Hello, does anyone know where I can get the fill plugs of the diffs for my '55 Olds and '59 Olds? Both fill plugs are in really bad shape and I want to change the oil on both diffs. Maybe other GM fill plugs also fit? I couldn't find any for '55 and 59' Olds.

 

Snap a picture of both plugs.
This will tell me (us) what tool is best for removing the old plug and the best source for locating replacement plugs.

 

59 olds is the same as all gm 8.5 7.5 etc. It is a ' innie ' 3/8" square drive ratchet.
55 olds... IS it in the third member or in the housing?
I usually hammer the plug with a 1# to shake the rust loose and to actual reform the square innie... then tap the ratchet 3/8" into the square and or put a 1/2" to 3/8" adapter and tighten first then reverse after you nudge it forward first.
IF the plug is REALLY messed up we weld a nut to the plug and the heat will penetrate from the heat in the thread hole of the nuts ... then let it heat sink for a minute and put a socket and try to work it loose forward backward...
YOU want to stay away from drilling...too messy.
Jim
JD Race

 

What's the third member? I took some pics of the '55 Olds diff. I can't put the '59 Olds on a car lift right now... the '55 Olds is currently a massive project and will block the car lift for a while. The other car lift is blocked too. I found a pic on the internet from a '59 Olds diff and I'm really sure the plugs are the same. The last pic is the 59' diff.

I measured the plug of the '55 with a folding rule. Unfortunately, my measuring tape with inches was at home and I don't have one in the garage.

Thanks for the tips!

 

This is a "third member"
Number of ways to extract that. Find a sacrificial 3/8" extension and tack weld it to the plug. A short extension. Like 2-4" max.
Start out using a 3/8" air hammer on low to gently coax it out. Might need a little heat around the circumference. Move up to a 1/2" impact with a 3/8 to 1/2 adapter and again gently let the hammering action work it loose.

 

Thanks for the tips but first I need to find a replacement. Maybe that fits?: https://www.ebay.com/itm/263538070046

 

Could try grooving the head with a cutoff wheel and using something similar to a large slotted screwdriver to twist. A chisel could work too....gently if it does not move after the first few strikes it's not going to move.

Were you the person who stripped it? If yes, the welding technique I mentioned will be the best option.
Another option if the welding method fails or you don’t have access to a MIG/TIG/Stick welder is to drop out the third member to get good access from both sides. Then drill out the plug just short of intruding on the threads, then gently chisel or EZ out the thinned out remains.

McMaster-Carr is your friend. Do you have something similar in Germany to avoid the shipping/customs? McMaster may have a European footprint. A hardware store over there may have NPT black pipe plugs??
https://www.mcmaster.com/

Two choices in this link external square or internal hex. My guess is that you have a 1/2" NPT Male with the 3/8" hex. Again, this will be SAE/NPT not metric.
https://www.mcmaster.com/products/st...pipe-fittings/

 

Thanks a lot! I've never heard about McMaster before. There are many shops here that offer parts for old american cars but I haven't found one like that and it seems like they've got everything for everything. Norelem is similar to McMaster but they don't offer plugs like that. The shops here don't have the diff plugs I need. Chevy, Pontiac, Buick, Cadillac are really common but Oldsmobiles are not that common here. Oldsmobile Cutlass are an exception, sometimes I see multiple at car meetings. So the demand for Olds parts is not as high which also means that you can't get many parts for Olds here. I think I'll have no other choice but to buy at McMaster and I believe it would be better if I use plugs with an external square drive because then I can use a wrench which is more easy. I also think that welding is the best option. The plugs have a square not a hex. If you are right, this one would fit too and what I like about it is that it has a magnet: https://www.ebay.de/itm/263538070046

 

The magnet is nice.
Keep in mind GM used many common parts within each division to keep cost down. You may find the plug in a Caddi, Chevy, Buick or Pontiac catalog.

 

You might place a Parts Wanted ad in the Classified sub-forum on this site. I know of numerous members w/ various rear ends some have them stock piled for years & some rear ends they might have a plug for you on a rear end they never intend to sell.

I think drolds is likely correct......

 

I also think that drolds is correct but I can't find any GM diff plugs here. I find plugs for Ford e.g. but no GM. Thanks for the tip with the sub-forum, I'll place a parts wanted ad soon.

 

The first thing I would do is try to get some "gun barrel cleaning solvent" to penetrate the threads. Use a small brush to apply a liberal amount. Do this several times and wait a day to try loosening. Keep in mind you are working with tapered threads that cause an interference fit when tightened. You can try to tweak the plug clockwise but its likely jammed up.
DrOlds is correct. McMaster-Carr has been around for at least 60 years. You may have luck with an "Industrial supply company" in your area. If you find an industrial parts place, make sure you get a plug with 1/2" NPT threads. NPT stands for "National Pipe Taper". You can visually see the threads are a smaller diameter on the end. DO NOT use NPS fittings. NPS stands for "National Pipe Straight" NPT and NPS are U.S. standards. Good luck.