3/4 piston/spring stack order

Mike T

Well-Known Member
#1
Didn't have the manual when building mine, when thumbing through one yesterday I found the spring/piston to be flipped opposite of mine.

After searching it seems our cars were case-piston-spring which should produce a softer shift?
Others came from the factory case-spring-piston (harder shift) ?

Seems backwards........... I would think a firm shift in the GN.
 

INEEDAGN

Seen Your Member
#2
Putting the piston in the case first and then the spring leaves the 4th accumulator active. If you install it the other way, the spring will hold the piston in the seated position, it will fill the cavity with fluid the first time and be the same as no accumulator. Since you're rarely allowing a 3-4 shift at full throttle there's no reason to put up with the harsh shift into overdrive and the associated strain on hard parts.
 

Mike T

Well-Known Member
#3
After putting it back together it's so soft I wasn't sure it was shifting. Don't really feel it bump until you manual shift around 55 mph. Has the extra clutches also. Has a 19 blade ptc so maybe it's cushioning the shift?
 

INEEDAGN

Seen Your Member
#5
I stand corrected. If spring was in case first, accumulator pressure would act against the spring and would move the piston allowing it to still be active depending on throttle position/calibration. Brain fart!
 

Mike T

Well-Known Member
#6
Sounds like it all depends on where the accumulator feed hole is located. If there were no spring at all (for illustration purposes) what direction would the piston bottom out with oil pressure? Would it go deep into the case or bottom out against the separator plate?
 

mikestertwo

Well-Known Member
#8
Sounds like it all depends on where the accumulator feed hole is located. If there were no spring at all (for illustration purposes) what direction would the piston bottom out with oil pressure? Would it go deep into the case or bottom out against the separator plate?
The piston will bottom out in the case from accumulator pressure until the 3-4 shift. Then it would move against the separator plate.
 

mikestertwo

Well-Known Member
#9
Seems like I've tried them both ways, couldn't tell much difference.
That's because the stock spring is relatively light. Most shift kits supply a stronger spring for the 3-4 accumulator. The Art Carr kit supplies a stronger spring that actually goes into coil bind when compressed shortening the stroke of the accumulator piston.
 

Mike T

Well-Known Member
#10
For a firmer shift it should be piston then spring in the case. In this scenario the spring assists the accumulator pressure giving a firmer shift.

I'm missing something. If the piston moves to the direction of the separator plate on the 3/4 shift wouldn't the firmest shift be spring in case first then piston (bottoming the piston to separator plate). The accumulator would always be filled with fluid which could not compress and in a sense totally eliminate the accumulator?
 

INEEDAGN

Seen Your Member
#11
The reason that's not the case (and the reason for my above brain fart) is the accumulator valve in the valve body that puts accumulator pressure on the opposite side of the piston. Depending on throttle position etc it may or may not have enough prressure to overcome the spring pressure so the piston will still be partially or fully toward the case when the shift occurs.
 

mikestertwo

Well-Known Member
#12
INEEDAGN is correct. With the spring in the case first then the piston it acts like a variable accumulator depending on throttle position. At light throttle the accumulator pressure is low so the spring will push the piston towards the separator plate giving little accumulation. As the throttle is opened accumulator pressure increases pushing the piston towards the case which increases it's travel giving you more accumulation which softens the shift. That's why a lot of shift kits will give you a stronger spring for the accumulator valve to increase accumulator pressure and then supply a stronger spring for the accumulator piston and in the case of an Art Carr kit a spring that coil binds relatively early and shortens the stroke of the piston to help give a firmer shift. With this setup the shift will be firm at all throttle angles.
 

Mike T

Well-Known Member
#13
Once again thank you guys for going into such detail. Takes years of doing it the hard way compile the knowledge that you guys are sharing.
 

David Husek

Turbo Buick Performance
#14
Didn't have the manual when building mine, when thumbing through one yesterday I found the spring/piston to be flipped opposite of mine.

After searching it seems our cars were case-piston-spring which should produce a softer shift?
Others came from the factory case-spring-piston (harder shift) ?

Seems backwards........... I would think a firm shift in the GN.
I have seen them both ways from the factory in a BR. Most are up against the spacer plate which is the only way to run this circuit in my opinion assuming you have a real calibration. It also helps with hard part failure when set up correct.
 

Mike T

Well-Known Member
#15
I have seen them both ways from the factory in a BR. Most are up against the spacer plate which is the only way to run this circuit in my opinion assuming you have a real calibration. It also helps with hard part failure when set up correct.
Assuming you mean spring to plate? Prefer the stock spring or stiffer ?
 
#17
What is the hole size for a GN for OD. I think the TransGo kit I did yrs. ago was .110 too big, harsh shift into OD. Eventually cracked the OD carrier.
 

David Husek

Turbo Buick Performance
#18
Stock size in an X plate is about .063". I do not like to see a hole size bigger than .078" even with upgraded parts. It just gets too abusive. You should also have a 3 clutch OD.