Diff issues

[phaetn]

I've been getting metal remnants in my diff oil for well over a year that I can spot sticking to magnetic drain and fill plugs.

Two years ago the drive shaft "clunked" a little when pulling away in first once in a while, but new U-joints and a guibo solved that.

Now the clunking is back, but it happens not just in first, but sometimes shifting into second, too, and even once into third. Uh oh!! It clunks in first almost every time I pull away from a stop.

My friend and Fiat guru slid beside the car and grabbed the driveshaft and said there was a lot of play and it was in the diff. He also said the old diffs are very strong but warned me not to take any long trips. He said I'd need a crush sleeve. So I've parked it for a few days to figure out what to do next. I was on my way out and forgot to mention the metal bits in the oil.

While doing errands in the family car today I happened to go by a transmission shop that had an old '50s Chevy as a customer car in front of it. I popped in and confirmed they will work on old cars. They said if I have metal in the oil then that means bearings are going.

Does it make sense to buy a bunch of bearingss and have them crack it open? Or should I try and buy a used diff? There's a '71 parts car near me but he wants to keep the wheels on it so it can get towed. I might bring it by the transmission shop tomorrow and just have them listen to it.

Thoughts/suggestions?

Thanks in advance!

Cheers,
phaetn

[baltobernie]

The rear diff may be one of the most durable moving parts on a Fiat Spider. You and a friend can swap the entire axle even in your driveway, so if you have access to a salvage yard, that might be the way to go. I'd choose a good used diff over even an experienced mechanic who's unfamiliar with the Spider.

Your signature indicates a couple of improvements to a stock '74 Spider. That being said, have you considered a change to a 3.90 rear ratio? You'd see about a 500 RPM drop on the highway, while still retaining a stump-pulling first gear. Spiders from 1978+ have the 3.90, and they were the best-selling years for our cars. You'll also need the Panhard bar and a 3.90 speedometer gear.

[phaetn]

Thanks baltobernie.

I'll see if I can get my hands on a used diff. I'm worried that with the clunking as is that I'll wear the yoke(s) where the u-joints attach; but perhaps I can get the diff and driveshaft...

I'm not sure I need to go to a 3.9 ratio, or maybe it's already there. My cruising on the highway is good: about 3750rpm at 70 mph and 4250 at 80mph. It's right in a sweet spot for power, too, given how my engine breathes. I don't know if I need gearing any lower than that, thereby sacrificing already somewhat stodgy acceleration by modern standards.


Cheers,
phaetn