Broken Tranny - The saga continues

B

backformore

New member
The saga continues. I discovered why I lost drive at the last (first and only) race with the Scirocco. The main/input shaft for the tranny sheared at the splines. Is there any particular reason this may have happened? Is this a common failure point? Any suggestions on fixing or who to have rebuild it. I'm in Gainesville, GA so I will probably contact OPM first since they are local and used to race VW.

Any input will be appreciated.

Rory
 
You must have some SERIOUS ponies in that Scirocco. I've never even heard of sheared splines on the input shaft of a VW. At least I've never had that happen to me in over 25 years of driving / racing VW A1 Chassis.

Who is your engine builder? I need to talk to him.
 
Yeah Tim, I'd like to believe that is the problem. That is the kind of luck I can have. The "wow, I've never seen one do that before" kind.

I'm heading to Topeka next week, but if I think about it after I get back, I'll try to post pictures of the pieces.
 
Rory,

020 VW boxes are pretty stout, and can tolerate abuse w/ an excess of 200hp. Honestly, there must have been something defective for the input shaft to fail. I've heard of people blowing gears and such, but in the 20+ years I have been involved w/ VWs, I've never heard of or seen and input shaft fail. OPM can probably help you out, but Stu and the folks at BSI have probably built more 020 boxes than just about anyone else in the country. That being said, you should be able to pick up a used box for your car for not a whole lot of money ($100 - $200).

As far as speculation as to what caused the failure, as I said, I can only imagine it was a defective part. Although, someone could have dropped the trans or possibly bent the input shaft as it was being installed. That's a really odd place for one of those things to fail.
 
If I pick up a used tranny, how difficult is it to swap the final drive. This one has non-stock parts (R&P). Is that something I should leave to the pros? I may actually have a spare tranny. I have to go take a look at it to see if it is an 020.
 
It's not an undaunting task, and if you're mechanically inclined, not that hard. Need some special service tools, but it's doable w/o them. Up to you if you want to tackle it.
 
Make sure you have your line up dowels in the block, the two bottom corners I think there should be dowels to line the tranmission up. Usuallu the clutch disc would break before the input shaft, and the case is probably shot the front bearing isn't really made to accept much load.
 
You'll want to be sure that the 'box you bolt your parts to has the right internal ratios. Check the GCR for the numbers, then refer to one of the online sources for the gearbox code you need to get the legal guts.

K
 
Do you have a welded diff?

I broke a beefy axle once due to low speed axlehop.. that would do it.
If its an open or LSD diff, then it truely is a mystery...
 
Spinnetti, I've run a welded front diff since 1993. The only failure I've had has been an inside CV joint while on track. I do change out the entire 1/2 shafts every year, along with the hubs/bearings.

I agree with Bill, that something else might have resulted in the input shaft splines being damaged.

What type of clutch setup are you running?
 
Let's try to do some failure analysis. I used to break input shafts for a living when I worked in the Test Department at Allison Transmission, so I do have some experience in this area.

Did the shaft break into two pieces? If so, look closely at the formerly mating surfaces. If the shaft broke from pure torsional shear, the entire surface on both pieces will be rough. However, if you can see where part of the surface is kinda smooth while the rest is rough, then the break started at the smooth area. If the smooth area is near the surface of the shaft, then there was some defect at the surface that started the crack which lead to the failure. The defect could be a stress riser left by machining, or a stress riser left by the hardening process, or a stress riser resulting from a material defect. You may see "beachfront" waves across the shaft--this is an indication that the shaft was cracked for a long time before it broke.

If the shaft is just cracked, not broken, then you can look closely at the crack to see where it started...usually at some stress riser. We used to do a lot of dye penetrant testing in our fatigue testing lab because it was quick and did a decent job of showing surface cracks. You can buy dye penetrant kits and I'd recommend using one periodically on parts that are subject to fatigue failures so you can see the cracks before you suffer a catastrophic failure.

Bear in mind that a shaft loaded in torsion is very dependent on what happens at the surface of the shaft. The stresses are greatest at the surface. That's why hollow shafts are used in some applications. Hollow shafts have almost all of the strength as a solid shaft, but you save some weight.

Torsional fatigue generally isn't a problem with the transmission input shafts on small cars. The loads are just not high enough. On the other hand, when you are getting 400 horsepower out of an 8-cylinder diesel engine that's only running about 2500 rpm, the torsional stresses passed to the transmission input shaft are massive.

As someone else said, make sure you use the dowel pins between the engine and transmission as they do most of the alignment, not the bolts. Any misalignment in this area will be hard on input shafts and bearings.

Bob...
 
When you say "sheared" do you mean it sheared the teeth off or the whole shaft snapped?
If the former check to make sure you got a clutch disk with the right spline diameter.
There are many being sold out there that have the right # of splines but barely engage the teeth.
 
This ain't Walt, its his dad.

Never seen a "stock" input shaft broken either.
Could be caused by:
1. Bent input shaft can cause a fatigue failure - could happen during installation.
2. Is there any chance that two (or more) gears were engaged at one time - can happen
3. Pinion shaft bearing failure - locks everything down and who knows what the next week link is.
4. Mainshaft ball bearing. This one can fail if improperly installed (preload, etc.)
5. Plus a lot of the things already mentioned, dowels, etc.

Rebuild involves a lot more than just replacing mainshaft. Bearing pre-loads on pinion shaft, syncro clearances, etc. Takes some special tools and ability to use a dial indicator.
 
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