I hope this is a simple question..

I'm trying to find out what the difference in spline count is on the 914-4 ver 914-6 torsion bars. I have a 1975 914-4 and want to go to 21mm but it seems most suppliers have mixed the 914-4 with the 914-6 and the 6's don't fix on the 4's splines.

thank you in advance with your answers..

The search function is your friend.
Answer found in this thread. Click me

914 torsion bars have 29 splines, 911 have 30.
Ask me how I know... I've been shipped 911 bars TWICE.

And they are "sided" so be sure you install them on the correct side of the car. They are stamped in the end with "R" or "L". At least the stock ones are...

epic troll but I had it coming... thank you

From what I have read, there is actually no difference in the L and R torsion bars as they are made. They do, however, "take a set" after being stressed in one direction for years, and reversing that stress tends to cause failures. So getting the "L" on the left side is only important if it came out of the left side...

Note that flipping the bar around front-to-back actually has no effect on the direction of stress through the bar, so getting them in the same orientation that way is not critical.

--DD

It does help to know what you're looking for in advance.
FWIW, the answer is also in Dr. 914's 700 Tech Tips book, as Ed pointed out to me a little while ago.

914 torsion bars have 29 splines, 911 have 30.

Why? That seems pretty arbitrary and pointless.

Those crazy Germans fell victim to the "more is better" syndrome...?

I find my car handles better, and is more comfortable when I leave the bars stock and adjust the suspension with an adjustable sway bar like Tarret...

if I understand you correctly, I don't see any possible way that is true. the torsional load through the long axis of the bar reverses if the bar is flipped front to rear on the same side

I scratched my head on this one, too. Ended up using a pretzel rod I was eating to prove it out:
Hold the pretzel as though it's the right (pass) side and you're standing on that side of the car. So the rear of the torsion pretzel is in your left hand, front in your right.
The rear (your left hand) is fixed in the cross member. As the suspension compresses, the front, outside twists up (clockwise, if viewed from the front). The rear is held in place but essentially, to counter the twist, it would otherwise have to twist counter-clockwise (if viewed from the rear).
I had to mark my pretzel (?!?!) to note the direction of the twist on each end.
Then I turned it around. Applying the same suspension force causes the same directional twist.
It took me a few times to prove this to myself but I'm with Mr. Darling: So long as you keep the torsion bar on the same side of the car, it's all good.

No.

Turning one end in one direction will turn the other end in the *same* direction when viewed from the other end.

Flipping the bar (on the same side of the car) will *not* reverse the direction of the stress.

The twisting on a torsion bar effectively is one way on one end and the other way on the other end. (Holding one end still and twisting the other.) So flipping the ends gives you the same directions of twist.

Think about a nut and bolt. Thread the bolt into the nut. You turn it clockwise, right? Then take it apart, turn it around, and thread the nut onto the bolt. Clockwise again!

That's basically the same deal.

--DD

ah, gotcha.
I fail at mechanics LOL

Nah, it's not a fail. It's just something that is counterintuitive at first. I think I had to draw some arrows on a paper towel tube to really get it...

--DD