My friend has a 2009 4l65E that has lost the forward sprag twice now. What would possibly cause such an issue?
It is behind a stock but transplanted LS3. I have rebuild more than a few of these and have never observed this issue.
BTW it wasn't me that rebuild the transmission.

 

his driving style is likely the issue

 

Could be, driving style, but the transmission only made it 200 miles. The owner is 70 years old, and likes spirited driving but it's not like he is 16 LOL. The old part looked more like a lube failure than anything. I only had pictures to look at, I would post them but I can't find them right now. The sprag was replaced with a new heavy duty part on the last rebuild. The car only has 8" wide tires. This transmission should be able to survive almost any hp with only the baby sized tires. Wouldn't you think so? These transmission were stock behind LS2 with 400hp with 12" wide tires. Originally this transmission was purchased over the internet. Didn't work out of the box. It has been assembled with force and the rings on the pump were cut in the installation process. So this is the third time this has been apart in 200 miles. Sorry should have included this information in the original post. I never recommend internet transmissions unless they have been dyno ran first. This has been a nightmare for the owner.

 

Servo, cushin springs, accumulator setup, orifice sizing all have an effect on this. How is the trans setup?

 

Not much is going to affect the input sprag other than shock load

the most likely cause is that he drives it around in D4 and likes to go WOT and the thing downshifts 4-1 and just slams the hell out of the input sprag

If the thing shifted hard enough 1-2 for the thing to break...it would be miserably firm to drive around...so I doubt accumulator and orifice size etc to matter much unless this thing has other serious driving complaints

Another common mode of failure is wheel hop. Because that loads and unloads the sprag in quick succession and again...slams the hell out of the sprag.

Are there other causes? absolutely. And we can get into the weeds if we need to. But I've seen this story play out before and 9/10 times it is just driving style and/or wheel hop

 

Reusing worn inner and outer sprag races can cause premature sprag failure also.

 

bbond
You're very correct

I sometimes forget to mention things like that because I assume it's common sense to not use worn out stuff but You're very good to mention it for the average reader
YES...don't use junk parts!

 

Also if you take a Sprag Assembly apart and find that only one of the Races were making contact with the Sprags...
Purchase new Races with your new Sprag Assembly.

I do not trust the Tolerance Stack-Up in that situation to handle High Power (it otherwise will probably operate fine under normal driving).

 

I would also suggest installing the Sonnax HD 2-3 shift valve and telling the owner to do his spirited driving in D3. With this valve AND in D3, the Overrun clutches are applied in all gears which reduces the load on the forward sprag.
https://www.sonnax.com/parts/2464-he...-3-shift-valve