[30mil]

turned around an got ready to make a pass 1/8mile and it just shut down on me

push it home and found this........cranks good,cylinders good....piston not so good...top case not so good

piece of the piston got stuck in between the case and crank and caused the hole in cases

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[30mil]

7mil billet crank

alky motor

squish .038

compression 210psi

+7 timing

cylinder to piston clearances is good also



motor was ran like this for about 4 months
fuel pump is also on the side that went out...(clutch side)

[Mikeac1984]

What happend here is your piston broke. So dif your case.

[Rotax Raptor]

Check the welds on the crank. I had a weld break out and cause the same thing. Or, lean condition caused detonation. One plug looked pretty lean in the picture. Did it have lean rev hang on?

[Radar]

Detonation , look at the piston crown. Can bust a skirt easy, tho usually not on a wiseco.

[30mil]

Radar, on 01 February 2012 - 09:15 PM, said:

Detonation , look at the piston crown. Can bust a skirt easy, tho usually not on a wiseco.

thats the light from the camera flash on the crown of the piston

[pipe-ster]

Show the dome. But my guess is this is what happened

30mil, on 01 February 2012 - 07:09 PM, said:

squish .038

compression 210psi

+7 timing

[R&R Performance]

pipe-ster, on 02 February 2012 - 08:33 AM, said:

Show the dome. But my guess is this is what happened

Agreed

[30mil]

dome

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[30mil]

maybe the squish was to tight and the piston head the dome causing the skirt to crack ?!?!

but that still doesn't explain the white plug

[rsss396]

I will venture to say detonation has caused the piston to crack maybe because of the wrong setup of the head.

The squish clearance between the piston and head seams fine but the squish band width looks pretty wide for a high rpm motor.

A wide squish band will make the squish velocities of the combustion chamber to high and will promote detonation especially on a long run.

The white plug is more than likely because as the piston started to fail and send pieces threw the motor causing sharp edges to be formed on the piston and combustion chamber dome

This will increase the likely hood even more for detonation to happen thus melting aluminum and depositing it on the plug

typical squish band "area" is in the 40-50% range with lower area lower squish velocity being for motors that have higher rpm & higher load.

The amount of load is a big factor, a 300ft motor can get away with more squish velocity than a 660ft motor can.

[pipe-ster]

30mil, on 02 February 2012 - 06:57 PM, said:

dome

This is where your problem is. 210 PSI kicking on a 7mm is like 20-1 That with a .038 piston to head (to tight in my opinion) and a squish band that looks to be 65% to 70% of the bore = detonation
Did a shop set this engine up or are you just bolting parts together ?

[J-Madd]

What does the "good" dome look like?

[pipe-ster]

J-Madd, on 03 February 2012 - 01:27 PM, said:

What does the "good" dome look like?

There isn't one quote "good" dome. A real builder builds around there port layout, timings, pipe and intended use. The domes sold over the counter are usually generic starting points off of computer programs or just someones idea. This is why its important to work with a reputable builder.

[J-Madd]

pipe-ster, on 03 February 2012 - 01:53 PM, said:

There isn't one quote "good" dome. A real builder builds around there port layout, timings, pipe and intended use. The domes sold over the counter are usually generic starting points off of computer programs or just someones idea. This is why its important to work with a reputable builder.

I meant what does the other dome for his motor look like. It may be easier to see deto on it since it hasn't be damaged by debris.

[30mil]

J-Madd, on 03 February 2012 - 01:27 PM, said:

What does the "good" dome look like?

looks good...i'll get a picture

[30mil]

pipe-ster, on 03 February 2012 - 09:04 AM, said:

This is where your problem is. 210 PSI kicking on a 7mm is like 20-1 That with a .038 piston to head (to tight in my opinion) and a squish band that looks to be 65% to 70% of the bore = detonation
Did a shop set this engine up or are you just bolting parts together ?

not saying your wrong about the setup,BUT this motor run 5.9's back to back and sometimes 5.8 at 111mph


port timing was 204 exhaust 133trans

alot of people i called said it wouldnt run good setup like that,but after taking it to the track.....i decided to leave it as it was....i thought it was running GOOD for a 7mil/68mm cub

OH i forgot to add that i had BIGBORE shearers when it was track tested....AND the day it blew i was trying out a set of SMALLBORE shearers

i dont know if the pipe switch could have cause this or not

[30mil]

good dome

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