Topic: Broken piston after 50miles CB 750 K2

[olasek18_18@o2.pl]

Hello, I was rebuild my engine in Cb 750, I used a new IMD high comp pistons + 1,00 and this happen. Engine heet up and thats what I find inside. Do You have any ideał what can be wrong? Maybe piston cylinder tolerance?

[spotty]

nasty.

i'm no expert but with that much carbon build up on the other pistons after 50 miles you'd have to say it was running way too rich

[madmtnmotors]

Bummer. 

Definitely looks heat related and appears that the piston crown has suffered a melt down. Too tight piston to cylinder clearance might cause it, but I have often seen this result from a too lean condition. Maybe the #3 carb suffered a fuel restriction? Inspect the exhaust valves for clues. Then see if there's enough left of the piston skirt and cylinder to determine what the clearance might have been. On the next assembly carefully measure the piston diameter and cylinder bores. Never assume the machinist "got it right".

At least you still have room to fit larger pistons since you only went 1.00 over.

[Tracksnblades1]

Bummer. 

Definitely looks heat related and appears that the piston crown has suffered a melt down. Too tight piston to cylinder clearance might cause it, but I have often seen this result from a too lean condition. Maybe the #3 carb suffered a fuel restriction? Inspect the exhaust valves for clues. Then see if there's enough left of the piston skirt and cylinder to determine what the clearance might have been. On the next assembly carefully measure the piston diameter and cylinder bores. Never assume the machinist "got it right".

At least you still have room to fit larger pistons since you only went 1.00 over.

+1 😳. Meltdown..Wonder if the spark plug has the tell tale aluminum spatter on it..?

[RAFster122s]

I'm leaning towards that cylinder was super lean to cause the meltdown. Rich running will typically be cooler running because of the excess fuel. A couple revs away from holing the piston.
Piston dome is likely to be showing signs of splatter or excess heat discoloration. If other domes have carbon while it doesn't then you have more evidence pointing to lean running cylinder. If you didn't back off the adjuster you should be able to figure out if it was tighter but maybe not now that it is apart.

At least you caught it before it started slagging that cylinder and ruining the head. Provided it didn't trash the combustion chamber dome.

[PeWe]

The melted piston has mark after ex valve as it look like.
Which engine is it?
F2/3 that need higher dome pistons?

EDIT:
I found your other thread about crank bearings.
K2 engine. Those wide higher domes seems to be high if chambers are stock.
392 pistons work fine in a K2. F2 looking pistons another thing.
That + ignition 2:3 wrong might explain the melt down.

Did it ping when engine ran?
Typical is when throttle from 4000 rpm on 5th gear. Pinging just before 5000, maybe 4600-4800rpm.

Pinging on other gears too, those numbers what I remember.

[olasek18_18@o2.pl]

The melted piston has mark after ex valve as it look like.
Which engine is it?
F2/3 that need higher dome pistons?

EDIT:
I found your other thread about crank bearings.
K2 engine. Those wide higher domes seems to be high if chambers are stock.
392 pistons work fine in a K2. F2 looking pistons another thing.
That + ignition 2:3 wrong might explain the melt down.

Did it ping when engine ran?
Typical is when throttle from 4000 rpm on 5th gear. Pinging just before 5000, maybe 4600-4800rpm.

Pinging on other gears too, those numbers what I remember.

What You mean the ping?
Yes I hear the strange humming, murmuring, just around 4-5 thousand revolutions!

[olasek18_18@o2.pl]

The melted piston has mark after ex valve as it look like.
Which engine is it?
F2/3 that need higher dome pistons?

EDIT:
I found your other thread about crank bearings.
K2 engine. Those wide higher domes seems to be high if chambers are stock.
392 pistons work fine in a K2. F2 looking pistons another thing.
That + ignition 2:3 wrong might explain the melt down.

And Yes PEWE its K2 cylinder head.
Thanks for help!

Did it ping when engine ran?
Typical is when throttle from 4000 rpm on 5th gear. Pinging just before 5000, maybe 4600-4800rpm.

Pinging on other gears too, those numbers what I remember.

Yes It's K2.
the manufacturer of the pistons claimed that they fit all versions of k0-k7
I should Ask here...

[Tracksnblades1]

Can you post a picture of the melted piston’s skirt and it’s cylinder bore ?

[PeWe]

Pinging is a knocking sound. Not as bad rods, sharper thinner metal sound.

These pistons?
https://imdpistons.com/products/honda-cb750-sohc-piston-kit-4-new-392-410-high-compression?variant=41323263590575

The compression number for F2 can not be compared with K2.
- F2 head has a chamber volume of around 28cc (ml)

- K2: 22.5 cc area

So compression in a stock K2 will be HIGH.
Higher compression pistons need hotter cam with longer duration and overlap where IN and EX valves are open at the same time letting pressure out.


It work the other way too when using hotter cams with more overlap. (The flow at higher revs for more power is the goal.)
Higher comp pistons to keep compression on a healthy level.

Higher lifting cams also need deeper valve pockets in pistons + better springs, harder and can compress more without coil bind.

[olasek18_18@o2.pl]

Pinging is a knocking sound. Not as bad rods, sharper thinner metal sound.

These pistons?
https://imdpistons.com/products/honda-cb750-sohc-piston-kit-4-new-392-410-high-compression?variant=41323263590575

The compression number for F2 can not be compared with K2.
- F2 head has a chamber volume of around 28cc (ml)

- K2: 22.5 cc area

So compression in a stock K2 will be HIGH.
Higher compression pistons need hotter cam with longer duration and overlap where IN and EX valves are open at the same time letting pressure out.


It work the other way too when using hotter cams with more overlap. (The flow at higher revs for more power is the goal.)
Higher comp pistons to keep compression on a healthy level.

Higher lifting cams also need deeper valve pockets in pistons + better springs, harder and can compress more without coil bind.

Everything is correct, unfortunately the manufacturer misled me with a description that the pistons will fit all CB 750 SOHC models.

[Tracksnblades1]

“Pinging” can be caused by detonation.

Sometimes referred to as spark plug rattle or spark plug clatter but usually pinging..

The link will throughly explain detonation and Pre-ignition and their differences.
https://resources.savvyaviation.com/detonation-and-pre-ignition/

[Tracksnblades1]

As others have posted “rich” air fuel mixture tend to have lower temperatures.

If you google “Adiabatic Process”,  “Adiabatic Heating/Cooling”, and “Latent heat of evaporation” in thermodynamics you can get into it at a different level of theoretical understanding….I won’t be able to explain it and won’t completely understand it either in 6 milliseconds…😁

[RAFster122s]

with the appropriate changes to compensate for the extreme compression of those pistons in a K motor, then yes...they might work. piston manufacturers create a product, they cannot expect to detail all the changes required to make that product work in every application. You would need the experience of DynoMan or APE or CycleX to maybe share if they have built a similar motor and the changes to make it work...
I don't agree with your argument that you were led astray, you failed to be knowledgeable enough to ask the right questions because of inexperience.
Just like car guys who drop overbore kits in their motor but haven't engineered the rest of the driveline and intake and exhaust to make the most of that piston set.

Very high compression motors run hot and are prone to pinging and predetonation, run them too lean or with low octane fuel and ugly results happen.

expensive lesson in motor building...

I am surprised your valve flycuts are not deeper in the piston. Did you assemble the motor yourself? Did you clay the valves and dome? How much clearance did you have between the valves and pistons? Apparently adequate as there doesn't appear to be kiss marks of valves and pistons.
You must have had compression ratio of north of 14:1 with the high domes in the K2 head.

 Sorry this happened...
David

[olasek18_18@o2.pl]

that's how I assembled the engine myself, I didn't realize that the compression would increase so much, piston manufacturers always give the compression ratio. Pistons in my k7 and F2 are also slightly convex, I was not aware of the difference in the cylinder head
painful lesson, but always a lesson

[Tracksnblades1]

that's how I assembled the engine myself, I didn't realize that the compression would increase so much, piston manufacturers always give the compression ratio. Pistons in my k7 and F2 are also slightly convex, I was not aware of the difference in the cylinder head
painful lesson, but always a lesson

Can you post some pictures of the side of the melted pistons, cylinder head’s combustion chamber, spark plug,  and the cylinder bore ?  Is this the same motor that you had some bearing questions on ?

[HondaMan]

From those domes and the inlet valve clearance cut on the pistons, I'd estimate the compression above 12:1. Pretty high for street use!

[Little_Phil]

High compression and low octane fuel is a recipe for disaster.

[olasek18_18@o2.pl]

that's how I assembled the engine myself, I didn't realize that the compression would increase so much, piston manufacturers always give the compression ratio. Pistons in my k7 and F2 are also slightly convex, I was not aware of the difference in the cylinder head
painful lesson, but always a lesson

Can you post some pictures of the side of the melted pistons, cylinder head’s combustion chamber, spark plug,  and the cylinder bore ?  Is this the same motor that you had some bearing questions on ?

I have only this photos, 2 and 3 are the worst condition. I send cyoinders to veryfication

[Geoff Hastings]

Interesting seeing peoples estimates of compression ratios etc, as if you can estimate it from a picture. Anyway, here’s my take on it. Three pistons are perfectly good, I wouldn’t even be concerned by their slightly rich appearance as I would assume you’d not be caning the life out of it in the early miles of a rebuild. Ignition timing won’t be an issue as two pairs of cylinders are common so that really only leaves the inlet system and carb for that cylinder. Definitely running lean will cause that over heating on that particular cylinder so check again the carb for cleanliness or stuck float valve or air leaks in the manifold or carb rubber.

[Tracksnblades1]

that's how I assembled the engine myself, I didn't realize that the compression would increase so much, piston manufacturers always give the compression ratio. Pistons in my k7 and F2 are also slightly convex, I was not aware of the difference in the cylinder head
painful lesson, but always a lesson

Can you post some pictures of the side of the melted pistons, cylinder head’s combustion chamber, spark plug,  and the cylinder bore ?  Is this the same motor that you had some bearing questions on ?

I have only this photos, 2 and 3 are the worst condition. I send cyoinders to veryfication

Thanks.
 
The spark plugs, cylinder head combustion chambers, and valves will tell the “rest of the story”… (Paul Harvey)

GH makes some good points. 1&4 tolerated their running conditions better. It does look like #2 piston skirt was showing signs of something where the #2 is wrote on the skirt. 2&3 ignition timing could still be different than 1&4 if using OEM point plate style triggering. If number 2 piston skirt is actually showing signs of scuffing it may have soon followed #3. I know by design of the air cooled inline four, 2&3 could run a little hotter than 1&4.  But I would think #3 excessive heat would have transferred to the adjacent #4. Prior to transferring through the cam chain tunnel to #2.

So similar to GH, I would be looking for Carburetor issues with #3, then #2. I would check for #3 actual slide opening at 1/4 throttle as compared to 1&4. Verify Synchronization. If #3 slide is considerably farther open than the rest it could indicate #3 was doing most of the work. Causing a different work load and environment for number 3.. Adding a lean mixture, an advanced ignition timing, and an increased work load may have contributed to #3 failure.

Without pictures of the spark plugs and cylinder head, is similar to trying to determine 60 foot drag race time issues without your tires your running…🤔

[olasek18_18@o2.pl]

that's how I assembled the engine myself, I didn't realize that the compression would increase so much, piston manufacturers always give the compression ratio. Pistons in my k7 and F2 are also slightly convex, I was not aware of the difference in the cylinder head
painful lesson, but always a lesson

Can you post some pictures of the side of the melted pistons, cylinder head’s combustion chamber, spark plug,  and the cylinder bore ?  Is this the same motor that you had some bearing questions on ?

I have only this photos, 2 and 3 are the worst condition. I send cyoinders to veryfication

Thanks.
 
The spark plugs, cylinder head combustion chambers, and valves will tell the “rest of the story”… (Paul Harvey)

GH makes some good points. 1&4 tolerated their running conditions better. It does look like #2 piston skirt was showing signs of something where the #2 is wrote on the skirt. 2&3 ignition timing could still be different than 1&4 if using OEM point plate style triggering. If number 2 piston skirt is actually showing signs of scuffing it may have soon followed #3. I know by design of the air cooled inline four, 2&3 could run a little hotter than 1&4.  But I would think #3 excessive heat would have transferred to the adjacent #4. Prior to transferring through the cam chain tunnel to #2.

So similar to GH, I would be looking for Carburetor issues with #3, then #2. I would check for #3 actual slide opening at 1/4 throttle as compared to 1&4. Verify Synchronization. If #3 slide is considerably farther open than the rest it could indicate #3 was doing most of the work. Causing a different work load and environment for number 3.. Adding a lean mixture, an advanced ignition timing, and an increased work load may have contributed to #3 failure.

Without pictures of the spark plugs and cylinder head, is similar to trying to determine 60 foot drag race time issues without your tires your running…🤔

Hmmm
The candles were checked, each had the same color, I think the mixture was good, I also did the synchronization at the beginning.

[olasek18_18@o2.pl]

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

[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

[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

Ok, sorry I didn’t remember reading it was pinging at 3500-5000 rpm..
High compression engines usually don’t ping, detonate, or experience pre-ignition at idle speeds,  no load,
or closed throttles. There is very little volumetric efficiency (cylinder filling) at closed throttle position.

Do you have your spark plugs ?
Do you have your cylinder Head ?

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

Everyone is trying to help….