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Con Rod Failure

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Don Holden

08-27-2002 16:56:45




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Hi folks! I had a 16 h/p cast iron Briggs rebuilt this spring and have been running it in a Simplicity Garden tractor to now my lawn. I have approximately 30 hours on the overhaul and the Rod Let loose from the wrist pin in the piston. It was full of oil, and the slinger is installed on the Rod cap end. What would be the most logical reason for a failure at the piston end? The piston bore was machined .020 over and new valves, valve guides and seats installed during the re-build. I returned the engine to the machine shop with all the broken rod parts to see if they could diagnose What Went Wrong and Why. What is your thoughts, experience etc etc etc...

Thanks in advance for your comments and replys
Don

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Lou

08-29-2002 05:37:31




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 Re: Con Rod Failure in reply to Don Holden, 08-27-2002 16:56:45  
Problem could be that the pin bore was not parallel to the lower end bore.



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Ryan

08-27-2002 21:34:41




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 Re: Con Rod Failure in reply to Don Holden, 08-27-2002 16:56:45  
Hello Don, Sounds like they didn't replace the connecting rod. My theory why this happened is: Installing a new set of piston rings adds quite a bit of friction if they aren't prepaired properly. A knowledgable mechanic will stone the edges of the rings with a small sandstone to knock off the sharp edges of the rings. I'm not talking about the end gaps but the edge of the face of the ring where it rubs the cylinder wall. The ring has about two thousanths gap in the pistons ring groove to move. The ring will rock slightly in the groove allowing the rings to dig into the cylinder and wear quite heavily until worn in. It used to be considered "good" when a fresh rebuilt engine turned hard, this was the rings wearing hard on the cylinder wall, better rebuilding techniques have taught us to know better these days. If your familier with rebuilding engines, you know the first oil change will be very dark (black). Next rebuild you do, try stoning the edges of the rings about two thousanths. Hold the stone at a 45 degree to the face, pull away frome the face as you stone it. This isn't anything that's going to affect the seal of the rings, unless you go across the face of the ring accidently, just take some care. Wash the rings in warm soapy water after and blow dry. Afer assembly of the engine you will right away notice it turns very easy. I allso suggest you final clean the cylinder wall with automatic transmission fluid, it is a very strong detergent and will remove most to the honing grit. Your first oil change will be clean is no heavy wear is happening as the engine breaks in. As to why the rod broke, aside from possibuility of more drag at break in allowing some excesive clearance which gave the wrist pin some room to continue expanding the small end of the rod until it failed. Piston wall scuffing from either too tight of a bore or overheating will also expand the piston causing it to drag on the cylinder wall causing the rod to fail. Examining the parts correctly will tell how this happened. If the connecting rods small end was inspected before assembly and known to be within tollerance, I would look at the piston for signs of scraping. Usually the rod will fail close to the small end as leverage from the crank tends to bend the rod at this point if binding of the wrist pin is happening. Another area to look at is the wrist pin hole itself. If clearance was excessive, the wrist pin would pound the material and mushrooming would be obvious around the outside of the pin hole. Investigating why & how a failure occured is somewhat of a science but if you really need to know what happened, the clues will be there if you know what to look for and understand what their tyring to tell you. Hope this helps.

Ryan

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Don Holden

08-28-2002 22:15:55




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 Re: Re: Con Rod Failure in reply to Ryan, 08-27-2002 21:34:41  
Ryan,I appreciate your very detailed response on my question. Prior to my retirement in the Die Cast Industry, I was the Quality Assurance Mgr in a job shop that manufactured small engine connecting rods and I have looked at porosity and poor fill on many Con Rods using real time xray. We generally would find the porosity in the area of rod that was "filled last in the die" and we are talking mili-seconds of fill time! Because that area would begin to solidify before the intensification phase of the shot profile was complete and the porosity could not be squeezed out of the part. The casting dies are usually gated to allow the part of the die least effected by porosity to be filled last and that generally was the middle of the rod and not either end that has to be machined to the proper diameters. My experience is that usually the porosity/non-fill in Die Cast Con Rods would become evident in the machining process and the part be considered non-conforming and not shipped to the end use customer because of risk of catastrophic failure to the engine. Having said that, your theory of not replacing the con-rod or potential piston drag in the bore is a possibility! I have asked the rebuilder to analyize the fragments and look at the point where the rod failed at the wrist pin to see if he can visually detect a non fill or porosity/coldshut area that potentially contribute to the failure. If he can see porosity or non-fill in the area of the break he should notify the purchaser of the rods so they can evaluate their inventory to assure conformance to standards on the current inventory and get corrective actions in writing for the failure from the casting and machining source. Thanks again for your response and interest in my concern

Don Holden

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