4 Speed 23 Spline to 18 Spline Conversion

[69Beeper]

Has anyone converted their 23 spline to 18 spline? I have a 23 spline numbers matching case from my 68 RR and I'd like to use an 18 spline input shaft and whatever else is 18 spline specific. Has anyone accomplished this before?

 

[greenmachine]

Has anyone converted their 23 spline to 18 spline? I have a 23 spline numbers matching case from my 68 RR and I'd like to use an 18 spline input shaft and whatever else is 18 spline specific. Has anyone accomplished this before?

Give passion performance a call. They should have the info you need.

 

[tallhair]

I'm sure Brewers or Passon can help unless you already have the parts. If I recall the 18 spline input shaft bearing is larger diameter then a 69 23 spline.

 

[Hemi Rebel]

I have inquired with passon on this. Its completly different all the way through. It would be easier and more cost effective to just buyan 18 spline. I got mine for 600 right here on fbbo. They turn up from time to time.

 

[68GTX4SPEED]

18 spline case is much heavier and stronger then the 23 splines case.

 

[62MAX]

18 spline case is much heavier and stronger then the 23 splines case.

That is not correct,the case is cast the same,it's the internals that are different.

 

[gford]

the case is the same if you have a 308 big input bearing I think 69 down have big bearing case

 

[BeastlyBirdie]

Umm, why are you trying to do this? The 23 splines work fine until you get under somewhere around 11 seconds with a 3500ish weight car. Below 10, and the 18 splines start failing. By both those points, you're running scatter shields, lockout shifters, etc. So, in a typical street car, a 23 is fine....

 

[Meep-Meep]

Agreed that you can get away with a 23 spline on the street with street tires just fine. But if you want to do the conversion it can easily be done after you buy the very hard to find 18 spline gear set, or the expensive 18 spline OD gear set from Passon. The only difference are the bearings. You will gut your old 23 spline and replace all the gears except the reverse idler, and use the 307 bearings. The result will be very much like the early hemi 4 speeds that used the smaller bearing.

 

[BeastlyBirdie]

There's a set over on moparts for sale right now...

 

[69Beeper]

Thanks for all the input, guys. Sounds like the 23 spline is a little more durable than I thought. What kind of horsepower would you be comfortable bolting up to it for occasional track use, in the neighborhood of 12-13 second passes? It would be a full weight car with drag radials (68RR).

 

[Cranky]

You should be more than fine running 12's. We didn't start killing stock boxes (23 or 18 spline) until very low 11's for the 23 and mid/high 10's for the 18 and launching at 6k with a 3000 lb car. Another thing you can do to make sure your trans last is to make sure the bell housing/trans fit is as close to '0' runout as possible.....face and fit. If you have more than .005" runout, you need to fix the problem and the closer to 0 you are, the better.

 

[69Beeper]

You should be more than fine running 12's. We didn't start killing stock boxes (23 or 18 spline) until very low 11's for the 23 and mid/high 10's for the 18 and launching at 6k with a 3000 lb car. Another thing you can do to make sure your trans last is to make sure the bell housing/trans fit is as close to '0' runout as possible.....face and fit. If you have more than .005" runout, you need to fix the problem and the closer to 0 you are, the better.

Excellent- Thank You!!