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New source to excessive oil leak, rather than rear main seal....'wrong 'threaded plug'

To All: After changing rear main seal twice…still leaked excessively…have retraced leak more thouroughly and found that lhe leak may be from what appears to be an incorrect ‘threaded plug’. Unfortunately this required dropping tranny, removing oil pan, pump, flywheel cover, housing etc. to expose rear of engine and the end of crankshaft and metal panel bolted on the rear of engine block

The excessive oil was around the plug and looked like it was leaking from there… the plug was removed and one can see that it did not match the threaded block hole. With engine on an upright position, the plug is located on the right sie of the block (picute below shows block upside down). When plate bolts and plate is removed, there is a soft freeze plug on the left and a ‘threaded plug’ on the right. The bolt size is 7/8" and the thread number is 14, with what appears to be straight type threads rather than sloted. Don’t know at this time whether the plug has a weep hole from top to bottom or just at the bottom or none at all. Again, since the one we removed appears to be the wrong plug, both in length and in type of thread, which may be why it’s leaking through the threads.

Anyone familiar with such a ‘threaded plug’ in mid year cars or later year cars…what is its function and where does that hole channel go to…? W30post, You mentioned a ‘blind plug’ that has a weep hole on it…is it located in the same area…? I couldn’t find any other threaded plug that has a weeping hole on it…? Anyone that has or knows where I can locate such a ‘threaded plug’, or anyone with an old '54 Olds engine that is willing to part out…let me know? Anxious to get this Olds on the road and cruise…!

I’m not very familiar with this engine although many decades ago my parents owned a '55 Olds. However, it was a ho-hum 4 door sedan. Your car is a real looker and that blue makes it. :slight_smile:

Generally speaking, threaded plugs in the block are used to seal off oil galleys. These plugs are often removed (should be anyway) when a block is vatted out before a rebuild. Maybe someone crossed one up at some point and that’s what led to the problem.
This should not be difficult to repair; either by replacing with the correct plug or by cutting new threads and inserting a plug that actually matches the new threads.

ok4450: Your assumption is probably correct, since the block went to a machine shop to be vatted then machine down all cylinders, crankshaft, main caps and all appropriate internal parts…then line-bored and centered-lined…to ensure that everything lined up and met specs. It was during the install of putting the internal parts, soft freeze plugs and other plugs that the machine shop somehow installed the wrong threaded plug. Although, they are now saying they didn’t, in any event I now have the problem of finding the right threaded plug or have one fabricate to match length and thread. I still woul like to know what this perticular plug’s function is…? I don’t want to create another problem by thinking I’m solving this one. As you say, plug maybe to seal off an oil galley or maybe to help relieve pressure with weep hole on the plug, as to not blow out rear main seal…? Thanks for your input, I’ll keep it in mind.

No bolt from the outside of an engine into a oil gallery would have a weep hole in it. Think about it.

If the threaded hole is messed up badly enough that the right size bolt won’t fit tightly enough I would use the same size stud sealed in the hole with JB Weld and put on a lock washer and nut. I have used this method ut save aluminum cased transmissions with stripped bolt holes.

I read this as the weep hole having to do with the rear main bearing cap, not the plug.

Oil galley plugs and block plugs (the latter incorrectly referred to as freeze plugs) serve one purpose. That is to allow removal of the casting mold material back when the engine block was first manufactured.

The cooling system is low pressure and a tapped in block/freeze plug (semantics again) will stay in place just fine. The oil pressure system has much higher pressure and is much more critical to the engine’s life so the plugs will be threaded instead.

It was my comment about an issue seen on a little newer Olds motor that was a weep hole issue. This just got Geomen on the track of looking at different things than the rear main seal.

This one I feel is most likely a pipe plug type thread and is used to seal an access hole for the factory to machine into the oil passages. Especially since it goes to the trans mating area of the block.

Compare the threads in the hole to pipe threads. Like the check/fill plug on rear ends and manual transmissions the block may have been tapped out and plugged with common pipe plugs. They seal on the threads, needing no flanges, seal washers, etc.

In the photo, I take it the plug(s) under discussion are hidden by the tin dust cover…Maybe the OP will join in the discussion… You can see one Allen head gallery plug but it looks correctly installed…This picture must have been taken during initial engine assembly…

This small block Chevy’s oil plugs are common NPT (national pipe thread) plugs.

and this note on a drilled plug is interesting

Rod Knox, that is how the Olds 350 and 455 used a weep hole in a plug in the back of the lifter galley to pee on the distributor gear for lubrication. That was my reference. I had never seen this motor he is discussing but knew my machine shop had overlooked this detail when they cleaned and assembled the freeze plugs in my motor.