Why did my ring break?

[DCSLADE]

(1973 Evinrude 50HP)
I need your thoughts on all the possible reasons a new ring (closest to crank) could break less than 10 hrs after being replaced.

I honed the cylinder walls, checked the ring gap tolerances, checked that the cylinder was round, ran extra oil in the gas, etc.
I can't afford to make the same mistake twice! Thanks

 

[ezeke]

Re: Why did my ring break?

Most likely that it caught on the port.

 

[DCSLADE]

Re: Why did my ring break?

yeah, but why would it catch if everything was per spec?

 

[Dhadley]

Re: Why did my ring break?

We'll need more info. What was the finished bore size of that cylinder? You said you honed it so we'll assume it wasn't bored. Are you sure it was within spec?

Now, here's some food for thought. That piston is symeterical. It'll flow fuel correctly when installed in either direction. However, if it's in upside down one ring end gap will be in or very near the exhaust port.

 

[DCSLADE]

Re: Why did my ring break?

Brief history:
My original block was destroyed when an acorn size piece of the lower piston broke off and flew through the side of the block. I found a used block with one good piston (top) and 1 bad piston (bottom). The crankside ring had broken on the bad piston but the cylinder was only slightly scored. I honed (not bored) both cylinders of the replacment block and reused the one good piston from my original block and the one good piston from the replacement block. Both which appeared to be in very good condition.
After I honed but before I reassembled I checked both pistons to be sure they were round. I also checked the bore sizes in multiple places to ensure they were also round and not too large. I didn't record the finished bore size but I do recall checking them against the specs in my Clymer and they were fine. I also placed the new rings in the cylinders and checked the end gaps..also within spec but on the higher end.
The pistons are stamped with "up" on the dome so I don't think I could have screwed that up.
When I compared the old wrecked piston to the latest wrecked piston I noticed that the failures were remarkably similar. In both cases the thin (maybe 3/16") section of piston wall between the ring and the piston port were broken out. I will try to post some pictures.

I'm having the bottom cylinder bored 30 over and putting in a new 30 over piston. Assuming the shop is good do you think this will prevent future problems?

 

[Dhadley]

Re: Why did my ring break?

Most likely you have an ignition problem. Anytime the corner of the piston is burned off (it'll look like it was knocked off, which technically it was) it'll happen right by the exhaust port -- hottest part).

 

[DCSLADE]

Re: Why did my ring break?

These are the pictures of the "old Piston" and the latest failed piston. (See above)
Is this just mechanical damage or could detonation be the culprit?

 

[DCSLADE]

Re: Why did my ring break?

These are the pictures of the "old Piston" and the latest failed piston. (See above)
Is this just mechanical damage or could detonation be the culprit?

 

[DCSLADE]

Re: Why did my ring break?

Now that I know how to post pictures, please take a read through and give me your thoughts...
Thanks, I really want to get this right this time.
http://i89.photobucket.com/albums/k....com/albums/k212/dcslade427/DSC04222.jpg[url]

 

[Stumedic]

Re: Why did my ring break?

Is the pin present in the ring land?

 

[DCSLADE]

Re: Why did my ring break?

yup, both pins are there. Is there anyway for something outside the block cause a ring to break. (carbs, ignition, timing, other) Or is it pretty likely a mechanical thing?