Determining prop shaft length

[composimmonite]

I'm changing the engine and gearbox in my Marina based Marlin kit car for Spridget 1275 items.
The original (modified) Marina prop shaft had a sliding joint but the Spridget gearbox takes a sliding UJ instead.
There is no floor in the vehicle so easy to see rear of gearbox and diff flange.
The engine and box haven't yet been installed.
Plan is to try (looks possible) to use the standard Spridget prop shaft - I'm changing the diff flange for a Marina item - UJ's are the same on both vehicles so easy job.
With prop slid into the gearbox and bolted to the diff I can then determine the engine and gearbox mounts.
Problem however is I have no idea what allowance I should make for the position of the sliding section of the prop into the gearbox. If too small then axle movement might push it into the gearbox until jammed solid and cause damage / handling problems. On the other hand if too large a gap there could be a chance it can pop out !
So question is what sort of position should the sliding aspect of the prop shaft be in the gearbox - I'm thinking / guessing, say, 3/4 in ?

Thanks.

Paul H

[ejenner]

I'm by no means an expert on this sort of thing but I do have experience in bodging for benefit.

I'd say the only way you'd be able to check off this item with absolute confidence would be to prove what extent of movement is possible in extreme circumstances. So the elasticity of your engine mounts is one place where you'll get movement. You can use a big lever like a long scaffold pole carefully positioned to lever the engine back and forward in the bay. Or if the engine isn't fitted you'd be able to check the flexibility of your engine mount by seeing how much you can make it move when you have it clamped in the vice. Same logic applies to the rear axle. The link arms on the rear axle will control the movement of it. Ideally if you can remove tension from the axle by removing the coil springs or whatever then you can flex the axle to maximum extent and measure the movement.

The other option would be to leave mounting of the propshaft to the last minute and with transmission and diff in their ideal positions you just offer up the propshaft and set it so the rest position is mid-way between all the way in and all the way out.

[colnerov]

Hi, I would tend to agree with ejenner, You have probably 2" of spline engagement but you would want more of the splines engaged than not. So as you guess 1/2" to 3/4" would be about right. But as he says wait until it is all mounted up before deciding.

Colin

[eternaloptimist]

Is there any kind of witness mark on the sliding section of the prop?

[DWDarkWheels]

Definitely mount the big bits in place, then measure the distance between diff flange and gearbox nose. Jack up the rear of the car so the axle is at full droop. With a second jack, articulate the axle in several steps to full compression. At each step, take a distance measurement again. Chances are the longest distance is mid-way in the travel of the axle.

Armed with max and min measurements, any prop specialist worth his salt will know what length to make a propshaft. As above, they'll probably tend towards having a bit more engagement in the sliding joint.