Joseph P. Indusi Posted October 2, 2018 Share Posted October 2, 2018 (edited) I removed the differential fill plug from my 53 Special to replace it with a better one (obtained from MrEarl) as the outside square points were butchered by previous owners even thought the car has only 66xxx miles on it as verified from the title document. The replacement plug came from a 1954 Buick and is solid steel. The original plug is hollow inside and has a hole through the threaded side near the head. Anybody know why the hole in the threaded side? I will try to attach a few photos. Joe Edited October 2, 2018 by Joseph P. Indusi mispelled word (see edit history) Link to comment Share on other sites More sharing options...
Beemon Posted October 2, 2018 Share Posted October 2, 2018 Looks to be about where the felt washer would be, most likely a breather. Link to comment Share on other sites More sharing options...
leon bee Posted October 2, 2018 Share Posted October 2, 2018 Isn't that so you can crack the plug and just have a stream coming out? Something you can aim, rather than just jerking the plug and making a bigger mess. This would make more sense on an oil pan than a differential, though I guess. Link to comment Share on other sites More sharing options...
leon bee Posted October 2, 2018 Share Posted October 2, 2018 Oops, it's a fill plug. Maybe it USED to be a drain plug. Link to comment Share on other sites More sharing options...
Joseph P. Indusi Posted October 2, 2018 Author Share Posted October 2, 2018 The drain plug is the very bottom bolt holding the back cover to the differential housing. Maybe the hole is there to slowly drain out lubricant if differential is overfilled? Joe Link to comment Share on other sites More sharing options...
gdmn852 Posted October 3, 2018 Share Posted October 3, 2018 Could be for a magnet to collect metal in oil , my MG has a set up like that but for engine oil drain.Tap hole on side is for set screw to help hold magnet ,that would be my guess. Link to comment Share on other sites More sharing options...
packick Posted October 4, 2018 Share Posted October 4, 2018 (edited) Stupid question, but if you removed this plug when the differential was hot, couldn't there be some built-up air pressure in it and maybe the hole is relieve that pressure before you take the plug all the way out? I know there is a small hole in my master cylinder cap which is, I assume, for the same reason. Or am I smok'n something? Edited October 4, 2018 by packick (see edit history) Link to comment Share on other sites More sharing options...
Joseph P. Indusi Posted October 4, 2018 Author Share Posted October 4, 2018 Packick: Your response sounds plausible. I use an aluminum bolt with a hole drilled through the center near the top of the back cover to use as a vent in case of a buildup of pressure. I am still not sure about the reason for the hole through the threads. Link to comment Share on other sites More sharing options...
Fr. Buick Posted October 5, 2018 Share Posted October 5, 2018 There should be a vent already on the top of the axle housing... IIRC, left side, half way towards the wheel. I would not install one on the cover, as there is a lot of oil being slung against it by the ring gear while driving. I imagine that hole on the fill plug is a vent for draining. Link to comment Share on other sites More sharing options...
1953mack Posted October 5, 2018 Share Posted October 5, 2018 (edited) All 1953 and early-1954 Buicks were not equipped with the bobble-head breather vent on top of the rear left axle's housing for relief of thermal expansion pressure which made the vent hole in the fill plug useless when checking fluid level. The solid steel fill plug most likely came from a later-1954 (after ± June 1, 1954) OEM Buick rear axle housing. OEM fill plugs had fiber washers . . . not felt washers. Al Malachowski BCA #8965 "500 Miles West of Flint" Edited October 6, 2018 by 1953mack (see edit history) 1 Link to comment Share on other sites More sharing options...
avgwarhawk Posted October 5, 2018 Share Posted October 5, 2018 My 54 does not have the vent in the axle. If remember correctly the plug was solid and did not look like the plug pictured above. Link to comment Share on other sites More sharing options...
Joseph P. Indusi Posted October 7, 2018 Author Share Posted October 7, 2018 I positioned the bolt with a hole through the center near the top of the differential rear cover near the top and well to the side of the ring gear. I never found any gear lube leaking out of the bolt head. So I think it is simple and functional. I think the hole through the the threads of my old fill plug was to vent the rear while draining the lube. Joe Link to comment Share on other sites More sharing options...
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