Help, Why did this happen?

[speedyauto]

On & off the throttle doing arond 70kmh on a farm track & 2klms short of finishing day 2, 1/2 way through 4th tank of fuel (40:1) & had this happen.
Seized but when it cooled 2 hrs later it turned over but with bugger all compression.
I'm not a tech guru just the average punter who enjoys riding my toy.
All my previous encounters with this sort of thing has seen the piston melt on top or crack.
The piston & Head colour are a tad light but not what I would have thought would cause this especially with running 40:1, the area where the piston has 4 scuff marks has grabbed the ring & is holding it in, I wasn't running wide open flat out, the coolant was still full & water pump is still in good order & I haven't found any air leaks.
So I'm at a loss as to what happened.
If anyone has a thought as to what happened I'm all ears.
Speedy.

 

[fran...k.]

That is pretty much what all of the used engines in my bone pile look like when that kind of piston is used. My theory is basically the stock piston to bore clearance needs be bigger when that kind of forged piston is used. You could find a similar thread over on ktm talk if you look hard enough. Part of my theorly is that piston construction has a lot more metal in a front to back way in the area of the wristpin/piston inter fit. This expands in a linear fashion and that is exactly where the seizure marks are. Your cylinder seems ported lessening the land the rear skirt has to rub against and those kind of piston generally have bigger side cut outs than the origionals hence don't have the same steadying effect as an origional one which is cast not forged. Curious as to other folks opinion but I have typed this stuff before at least most of it.

Fran

 

[ruwfo]

Was the piston/top end broken in by heat cycling, 1st? It's important to properly break the top end 1st.

Husky John

 

[Ron]

Four corner seize is due to not enough piston to cylinder clearance. A long slow break-in and it might have survived. But not likely.

 

[Ex HVUK]

Is that detonation damage on the front edge? If so could either be too lean or ignition timing problem.

Dave

 

[dukkman]

Bummer
With the marks all round the piston it looks like a clearance problem and a 2/stroke can give problems on a shut throttle overrun because there is no fuel to keep everything working especially if it's running hotter due to clearance.
Forged piston needs a lot more clearance than a cast one.
The clearance is written on the box and is measured at the BOTTOM , front to back , normally.
Mahle pistons are usually smaller than the bore size , in other words you just bore and hone the cyl to the nominal size ie- 68.75 and the piston does the clearance for you.
Other makers make the piston 68.75 and you have to add the clearance.
It's easy to get mixed up and stuff up when you are doing the boring.
When making a drag car engine the clearances went up to .006" , on a forged piston , which is past worn out in a normal engine [ .0015" is normal ]
A forged piston expands a lot more than a cast one but can also take a lot more punishment.
Give the bore a light hone , tidy the piston up making sure that the ring is free in the grove and slap her back together.
Might rattle a bit but should be ok.
If it was a cast piston she'd be stuffed.
Cheers

 

[speedyauto]

Firstly thanks too everyone for sharing their thoughts,

secondly there are a few things I forgot to mention the other day.
We had been riding for about 2hrs when we stopped for 25 mins due to someone else's flat tyre, then restarted & 2k's down the track the motor started to make a crackling noise when under power (like a cracked exhaust header,which it isn't) so I reduced to about 1/3 throttle in top gear as the homestead was just up the road which is when it stopped. when I pulled up I noticed that all 4 cyl stud nuts had very minor coolant weep marks.(the tension felt tight when stipping it down)

Thirdly to answer a few Q's,
This was a very low hr original engine when I bought it & before it was sent to be ported (was checked by the tuner when he stripped the motor) & this was the 4th tank so I dont think heat cycling or run in is the problem.
The piston is the original from what I was told so I'm not sure whether that makes it a cast or forged one, I don't
know how to spot the difference.
The discoloration at the front/top of the piston is alloy shavings from when I have turned it over by hand & has not been melted it's just like alloy dust.
With regards to the timing I'm going to have to check that I did it right the first time & make sure it also hasn't moved.

Now to my Q's
with the coolant weeping out through the cyl stud nuts could this have some effect on the cyl burn?
Although there dosent appear to be any gasket failure could the head or base gasket contribute to this?
Could the timing in anyway result in this type of failure? (starting was normal & no kick back)
Is my piston a cast or forged one?
Could all this be just from incorrect jetting? ( I raised the needle, 1 clip to make it richer than I had it before)

looking forward to reading any responses I get.
Thanks,
Speedy.

 

[Kartwheel68]

Sure looks like detonation marks on the exhaust side to me, it will look kind of dull and powdery and the edge will be eroded almost like it was sand blasted.

 

[Tinken]

You definitely over heated your engine enough to expand the piston to the point of seizure. Your cylinder bolts were weeping due to heat expansion. When you port and polish an engine, it allows more air/fuel into the cylinder. You get more power, but you also get additional heat. If your cooling system remained stock, it may not be able to handled the additional heat, especially after riding hard and coming to a stop with no cool down. I have no doubt with the expansion you had that you developed an air leak, more than likely with the base gasket where the cylinder meets the crankcase. Detonation may of occurred due to excessive heat increase. You might look into a alternative coolant, they tend to run cooler than the regular mix. And maybe your air/fuel mix.

 

[Wildebeest90210]

4 corner seizing can be caused by over torqueing. Distorts the barrel.The witness marks may not be related to the seizing itself as an engine can run over torqued but worth looking in to?

 

[dukkman]

To tell the difference between a forged and cast piston.
Turn your piston upside down and look inside.
A forged piston is stamped out in a big machine and leaves the inside of the piston smooth.
A cast one is cast and the inside has lines and mold marks.
Cheers

 

[jimspac]

A forged piston has a much higher expansion ratio because of the denser molecular structure thus the requirement for higher piston to wall clearace at bore time. My father found out the hard way in the 70's using Wiseco pistons to replace the Mahle that was in his 72 Puch 175GS. He had more seizures until he called Wiseco and found Wiseco clearance was double the Puch/Mahle clearance

 

[speedyauto]

You definitely over heated your engine enough to expand the piston to the point of seizure. Your cylinder bolts were weeping due to heat expansion. When you port and polish an engine, it allows more air/fuel into the cylinder. You get more power, but you also get additional heat. If your cooling system remained stock, it may not be able to handled the additional heat, especially after riding hard and coming to a stop with no cool down. I have no doubt with the expansion you had that you developed an air leak, more than likely with the base gasket where the cylinder meets the crankcase. Detonation may of occurred due to excessive heat increase. You might look into a alternative coolant, they tend to run cooler than the regular mix. And maybe your air/fuel mix.

You might be on to something with the base gasket as 1 corner was loose & had an oily patch on it & the rest was stuck down & clean.