This guy I know has a 700R4 and he says it has a broken sprag. What is that? Are there more than 1 inside the trans? To fix it, do you have to take apart the entire trans?

Thanks

 

A sprag is a one way clutch that locks second gear. It's a little hard to describe. Think of a pull start on a lawnmower. When you pull the cord, 2 little dogs come out and grab the flywheel to crank the engine over. They only grab in one direction. The sprag in the tranny works very similar. It's designed to grab in one direction but release under decelleration.

The downside is that there are something like 9 little dogs that grab in the TH350 and I think also in the 700R4 since it's a similar tranny. It doesn't take much abuse to destroy the sprag. A TH400 has 34 and is much stronger. The tranny has to be removed and disassembled to replace it. Depending on how much damage there is, more parts may have to be replaced.

I destroyed a sprag in a TH350 early in the season last year at the track. I had to replace the sprag, second gear clutch hub, decelleration band and oil pump in the tranny. $50 for a used tranny from the junk yard got me all the hard parts I needed. All the clutches were new so I didn't have to do a complete rebuild. I raced the rest of the season with that rebuild.

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A sprag is a one way clutch, locks one way and frewheels the other (sort of like a ratchet).

A 700 uses an input sprag the holds on acceleration in 1st-3rd, then overruns in 4th. A sprag is strong but the downside is that it wears over time. This is a common wear part/failure in 700's and it is agravated by low rear end gearing.

They also use a roller clutch for hold in first gear acceleration that except for when people do neutral drops, seems to last for ever but isn't as strong.

The very earliest 700's' used a roller clutch for the input but it didn't hold up. They have gone through three different sprag designs since that, a 26 element, 28 and now 29. The latest 29 when used along with the latest O/R clutch hub is a real winner. I don't think I have EVER seen one of these fail, even in high mileage cars and until I see different, is the setup I use for special jobs.

 

Leo, I have seen one 29 element bad, but is had to have been a manufacturing defect because it came from a 2.2L Sonoma. I really doubt the Torque killed it. I get to see alot of funky transmission failures, usually low milage factory bloopers I guess.


The earlier sprags were also shorter.

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I get to see them on the other end, 100-200K. I am sure that yours was probably more of an anomoly. Was it by any chance an earlier one, you know, not using the flat O/R clutch hub? Those are the ones I am refering to. I have heard that the early setup didn't always work well. I don't think it was so much the sprag as that hub it was against.