Checking Sprag

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727 PART - A&A Transmissions

Here’s the A&A piece... personally I’d call Pro Trans (they’re now in Waco TX) and see what they have to offer, but that’s just me as Dave and Darren treated me well when they were in CA.
 
moper said:
“, the stock drum can turn 15k rpm and at such rpm is a ticking time bomb”

The drum speed is normally much lower. About 1/2 engine speed when the sprag is functioning. If the sprag breaks or fails, the drum accelerated to 2-3 times engine speed. It explodes normally around 7-8k, or 3500-4K engine rpm.
Best defense is not a bolt in sprag, but the billet drum.
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If the sprag lets go, the drum will spin at 2.2 times the engine rpm
So if it lets go in say the water box at 7k, that is 15,400 rpm... and the reason i said 15k in my post.
TCS has tested various drums.
Stock drum let go at 14,000 rpm
Steel drum 25,000 Rpm
Aluminum drum 36,500 rpm
Cars see 7 k a lot, sprag failure at that or anywhere near it results in the drum turning very fast in the opposite direction( over twice that speed).... equals not good. And lots of the parts in these trannies are 50+ years old, so might not hold as much as test results would suggest
 
Also - you can't see anything by looking up through the pan opening. It has to come apart and the parts looked at "from the pump end" without the low reverse drum in place.
 
B3422w5 said:
If the sprag lets go, the drum will spin at 2.2 times the engine rpm
So if it lets go in say the water box at 7k, that is 15,400 rpm... and the reason i said 15k in my post.
TCS has tested various drums.
Stock drum let go at 14,000 rpm
Steel drum 25,000 Rpm
Aluminum drum 36,500 rpm
Cars see 7 k a lot, sprag failure at that or anywhere near it results in the drum turning very fast in the opposite direction( over twice that speed).... equals not good. And lots of the parts in these trannies are 50+ years old, so might not hold as much as test results would suggest
Click to expand...

Sorry - the info I had is very different on the rpms the drums would survive at. It's a lot lower than what you're saying. I know of two that have let go at engine speeds below 4K. One while on the two step, one as the car was moving in 1st gear.

Maybe I'm confused...lol. Been a long day.
 
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moper said:
Sorry - the info I had is very different on the rpms the drums would survive at. It's a lot lower than what you're saying. I know of two that have let go at engine speeds below 4K. One while on the two step, one as the car was moving in 1st gear.

Maybe I'm confused...lol. Been a long day.
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I'd say it depends on the drum. As Don mentioned these things are old and who knows how much abuse they've seen.

The only one I've seen let go was at the drag strip on the hit. It survived the water box but exploded at the hit.

My 727 has a steel drum in it.
 
drum1.jpg


I ran the one on the right into the early 90’s.... was also lightened on the back. Wonder what it would spin up to? :)
 
guys
do you not hold with the band and not the sprag?
sprags kill (with the front drum)
at the 904 conversion in the 727 case is another way to make a safer trans- and lighter
as for the water box
hitting traction with the tires lit up from the water box is really dangerous and hard on parts
back off while doing the burn out
we learned this on the max wedge c.a. 1962 doubt much has changed
 
In 1st gear the only “fixed” part is the low reverse drum. In breakaway first the low reverse band is holding that too. In most bargain manual valve bodies and automatic valve bodies in Drive, the band is not applied in 1st gear. The kick down drum is spinning because the forward clutch is on, and it’s spinning slower than the input rpm because of the planetary gear between them.
 
SBeeMan said:
So really no way to check the sprag other than taking the trans back down?
I kinda thought that lol
Knowing there are some really smart members on here I just wondered if it could be checked in the car.
Click to expand...
If it has a stock front drum it is not worth the risk to guess. I know it's a pain but you gotta take it apart and check everything, your legs and your life are depending on it. And while you have it out, if you haven,t already, spend the money on a steel billet or aluminum drum. It's just not worth the risk. IMHO
 
Thanks for all the info.
I'll be pulling it out today & will add a new drum.
 
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