> No one has been able to give a good reason on why the piston broke. Many say ignition timing, some say over-heating. Even Dick Paterson couldn't
> give a good reason. I know my timing was set correctly. It remains a mystery.
Ray,
This is Thanksgiving Day, and as my daughter and family are arriving soon, I have to stay clean. That means I can't go out to the shop and work on my
own coach. So, I stopped to look at your case in greater detail. While I know I could do better with the parts in my hand and my own microscope, I
think I can shed some light on this for you. Even without seeing the crown, there are some good indicators available.
If it makes you feel better, you didn't stand a chance. The critical part of the failure occurred in about three turns of the crank and the entire
process was probably less than 100.
Stop for a moment and look at frames A12 and A111. While this is at the pixel limit, both show signs of fatigue. This is particularly evident in
lower corner of the break (Piston lower). It is that "clam shell" appearance. The failure started there or possibly in the other corner, but part
that looks to me like it happened too fast. Once that crack reached a critical point, that side of the support for the thrust skirt was gone. Then
the skirt collapsed and pealed it self off the remaining crosshead (bearing boss) carrying the cast in reinforcement with it.
The rings appear to still be free, so it was not a timing issue causing detonation.
You wrote that there were no marks on the crown, and that is also consistent with not having a timing issue.
The coking under the crown is actually lighter than I might expect given the engine's service. So, it sure was not an over heat problem. Even in an
engine equipped with a lube oil cooler, an overheat causes the lube oil to go over usable temperature and it will coke where is can do so.
Well, I'm glad that you were able to wrangle the parts you needed to put a good engine back together.
I hope you also had a good Thanksgiving with friends and/or family.
Matt - the engine lab refugee
--
Matt & Mary Colie - '73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan
OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Twixt A2 and Detroit
> give a good reason. I know my timing was set correctly. It remains a mystery.
Ray,
This is Thanksgiving Day, and as my daughter and family are arriving soon, I have to stay clean. That means I can't go out to the shop and work on my
own coach. So, I stopped to look at your case in greater detail. While I know I could do better with the parts in my hand and my own microscope, I
think I can shed some light on this for you. Even without seeing the crown, there are some good indicators available.
If it makes you feel better, you didn't stand a chance. The critical part of the failure occurred in about three turns of the crank and the entire
process was probably less than 100.
Stop for a moment and look at frames A12 and A111. While this is at the pixel limit, both show signs of fatigue. This is particularly evident in
lower corner of the break (Piston lower). It is that "clam shell" appearance. The failure started there or possibly in the other corner, but part
that looks to me like it happened too fast. Once that crack reached a critical point, that side of the support for the thrust skirt was gone. Then
the skirt collapsed and pealed it self off the remaining crosshead (bearing boss) carrying the cast in reinforcement with it.
The rings appear to still be free, so it was not a timing issue causing detonation.
You wrote that there were no marks on the crown, and that is also consistent with not having a timing issue.
The coking under the crown is actually lighter than I might expect given the engine's service. So, it sure was not an over heat problem. Even in an
engine equipped with a lube oil cooler, an overheat causes the lube oil to go over usable temperature and it will coke where is can do so.
Well, I'm glad that you were able to wrangle the parts you needed to put a good engine back together.
I hope you also had a good Thanksgiving with friends and/or family.
Matt - the engine lab refugee
--
Matt & Mary Colie - '73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan
OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Twixt A2 and Detroit
