Author Topic: Which piston?  (Read 847 times)

Online RichardL

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Re: Which piston?
« Reply #15 on: 30.06. 2016 13:29 »
It is called a 4 square defect, google it for the long detailed description.

Trevor,

I'm interested in learning more about this but having trouble locating a direct reference to "4-square defect". Found stuff about bore distortion and torque plates and sleeve interference fits, but no reference to "4-square defect". Could you steer us to a link? Thanks.

Richard L.
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Online Triton Thrasher

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Re: Which piston?
« Reply #16 on: 30.06. 2016 17:42 »

You may find it if you look for 4 corner seizure.

http://www.britbike.com/forums/ubbthreads.php?ubb=showflat&Number=569968

Quote from: J Healy on Britbike
The one thing that is helpful when diagnosing these kind of pistons seizures is the wrist pins.

These kinds of seizures happen because of lack of clearance. But there are two reasons it can have too little clearance:

The cylinder was finished with too little clearance for NORMAL operation. The piston did not have enough clearance for what would be considered normal piston expansion. It is normal for the heat of combustion to migrate into the piston and the resultant piston expansion is compensated for with the factory recommended piston clearance.

It is when we have abnormal combustion that we get the other reason for this kind of piston seizure. When a piston reaches normal operating temperature, and expands in the bore, there is very little extra clearance available for additional expansion. If the engine suffers prolonged detonation, which can be inaudible to the human ear, we are going to drive additional heat into the piston. If this is more heat than the engineers planned for when they specified piston clearance the piston will become "to big for the bore." We have a seizure.

So what can the wrist pin tell us. Well for one thing it was there during this event. It is also polished steel and discoloring because of it being exposed to heat will be available for us to examine. For centuries metal workers have been using the color of polished steel to tell them the temperature of the metal as it heats up. Long before the pin starts to glow red the surface will start changing color. At around 390 degrees F the bright steel color will start to turn a faint straw color, 445°F it becomes light straw, 465°F it becomes dark straw, 480°F it becomes Brown, 520°F it becomes brown/purple, 520°F it becomes purple, 540°F it becomes dark purple, 575°F it becomes blue. The thing that is handy about this is when the pin cools the color the pin reached remains there for all to see.


The bottom line to all this
If the pin comes out of the piston the same color as it was when installed - and thus only exposed to normal engine operating temperatures - the cylinder was set-up with too little clearance.

If the pin comes out displaying ANY color the engine was operating in conditions where there was abnormal temperatures and detonation is always a prime suspect in these engines! All this is especially true if there is also signs of detonation on the top of the piston or spark plug.

Why do I think the problem was detonation, not lubrication? Because lack of lubrication seizures typically start at the bottom of the skirt and go up. This seizure started just under the oil ring, a point where abnormal heat causes the most problems

Online bikerboy

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Re: Which piston?
« Reply #17 on: 01.07. 2016 01:21 »
Wow I dont think I have ever seen a piston that bad before. Go to a good machinist and get him to measure the bores and if necessary hone or bore. New pistons obviously  but I would definitely want to check oil flow and ignition timing before I started that motor again.