Third broken input shaft

[Tusabes]

I totallyagree with just getting a stock tranny on eBay and trying that

It's your lowest cost possible fix

[dratts]

I want to thank you guys for the responses I've gotten.  I don't know if missalignment is the problem, but I for sure will look at it and take it into consideration.  I was totally unaware of this possibility until you guys spoke up.  Thank you again!

[SchulzeA]

I talked to someone who had a similar problem as TonyPaul described. The guy kept having issues with the seal around the output shaft. It was replaced 3 times under warranty on a brand new transmission. Finally the dealer replaced the transmission and sent out the original one to be inspected. Turns out that the bored holes for the output shaft and seal were out of round slightly.
So maybe that's your issue and it's causing the shaft to walk or bind, then break...? Maybe finding a new transmission case will solve your problems.

[tonypaul]

I completly understand your frustration, kinda had the same situation many yars ago on a drag car I had purchased. I kept breaking the input shaft on a powerglide. Had it rebuilt 4 times and it kept breaking. Finally the trans guy said get a whole different powerglide, and he would build one from scratch. Sure enough I got one from a salvage yard, he built that one and I never had another transmission problem. Come to find out the 1st powerglide case was warped, dont know how it happened but under normal daily driving it was fine. Put some power to it and it would snap the main input shaft.

I know it sucks, but maybe you could just start with a whole different transmission core and start fresh. Good luck what ever you decide....

[Onewickedsvt]

You have any pics?

I am interested to see this. 

[dratts]

Is it possible to misalignment the input shaft?  Kevin at cartuning sold me the turbo kit and I sent the transmission to him for beefing up.  It has a shift kit and an oversized piston for better clamping in third gear as well as better clutches and a few other things.  He is going to send me more information, but he's leery of giving away his secrets, which I understand.  We put a performance torque converter in the last time, but we had Kevins tune on it and I believe that there was nothing weird or bypassing done on the converter.  Thanks for the suggestions!

[jdinner]

So, there is no chance of a misaligned input shaft?
When you say "built" transmission, do you mean it has a harsh shift kit in it?
If so, you might want to go with a stock shift pressure to reduce shocks.
Are you bypassing or doing something weird with the converter lock-up?

I find it very hard to believe that is happening with an auto trans.
Try a scrapyard transmission before spending any more money on it.

If you want to cryogenic treat it - send it to me, I'll perform the voodoo here and charge a fair price. (kidding of course)
Cryogenic processing really only helps wear, not strength. So the surface protection may improve but it will still snap under shocking conditions.

[dratts]

The 300m and aermet 100 alloy shafts are far stronger than any cryo treated shafts.  People are putting 600 hp through the 300m shafts with few failures and the aermet 100 was engineered for over 1000 hp.

Did you consider having the shaft cyro-ed before you install it?

[dratts]

The previous shafts just sheared at the narrowest part.  I appreciate the offer of your technical guys and I'll stay in touch when I get the tranny out again.

You gotta have something else going on other than the power breaking the shaft. The odds of you breaking 3 input shafts in a nonracing situation is almost unthinkable.
Can you post up some pics of the other shafts where they broke? Are they twisting or snaping apart? If you put some pics up I will show them to a guy I know that used to be on a crew on a GM sponsered NOPI drag car. He is the man on performance FWD GM transmissions...

[dratts]

I should have been a little more clear.  The current transmission is a built version of the 4t65ehd that came with the ls4.  I started out with an isuzu hooked to a 350 sbc.  My son in law broke that one.  No surprise and no blame.  The next one to break was the stock 4t65ehd input shaft.  Replaced with a 300m input shaft and a 7/8" chain.  Broke that one immediately.  Next one was with a $1000 aermet 100 alloy input shaft and the built transmission from cartuning.  It went when the torque converter let go and filled the tranny with debris.  Then we cleaned the transmission out and installed a built torque converter.  That was what I was running with a fresh tranny tune from cartuning when it let go again.  I put the car away and haven't done anything yet out of disgust.  I strongly suspect the input shaft, just because it felt then same as the last time it broke.  Under hard acceleration, not from a stop when I lose all forward gears and reverse.  Plan on replacing the input shaft with another 300m because there is nothing stronger and the. Just drive it with severely reduced throttle (like an old guy, which I am anyway) until someone can figure out what is going wrong.  I know that Fiero guys are doing low 10 second quarters without all the mods I've done.  They have to be putting down similar horsepower, but with a shorter wheelbase which should put more weight transfer, but 600 lbs lighter weight than mine.  Anyway I will be back on the road soon with a lighter right foot.

[jdinner]

It sounds like you are running a stock Fiero transmission. (Dinner Plate is for the Audi).
I believe this issue is caused by one of two things or both.
1) Is the clutch friction disc equipped with torsional springs? If not, replace it with one that does.
2) Are you trying to put down a patch of rubber?

Keep in mind you are now heavier, you have more rubber contact on the road and you are running more horsepower. It is going to break. This is not a matter of 'it might break' it is going to break regardless. You need to NOT shock the driveline.
Let the clutch out easy, get rolling and then hammer on it. It is the sudden shock that broke it and no person will convince me otherwise. I have seen it way too many times.
The only other thing it could be is a block of steel floating around inside the transmission that is jamming the gears.

[msaby]

Did you consider having the shaft cyro-ed before you install it?

[SchulzeA]

What's the Dia of the shaft?
Some issues I found while researching transmissions comes down to bearing support within the transmission. Not having enough support allows the gears to push apart when you apply a lot of tq, which causes the shafts to bow. Normally it just breaks the gears but maybe your gears are strong enough and it's the shaft giving way. I think Jdinner made a plate for this issue..? The "Dinner Plate" 

[tonypaul]

You gotta have something else going on other than the power breaking the shaft. The odds of you breaking 3 input shafts in a nonracing situation is almost unthinkable.
Can you post up some pics of the other shafts where they broke? Are they twisting or snaping apart? If you put some pics up I will show them to a guy I know that used to be on a crew on a GM sponsered NOPI drag car. He is the man on performance FWD GM transmissions...

[cementhead]

It is probably not so much the horsepower, but the torque. I was having the same kind of problem with my Porsche. I built a new motor now with a different torque curve. Less low end torque with more mid and high end. High horsepower low torque if that makes sense. That and no hole shots. It seems to be working for me.