The car I am working on was originally a 23 spline 833 4 speed transmission. I have read many times on here that the 18 spline unit is much stronger. Could someone clarify what specific areas are beefed up?

Also this is probably a dumb question, but isn't more splines on an axle stonger? Why is this different for the trans shaft, I.E 18 splines are stronger than 23?

Thank you for any input!

The 18 spline input shaft is thicker than the 23 spline on the A833s.

OK, that makes sense. I was unaware that there was a difference in diameter. Thank you!

The input and mainshaft diameter is larger, the gear teeth are also thicker than the gears on a 23 spline. The bearing diameters are also larger. It's just overall, a heavier duty gear box.

They made hemi 4 speeds with the smaller bearings only in 1966 and 1967. That's why they have the small pilot hole in the bellhousing.

The main shaft is the same between 23 and 18 spline Trans.

Yes the input shaft is larger diameter. Which makes the front bearing retainer and throw out bearing larger also.

All the NP833 18 spline trannys I've work on, more than 3 , all had bushings inside the gears also.
NP offered multiple different gear ration in both the 23 spline and the larger 18 spline, The Blue Stripe 18 spline ratio was suppose to be very close to the early BB 23 spline ratio, the red stripe, I may have these ratio offerings backwards, had closer 2nd,3rd ratio for drag racing.
Are you going to drag race this tranny? If so the clutches offered today would probably work just as well on either tranny size saving you from spending money on the larger more expensive 18 spline parts
I've worn the sliders, brass and small ears off of both sets of 18 and 23 spline gears in the 2.65 and 2.66 ratio as well as the early A body 3.09 gear ratios
I switch from the stock Slick shifted gears to the Pro shifted gears and still had a lot of maintenance on them
I haven't ran any of the newer face plated gear sets, their suppose to be better and stronger than the slick and Pro shifted parts as well as your able to shift them without using the clucth

Face plating refers to the engagement type installed onto the gear and has no bearing on the gear strength. This DOES NOT make it a clutchless transmission and YOU WILL destroy the gears if you try it. It is designed to be a very low maintenance setup as opposed to Pro Shifted gears.

X2

Also, the difference in strength between the two is about 100ft/#'s and HP. 23 spline is supposed to be good for up to 450 hp/torq. The 18 spline is supposed to be 100 better.