727 explosions like the pic above are caused by after market valve bodies that don't apply the low band. A bolt in sprag is a good idea but it would not have prevented that.
Here's some good info. from John Cope from Cope Racing
There has been a lot of talk about aftermarket manual valve bodys with the low band apply feature,and for good reason. First, you should understand how the low/reverse band in Chrysler transmission works. In a stock 904 or 727, the low/reverse band is applied when the gear selector is in R / reverse and/or L / first gear, sometimes called low one.When the gear selector is in the D position and the transmisssion is in first gear, the low/reverse band is NOT applied. So, the low/reverse band does not have to be on to get first gear. When the low/reverse band is on or applied, the band is applied around the low/reverse drum. The low/reverse drum is splined to the overrunning clutch, also called the sprage.
727 low/reverse drum,low/reverse band,sprage race
When a 727 or 904 transmission is in first gear, D, or L the sprage is in the locked position. ( note: the sprage free wheels in 2nd & 3rd gear) Having the low/reverse band on will keep the sprage in the locked position under heavy or high performance loads. Now, the sprage AND the low/reverse band are working together to keep the low/reverse drum in the locked or stopped position. You can try this in your rear wheel drive mopar or your Dodge ram. Test drive the car or truck with the gear selector in D. Note the feel of the 1-2 shift, then put the gear selector in L, shift to 2, and you will feel the bands overlap. The shift will not be as quick and or firm. (note: the KD band comes on / is applied in second gear) This is the problem that aftermarket valve bodys had when making the 1-2 shift in racing transmissions. As we all know, when racing we want quick/firm shifts, if not, we would have probs with burning up bands because band over lap is having both bands on at the same time, or the timing of the bands coming on then off to make a clean and firm 1-2 shift. So what was the only solution (at the time) to make the 727 & 904 have a clean and firm 1-2 shift? Leave the low/reverse band off/unapplied in first gear. Remember, the low/reverse band does not have to be on/applied in first gear, to get first gear.
Now, by leaving the low/reverse band off, the sprage has to do all the holding of the low/reverse drum. This now makes the sprage a weak point in the 727 trans because the low/reverse band is not applied along with the sprage.
What happens to a 727 when the sprage fails? The transmission will free-wheel at 2x the eng. RPM. When this happens, and the stock steel front drum is in the transmission, the front drum will explode.
This 727 was in a 11 second bracket car,that did NOT have low band apply.
How fast are you going ?
Now that you know what can happen when having the low/reverse band off/unapplyed when running a stock steel front drum, it's like having a time bomb strapped to your car. Do you want a valve body with low band apply? I know I do.
How can the 727 sprage fail in a racing transmission ?
1. Any time the sprage is shocked, you break a U-joint, axle, or ring&pinon, it can cause sprages to fail
2. Driver error. In the heat of competition/excitement a driver could accidently nuetral-drop the transmission. I know what your thinking, I've never done that. But it could happen.
3. If the carb or ignition causes a back-fire and the car noses down and picks back up, or if you get in and out of the throttle while in first gear, you can damage the sprage.
4. If the car hooks, then the tires spin, then hooks again, the sprage can fail. And I know none of us have EVER had traction probs. (yeah, right)
5. Doing burn outs, starting in first gear is the biggest no-no of all. Burn outs should be done starting in 2nd gear and shifting into 3rd gear.
All 5 conditions are while in first gear.
How can you tell if your manual valve body does or does not have low band apply in first gear?
With the transmission in first gear and driving about 20 MPH
, let out of the throttle, if the car goes to idle and the motor does not help the car slow down, (engine braking) you do NOT have low band apply in first gear. If you car slows down and the motor helps slow the car down, you DO have low band apply in first gear. Note: If you know you have a valve body with low band apply in first gear and your car does not have engine braking, your low reverse band may be out of adjustment or you may have a broken band strut, or you have a cracked low reverse pistion. Checkng to see if your low band apply is working ,you can test this by checking the engine braking in first gear.
I must let you know that Rick from A&A transmission is the leader in low band apply technology. All CRT transmissions with manual valve bodys and trans-brakes, come with the low-band technology.
CRT is a authorized A&A dealer.