904 18 spline input shaft strength

[Barra_67]

Hi all.
I have a rebuilt 904 which has the early 18 spline input shaft. What sort of power is the smaller shaft good for. Wondering if it’s worth the effort to change to the later 27 spline? 318 that makes roughly 400 HP.
Street cruiser. Will the 18 spline input shaft be the weakest link?

Or should I look at a 727. 727 will last but takes power and if not built properly will be a bit of a slug.

 

[moparlvr166]

I would not worry about the strength of the shaft, but I would upgrade to the other shaft and pump due to a better selection of Converters.

 

[Charrlie_S]

If you have a "good" converter to go with the early 18 spline, I would stay with that. If you need to get a converter, then, convert the trans to 27 spline, and get a 27 spline converter.
Forget the 727

EDIT: What size is the converter pocket in the end of the crank. In the USA, the crank pocket changed size between 1967 and 1968. If you have the early small pocket, you will have to use the 18 spline converter. If you have the later large pocket you can use either converter. The 27 spline converter fits the crank "as is". The 18 spline converter requires a adapter bushing in the end of the crank.

 

[Barra_67]

If you have a "good" converter to go with the early 18 spline, I would stay with that. If you need to get a converter, then, convert the trans to 27 spline, and get a 27 spline converter.
Forget the 727

I have a new good converter that come with the box. 727 no good. Overkill?

 

[Charrlie_S]

727, Not as efficient, and extra weight. Plus you would need to spend the money for all the extras. Even a shorter driveshaft.

 

[Charrlie_S]

When I say "Good" converter, I don't mean "not broken", but one that is stall speed matched to your particular engine.

 

[Barra_67]

When I say "Good" converter, I don't mean "not broken", but one that is stall speed matched to your particular engine.

I do.