55.2-59 1955.2-1961 Ring and Pinion Question

[Steveoreno]

I am restoring a 59 NAPCO 3100 and discovered that the splines on my 3.90 pinion are toast. Does anyone know of a manufacturer that carries the 3.90 ring and pinion? I have been able to find other ratios but not the 3.90 and because it's a NAPCO I have to have the same ratio as the front differential. Thanks! -Steve-

[dsraven]

if the rest of the parts are in good shape, gear faces, shaft sizes, runouts etc, possibly your spline area could be built up, machined to size and re-splined to match the yoke? a good machine shop could likely fix you up. not sure on what that would cost you but if you have no other choice.....might be worth a drive and quote. another option would be to check with the local gear vending and differerntial repair outfits to see if they have encountered anything like it before.

[Steveoreno]

Excellent recommendation dsraven, I will give that a shot! Appreciate your input ! -Steve-

[leegreen]

I had a similar problem with a off topic SUV, The nut holding the spline together would not stay tight, when it loosened driveline vibrations would set in. Replacement parts for this specific model were hand forged by elves out of unicorn whiskers and cost more than the SUV was worth.

I tightened the nut, welded the nut to the splined shaft and the splined yoke to the nut. Problem solved. At least the immediate problem was solved. If it had not been rear ended a couple years later eventually I would have had a larger problem when something needed servicing.

[MJN]

3.90 was the standard ratio for all half tons (3100's and 3200's) and the same rear axle was used on 2wd trucks, so you should be able to source another axle or even third member with a better gear set and it would be a direct swap. Nothing special about the rear axle just because the truck was equipped with Napco parts up front.

[Accelo]

Re-maching the spline will be cost-prohibitive. Welding on hardened steel without heat treating afterwards is not a long-term solution. Welding isn't out of the question, as you have little to loose. Consider epoxying it together and driving it if you can't find the correct parts. They are out there somewhere.

[MJN]

Parts book says 3741092 (later replaced by 3747808) is the 3.90 ring and pinion set- happens to be one on EBay now https://www.ebay.com/itm/375416815875

[mr48chev]

Probably the best option is finding someone close to you who has pulled a good 3.90 Six lug rear out to swap to a newer 5 lug.

I'm not finding a source for a 3.9 ring and pinion right off hand.

[Steveoreno]

Thanks guys....I took it to a shop and they felt that my yoke was slightly wallowed out a bit and recommended that I get a different yoke and then see if conditions improve. I was lucky enough to grab one from ebay...so we will see how that works out. If that doesn't take care of it I will look for a good used set. -Steve-