More prop shaft woes

Removed the remains of the prop shaft today and had a good look at them. Clearly the welding between the front CV-joint and flange failed completely. And on closer inspection it became clear that a fair part of the crack (almost half of it) is older than the other half!

Another interesting bit is that there are clear marks on the inside of the broken off part, where the centre of the CV joint made contact ...

I can only explain this as the prop shaft being too long, resulting in the centre shaft of the front CV joint touching the front flange. Which reminds me of an “incident” that happened while restoring ‘t Kreng in the mid 90’s. As I didn’t (and still don’t) rate the CV jointed TR7 prop shafts very high, I decided to have one made locally with standard UJ’s, using the original one as template. When I fitted this prop shaft it turned out to be too long, with no movement left on the sliding splines. Didn’t think much of it then because they didn’t charge me for shortening the prop shaft ...

But with the original old shaft removed from the DHC I compared it with two spares I have lying around. They are both from late Solihull built FHC’s. The one in the middle is the same as the one that failed (PKC1979) and the one on top is the prop shaft that was fitted to ‘t Kreng when I bought her in the early 90’s (could be part number PKC1420, but not sure). I fitted the last one for obvious reasons!

When fitted, the sliding splines are well over an inch extended from the shaft. So enough travel left. But this brings me to the reason why I think the original shaft failed!

When I bought the DHC it was sitting at the original ride height at the rear for a TR7, so with a little more room for the prop shaft to move. But I lowered it a little and after that I mostly used her for long holiday and touring trips. So with the car fully loaded. And I think the stresses that were put on the prop shaft as a result, killed it in the end. And this might also be the reason why the gear changes were sometimes rather awkward. With the prop shaft pushing onto the gear box’s main shaft, this might lock up a few bits inside the ’box. Test drive scheduled for tomorrow afternoon or Friday to see if my assumption is correct!

Another problem with the original CV jointed prop shaft is its alignment. A constant velocity joint works best in a straight line, where a universal joint works best at a slight angle. When you look at the position of the engine and the back axle you'll see that they are not in line, but slightly angled. As a result of which their centre lines run more or less parallel. And as such ideal for a prop shaft with universal joints. And probably the reason why the CV-jointed prop shafts wear out pretty quickly on these cars.