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MG MGB Technical - Oil Galley Plug Repair

Has anyone removed the press-fit brass plugs in the block and threaded the holes? If so, what size thread did you go with? Looking specifically for the one under the oil relief valve. I'm probably going with NPT, either 1/4" or 3/8".
Steve Simmons

Steve. I have done this a number of times over the years. You should get a copy of the "University Motors, Ltd Technical Book" from John Twist via his University Motors Ltd website. It contains all of the information you need to do the job and will tell you which sized plugs need to be used in which holes. Also contains information of a number of other topics, and BMC/BL vehicles which might be of use to you.

When drilling out the old plugs (unless they are ready to fall out) you need to drill and tap them to 1/4" UNC, then a flat plate with a clearance hole drilled in it. The plate is placed on top of a socket (or a piece of tubing), the socket head cap screw or hex bolt placed through the hole in the plate, and the assembly placed over the plug to be removed. The screw/bolt is threaded into the plug and tightened. When fully tightened, the socket/tubing will rest against the engine block. Then, further tightening will pull the plug from the block quite easily.

One thing to remember when doing this is that there will be swarf (i.e. metal shavings) presented into the oil passages of the block. These need to be fully cleaned out using rifle cleaning rod with various patches and brushes screwed onto the end. A slotted cleaning tip, preferably metal, and a .45 rifle brush (you can use the shorter pistol brush if you cannot find a rifle brush) and plenty of good quality cleaning solvent should be used. I prefer to have the block hot tanked (if you can still find someone who does this), then thoroughly clean the passages with the rod, brush, and plenty of patches. You do not want any swarf left in the oil passages.

The tapered pipe fittings (plugs) are, by definition, tapered. Normally, this requires that the proper taper reamer be used before tapping the cast iron holes. However, with the five plugs on the front and rear of the engine block, the use of the reamer is not required and simply tapping them with the tapered pipe tap will work just fine.

Les Bengtson

Thanks Les, the plug is already out. It found its way out while I was driving. Darn good thing I pay attention to my car and don't have a radio blaring or I would have ruined a very expensive engine.

I'm hoping to make the repair it in situ. There is no room to press another plug in but there may be enough to get a tap down there, plug the hole with grease, carefully tap into it, then vacuum and clean the hole before threading in the plug.

I think there are two different sizes of plugs. Will a 1/4" NPT tap work without drilling on the one below the relief valve? Guess I should go dig for a spare block to test it on first.
Steve Simmons

I'm sure you have thought about it, Steve, but that seems like a bad place to introduce swarf into an expensive engine. Can you undo the trans cross member bolts and engine mount bolts, and a few other things and move the engine forward to get access? That would be somewhat less work than completely removing it.

C R Huff

I figure I can get it all out if there is a glob of grease behind it, then run the engine to "spray" anything left in the threads out of the hole. But I'll admit it still makes me nervous. The only other way I can see to do this is to pull the engine half apart and run cleaner through the oil lines. Or replace with another brass plug but that makes me even more nervous.

Anyone know where the oil goes after passing that plug? If into the sump then if something got left behind, it wouldn't do any real damage. If into the bearings however....
Steve Simmons

Steve. No, a 1/4" NTP plug would be too large for the small hole under the pressure relief valve. Also, looking at its location, it would be almost impossible to ream and tap the hole with the engine in situ. The hole needs to be reamed along the axis of the hole, then the reamed hole needs to be tapped to the correct depth. Getting this done correctly, much less getting any swarf that may be deposited into any oil passages fully removed, cannot be done, in my experience, without removing the engine and stripping it down.


Les Bengtson

This thread was discussed between 09/04/2015 and 13/04/2015

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