A cautionary tale

The takeaway:

  • If the specified fastener is a locking nut (nyloc, cleveloc), use the same locking fastener.
  • If you completed a task in haste, go back over your work thoroughly.
  • Prop shaft vibration is scary.

I reassembled my 1970 FHC about 2 years ago, and as I attached the prop shaft to the gearbox, I found I did not have new nyloc nuts (3/8-24, if you’re interested). I did have some regular nuts, so I used them and ordered the correct ones from Bolt Depot intending to replace the incorrect ones the following week.

Of course, I forgot about the incorrect fasteners. They did not forget about me.

It took about 10 hours of driving and then I noticed a bit of vibration at the gearshift lever and noise from the rear of the car. This was not bad at low speed but became severe, about 30-35 mph, and I began investigating. This vibration became more severe and toward the end there was a metallic tapping sound. Not good.

I had recently replaced the 3.54 final drive unit with one I rebuilt using a 2.88 crown/pinion. I had taken my time and was very careful to get everything just right, and I could not imagine this kind of vibration from the final drive unit as it was nice and quiet.

There was no vibration with the car stationary and clutch engaged or when out of gear. Seemed to be the half shafts or prop shaft and likely one of the new U joints.

I removed both the half shafts at the brake disc, and both sides seemed fine. No play, no seized joints.

I then disconnected the prop shaft from the final drive unit and confirmed the final drive unit turned easily with no noise, just as I had prepared it. I also confirmed the rear prop shaft U joint was good – no play, and easily movable.

As I rotated the prop shaft, there was apparent sloppy play somewhere forward, and the culprit must be the front U joint.

After you have installed a nice Suffolk and Turley interior, one of the last things you want to do is take it back out to get to the front of the prop shaft.

After removing the carpet, seats, center console, and gearbox cover, lo and behold the prop shaft U joint was fine.

However, there was only one bolt with a nut on it holding the prop shaft to the gearbox flange, and this was ¾ loosened. There was 3/8 inch play between the prop shaft and gearbox flange. The other three bolts were in place, but without nuts, and there were some scrapes to the rear gearbox oil pump cover, thankfully nothing serious (I hope, fingers crossed). The bolts were not damaged.

Another mile or two and the prop shaft would have gone its own way. Yikes.

I replaced the prop shaft nuts with locking nyloc nuts, well tightened and checked. And checked again.

I reassembled the interior, reconnected the prop and half shafts to the final drive unit (using the correct fasteners), took the car for a drive and all is well, no vibration and a quiet final drive unit.


A major bullet you dodged!

i’m guessing you didn’t use lock washers on the standard nuts, they would have held


No, I did not use lockwashers. I intended to replace these regular nuts with the nyloc nuts in a few days when I received them and long before I started driving the car. Just forgot to replace the nuts.
I can’t tell you how much better the car drives with the prop shaft connected…


I think we have all been there. After a recent front suspension rebuild and shakedown I was checking the safety wires on the brakes, found a sway bar connector bush pushed out and the connector hanging, for lack of a proper sized washer. Not like a loose driveshaft, but still…

I use loctite on them too. Just to be sure

Ever since about the early 70s, I did away with all types of split lock washers, and used nylock’s, and Stover nuts: I always used aircraft grade flat washers on Stover nuts, on propshafts, and halfshafts, and never, ever experienced a nut coming loose.

as the saying goes> an ounce of prevention is worth a ton of cure