broken con rod

[mumptia]

Had a new one in the shop the other day.

A couple of my students reconditioned a 3 1/2 Briggs from 3 other engines and got it running. And running smooth I might add. I was really impressed.

They measured all of the specs as they went but the con rod at the wrist pin broke as I started to power it up.

Initially, they let it idle for 5 or so minutes to help burnout the prelube we use for assembly and when the smoke laid off a bit I throttled it up just a bit and well, we all know the sound, snap and clinkle. The con rod gave way with the piston was at TDC

Spark-plug is the right length ( these are green high school students you have to check the things we normally assume:tongue

No water in the cylinder, no significant amount of prelube in the cylinder that would impede the piston stroke ( we had been running for 5 minutes already), inspected the piston and rings ( thinking maybe a ring jammed) all was as it was installed, cylinder was smooth ( no reason for a jam up)

Now I'm told the only thing they didnt check was the wrist pin dia. :wink:

Any ideas? Fatigued con rod atthe wrist pin?

Been doing this for years an have never had a con rod break there.

So now I'm stumped.

 

[KennyV]

Was the wrist pin out? There is very little movement at that point when it is operating, if it's tight or seized when installed ..it could fatigue and break within a few minuets run time... That's the only thing that I can see that would cause that type of failure within the time frame you experienced...
Press the pin out and look close for gaulding or temperature coloring...
If it was too tight you may not have much evidence of a jammed pin, but there should be flex fatigue at the break... If too loose it will have a lot of hammering indication. :smile:KennyV

 

[reynoldston]

Being a small engine and as I recall isn't the wrist pin free floating into the connecting rod. Not pressed. Also being from a used engine could the connecting rod have been cracked? As said earlier if it was too lose it would have made noise. The wrist pin should just push out with finger pressure and if not I would say it is too tight.

 

[mumptia]

No temp colors on wrist pin.

After looking at it again and again, there is definite wear spots but I noticed that the breaks aren't nice and bright like a metal break should be. Con rod might have been fatigued at that point and snapped on load I guess.

When we re-assembled it there was no noticable slop at the wrist pin so who knows

No worries, it was a dump pick-up and to see the students eyes light up when it ran was worth the hassle.:thumbsup:

I'll double check the wrist pin to see if it seized up.

 

[reynoldston]

Was I right in saying that the wrist pin is free floating into the connecting rod? It just seem in my memory the only time I pressed wrist pins into the connecting rod was in larger engines like car or truck engines ???

 

[mumptia]

Was I right in saying that the wrist pin is free floating into the connecting rod? It just seem in my memory the only time I pressed wrist pins into the connecting rod was in larger engines like car or truck engines ???

Yep. freefloating and not siezed.

The same student dropped the other con rod from the parts mower in it this morning and was beaming when he got the engine running again. (this time the con rod held after a warm up and a throttle test:biggrin

That kid is going far....

 

[reynoldston]

Yep. freefloating and not siezed.

The same student dropped the other con rod from the parts mower in it this morning and was beaming when he got the engine running again. (this time the con rod held after a warm up and a throttle test:biggrin

That kid is going far....

Good for him, maybe when he gets to be a old retied man like me he will be able to run a small part time repair shop like I do. I know I sure enjoy it. The only thing is when he gets my age he will need a good education to be a mechanic because I can see a lot of electrictronics in the future coming.

 

[mumptia]

Funny you say that.

I decided to take a small engine course to upgrade the electronics data base in my head.

My training is over 20 years old and some of the newer stuff coming in the shop has a bunch of wire and digital readouts.......

Looking forward to learning something new.

 

[ILENGINE]

Electronics on the horizon are EFI ( Kohler for about last 10 years) Spark advance timing built into ignition modules instead of a separate controller, EFI systems that control all aspects of the mower including optimum blade speed for conditions. Flat head engines going bye-bye and becoming all overhead valve or overhead cam engines. MTD brand engines made in china for all of there equipment. MTD mowers with the Murray name on them ( starting this year), all engine and equipment training completed online and not at a school (future Briggs, Honda, MTD). Equipment recertification completed online by annual testing.

 

[reynoldston]

Yes I can see it coming. I had a lot of training in electronics because I repaired electric fork trucks for 10 years. One of the very first things to learn is how to read a set of electric schematics. Ones you have that mastered you can do a lot. I am finding that the newer ATVs and motorcycles have gone to more electronics then the lawn equipment. I just repaired a Honda ATV this pass summer that shifted with a computer. This is going to be the place where the money is going to be made.