ESE's works engine tuner

[TZ350]

That piston markup on the dome is all the way round, the width of the squish , so that isnt deto, its been smacking the head big time.
I would say that the crank is flexing - is it a hollow pin or solid - Jan has said that the Aprilia would not hit the head at 14,000 with 0.4 squish. And no, Honda have used the tongue idea for ever with no issues - and that hole in the piston skirt starts well above the tongue at BDC. At 0.6 squish depth, and that very wide squish width, what is the MSV in EngMod.

Hollow Pin and 37.5 MSV

[jonny quest]

Is the ring pinched in the piston?

[wobbly]

Squish looks wider than 6.9mm in the pics is only reason I asked.

[husaberg]

Is the ring pinched in the piston?

When I zoomed the pic way in it looked that way. just by the ring not poking out but the land groove appears to have gap?
I couldn't imagine it whacking the head hard without it doing this either...

looking at the pic though is it pinned at 7am or where the ring is?
is 7am debris?

Also Hey Rob is that the strort stroke engine with NSR top end if so its not impossible for that cylinder to catch due to the dimensional changes you made taking 6mm? off the stroke?

[ken seeber]

Looking at Hooser's pic, the crown damage/ring jamming is obviously due to shrapnel from the busted out bit. However, as there is no other longitudinal skirt damage that might have caused the "punched out" bit, I reckon it started with the small crack from the bottom which progressed upwards and you can see is aligned with the LHS of the hole. Then, being on the thrust face, this allowed the area to flex = fatigue = breakage = failure.

As to why the small crack got there in the first place is a guess. I'm sure TeeZee doesn't hammer his pistons too much.

Surprised that there wasn't much more destruction.

[husaberg]

TZ
Following what Ken has said, whats at 9 oclock have you moved the pin and its pinned through at the top? or is it like a casting or makers mark?
becasue if that is sticking out of the top of the piston? why is that not hitting the head?

[Condyn]

Both times that I have seen cast pistons hit the head, the force damages the ring land and the crack propagates from the ring land down to the wrist pin bore. In your case it almost looks like the problem started in skirt. Does the shrapnel appear to have gone outward or in towards the rod? Maybe something was ingested and got between the rod and the piston skirt?

[TZ350]

.

When I zoomed the pic way in it looked that way. just by the ring not poking out but the land groove appears to have gap?
I couldn't imagine it whacking the head hard without it doing this either...

looking at the pic though is it pinned at 7am or where the ring is? is 7am debris? Also Hey Rob is that the short stroke engine with NSR top end if so its not impossible for that cylinder to catch due to the dimensional changes you made taking 6mm? off the stroke?

Ring jammed. The apparent side gap is just an optical illusion because of the slight ring groove chamfer. Yes 45mm stroke NSR250 cylinder on a short 48mm stroke bottom end with a long rod. Cylinder shortened, plus head insert inserts down into the cylinder. BDC for the piston is not go all the way down like the original NSR.

TZ
Following what Ken has said what's at 9 o'clock have you moved the pin and its pinned through at the top? or is it like a casting or makers mark?
because if that is sticking out of the top of the piston? why is that not hitting the head?

Original Honda RS125 pistons have the ring pegged at 7 o'clock so that the ring ends run up the cylinder wall between the B and C ports. Works the same on the NSR cylinders that we use for the 110cc engines.

Piston crown definitely crushed down trapping the ring.

[TZ350]

Squish looks wider than 6.9mm in the pics is only reason I asked.

Squish band 9mm wide so 55% not the 45% Engine mode was told. I do find it challenging to cut metal to match the plan.

The squish gap would probably have been fine if it was not for the debris getting jammed in the squish gap. Although I have opened it up a bit for this rebuild.

[diesel pig]

. Yes 45mm stroke NSR250 cylinder on a short 48mm stroke bottom end with a long rod.

Sorry to be pedantic. But don't you mean 54mm bore on a NSR250?

[TZ350]

Sorry to be pedantic. But don't you mean 54mm bore on a NSR250?

Yes you are right. 54mm bore, 48mm stroke.



Also it came to me in a dream last night. I had over tightened the piston in the mill vice and cracked it. We have to take a bit off the pin boses to get the piston to fit over the little end of the RD400 rods we use.

[husaberg]

Yes you are right. 54mm bore, 48mm stroke.



Also it came to me in a dream last night. I had over tightened the piston in the mill vice and cracked it. We have to take a bit off the pin boses to get the piston to fit over the little end of the RD400 rods we use.

My dreams seem to be a bit more exciting than yours Rob.......
Oddly though they both seem to involve vice and short tight skirts....