the other pistons would have quickly followed suit. Variations in the air/fuel mixture between cylinders could have led to the early failure of piston number three if it were running lean (or rich? Not sure how A/F affects detonation) when compared to the other cylinders, unless of course the air/fuel ratios were carefully calibrated using O2 sensors on each cylinder and the carbs were adjusted accordingly.

[olasek18_18@o2.pl]

Gentlemen, as soon as I am in the garage, I will take pictures of the spark plugs, piston number 2 also began to slightly collapse on the dome, which indicates strong overheating. I checked the ignition and set it to the lamp (this method has always worked for me)
Sorry for my English, technical words I write with google translator

[PeWe]

I hope you can quickly repair, get it running and enjoy coming bike season.

I have got similar issue back in the 80's. After 10.000 km. I was happy it happened not far from home instead of former Jugoslavia or somewhere on the road to/from Sweden.

Also 2-3 that melted.
I think timing was slightly off. 2:3 easier to get wrong.
Sloppy advancer springs will full advance to happen earlier too.
That was with RC836 pistons with rather high compression.
I had to fill correct gasoline, not cheaper lower octane.

So  not much could be off to cause issues. One piston had a small hole in inlet valve pocket. The piston beside had a bump.
Rods small ends bluish, a few guides overheated too.
Cut the advancer springs 1/2 wound is a must to get it right is my experience.

[Geoff Hastings]

I hope you can quickly repair, get it running and enjoy coming bike season.

I have got similar issue back in the 80's. After 10.000 km. I was happy it happened not far from home instead of former Jugoslavia or somewhere on the road to/from Sweden.

Also 2-3 that melted.
I think timing was slightly off. 2:3 easier to get wrong.
Sloppy advancer springs will full advance to happen earlier too.
That was with RC836 pistons with rather high compression.
I had to fill correct gasoline, not cheaper lower octane.

If number two is also showing signs of melting then it’s got to be ignition timing as both those cylinders have a common set of coil and points.

So  not much could be off to cause issues. One piston had a small hole in inlet valve pocket. The piston beside had a bump.
Rods small ends bluish, a few guides overheated too.
Cut the advancer springs 1/2 wound is a must to get it right is my experience.

[dave500]

its not a common occurance as much as it should be?i mean with all the pods and bad points and what not?is it not poor metalurgy?ive had like so many others a heap of these hondas and nothing no where close to this happening?correct me but i think this is the first post ive seen like this?

[olasek18_18@o2.pl]

The darker are cylinder 1 and 4, on 2 3 I think it was too hot...

[dave500]

they look like any plugs out of any engine?i think crap pistons?

[Geoff Hastings]

I think crap pistons is a bit of a leap, why just one crap piston and three good ones? Any alloy will melt given the right circumstances.
Let’s review the information, one piston has over heated and melted, so we should look for what will affect only one piston, octane affects all pistons, timing will affect two pistons, an air leak or faulty carb or fuel supply to one cylinder. The owner now says that possibly a second piston has shown signs of distress so that would suggest looking at the timing on that pair of cylinders.

[newday777]

Why different heat ranges of pistons and why resistor plugs??
What are all the plug numbers you have there?

[olasek18_18@o2.pl]

Why different heat ranges of pistons and why resistor plugs??
What are all the plug numbers you have there?

Omg!
I didn't see this before... I was sure they are the same.
But it was on 4 number, this type of spark plug should handle overheating better, am I right? it shouldn't have that much of an impact.

[madmtnmotors]

Why different heat ranges of pistons and why resistor plugs??

Wow, I didn't even notice the mish mash of spark plugs. So potential issues identified so far:

>High compression pistons used with:
        *Potentially low octane fuel which is a very common cause of pinging in high compression motor. The fuel detonates from heat before the spark event.
        *Varying spark plug heat ranges and resistance values used
        *Spark timing potentially off?
        *Potentially lean or rich fuel?

High compression pistons would be more sensitive to any of these variables.

[newday777]

Why different heat ranges of pistons and why resistor plugs??
What are all the plug numbers you have there?

Omg!
I didn't see this before... I was sure they are the same.
But it was on 4 number, this type of spark plug should handle overheating better, am I right? it shouldn't have that much of an impact.

They should all be D8EA

[Stev-o]

Why different heat ranges of pistons and why resistor plugs??

        *Potentially lean or rich fuel?

High compression pistons would be more sensitive to any of these variables.

Good point Wilbur - who knows what kind of fuel [and quality?] he used in Poland?

[Dunk]

I'd suspect piston to valve contact, looks like a witness mark. What was your piston to valve clearance? Valve to guide clearance on that cylinder? If P/V clearance was plenty then maybe that cylinder had a tight valve that was hanging in the guide.

Detonation, on car engines at least, usually cracks the piston around the perimeter above the top ring land. Lean melting I'd think would be more of a melted area/hole and not a crack. Lean to taht point would also typically result in a badly scuffed or scored skirt and bore too, probably before melting through the piston.

[olasek18_18@o2.pl]

The clearance was standard 005 and 008. I think that the collision occurred only after the pistons overheated (they expanded) \. I checked the head and the valves were not bent or damaged (I suspect that at the time of contact of these elements the piston was hot soft and plastic)

[Dunk]

I meant piston to valve clearance, generally .100" is a ballpark minimum for most automotive engines but you can get away with less if you have a light valvetrain, high spring pressure, etc. Running tight piston to valve clearance will cause problems if you ever get into valve float.

I'm guessing maybe you're referring to valve to guide clearance. If those numbers are inches that's super sloppy worn out, no way a valve hung in a guide. Likely be a smoke show after the seals gave up. If that's mm, that would be way too tight. Valves would probably be stiff to move by hand and would be pretty obvious on assembly.

[scottly]

Are you using the stock points ignition system?