The XPAG Core Plug: a Temporary Solution

The subject of core plugs is seldom on the T-Type owner’s mind until things go wrong; then the consequences can be nasty if you lose one or suffer a slow leak of anti-freeze from a corroded core plug disc – especially if it happens to be in the worst place possible for replacement in situ and you do not relish the thought of an engine out job.

First we refer to the large 48mm diameter core plug at the rear of the engine block on practically all T-Types. Solutions have been mooted over the years in the past in our journals etc and some with moderate success, but is there a solution which, given a little D.I.Y. time and tools, could result in a temporary remedy and get you going on your journey say within the hour when you are in the middle of nowhere!! and, of course, if you carry extra water?

I remember reading an article in the Octagon Bulletin some years ago on this subject; the idea was to use two core plugs bolted together and as you tightened the centre bolt it expanded the core plug to make a temporary seal. This got me thinking about a similar method and one which could be accessible to fix in situ.

The problem, as many owners will be aware, is that there is very little room in which to work on all the core plugs, least the rear large one. As luck would have it, Peter Harrington, a very good friend of mine and M.G. owner of three T-Types had an engine waiting for overhaul in his workshop. I was therefore able to bench test a device in principle which would solve the problem and be an easy to fix as a kit.

This was done by first making a special core plug 48 mm diameter (1 7/8”) which was domed but with a flat outer rim 5/32” width with a centre hole 3/8” diameter (photos 1 and 2).

Photos 1 & 2: on the left, the domed side of the special (brass) core plug; on the right, the dished side.

Next a steel bar was designed which has a central set screw welded and reduced head thickness which can be domed, the bar radius at the ends so that the bar would then fit inside the water jacket of the block.

Photo 3 – steel bar with welded central set screw

Finally a special aluminium disc was made with a threaded centre hole 3/8” diameter Whitworth or B.S.F. and also domed to suit the profile of the special core plug; its width was 1 1/16ths inch and on its outer diameter was drilled four tommy bar holes 3/16ths inch diameter (photos 4 & 5).

Photos 4 & 5 – showing both sides of the special aluminium disc
Photo 6 – the complete assembly. Not previously mentioned in the text is a cork gasket (located between the special brass core plug and the steel bar – later referred to in the text as the metal anchor plate) and a rubber grommet which is located between the cork gasket and the steel bar.

The outer diameter of the aluminium is only about five to ten thousands of an inch smaller than the core plug diameter, so to fit the kit in place, first we have a good quality sealant which is used on the gasket to core plug and engine block aperture, the steel bar, a rubber grommet 3/8ths bore to fit over the centre set screw, cork or paper gasket with a centre hole 3/8ths and outside diameter the same size as the core plug, then fit the special core plug in its rightful place in the engine block. In most cases the metal anchor plate will only fit correctly in one position for example the rear 48 mm core plug the anchor plate will only fit at five to five (as on a clock face), so you line up the slot cut in the threaded pin to this position and spin on the threaded aluminium disc and moderately tighten to give a leak proof joint.

A small lock nut can be used as security if required but generally if you apply plumber’s PTFE Tape to the threads things should be O.K. Also I did apply sealant to each side of the rubber grommet which would help to seal the threaded centre screw and grommet to gasket centre. Now in an extreme case you may be able to use the standard 48mm core plug with a good gasket, preferably cork or steam gasket also a different profile threaded Aluminium disc is required which is more simpler to make for which a drawing has been produced, and if needed the same set up can be used on the smaller diameter core plugs but in both cases a centre hole 3/8ths inch diameter has to drilled accurately finally using a good quality sealant.

Ed’s note: At this point, I think a few photos will help to explain the set up and how it is fixed.

Photo 7 – the complete assembly as described in the text. Note the rubber grommet on the steel bar and the special spanner which locates in the 3/16” tommy bar holes on the face of the aluminium disc (shown in photo 5)
Photo 8 – steel bar with welded central set screw ready to be inserted in back of block
Photo 9 – steel bar with welded central set screw now inserted in back of block
Photo 10 – cork gasket fitted
Photo 11 – special brass core plug fitted
Photo 12 – aluminium disc being tightened using a tommy bar engaged in 3/16” holes

Coming on to the M.G.TF which is the most awkward car to work on in this respect some American owners have cut the bonnet side to the same profile as the outer wing and made the bonnet sides removable with the aid of M.G.A. large washers, in all about six bolts in order to make work on them a little easier. For the smaller core plugs of 1.3/8ths inch diameter a similar system can be employed, but replacing the core plug, which is the rear one in the water channel (which also has a small drain hole which must be clear) which you should check when replacing the core plug, is difficult because it is in direct line with the steering column.

So to aid the installation of this core plug the Aluminium threaded centre disc has four peg holes 3/16ths diameter drilled on its surface so that a special “C” Spanner or circlip spanner can be used (see photo 7) and it’s a system which can be used for the core plugs on the side of the engine block after you have removed the old core plug and cleaned its aperture before fitting this temporary kit. When fitting standard core plugs it is always best to make sure that they are a tight fit in the engine block aperture before you use force to seal them, and to do this place the core plug on a flat steel surface gently tap the radius surface around halfway towards the centre of the plug until you get a good fit, finally fitting using force to seal, avoid flattening the centre of the plug to much because in extreme cases you can actually reduce the diameter of the core plug.

A set of drawings is available for members wishing to make this core plug kit if so desired. Finally I would like thank Peter Harrington for his patience, practical knowledge and advice in fitting this temporary core plug to the XPAG and XPAW engine blocks.

I hope this article is of interest to our members which may create sufficient need that manufacture of a limited number of these kits should be possible.

A. Atkins

Ed’s further note: Whilst we are on the subject of core plugs (sometimes referred to as “freeze plugs”) the correct sizes for the XPAG are 6 x 35mm (1 3/8” will fit), 2 x 48mm (1 7/8” is too small), 1 x 45mm (for the cam). I am indebted to Bob Grunau for this information.

Regarding corrosion, it helps to paint the inside of steel core plugs with red lead (if the ‘Elf’ and Safety brigade haven’t banned this) or a similar rust inhibitor.

A supplier of brass core plugs is TTT 2 member Tom Lange. Tom’s contact details are as follows – Website: E-mail: tlange’at’

When fitting core plugs to the MPJG block, Brian Rainbow cautions to exercise extreme care as these blocks are notoriously weak and you may well end up cracking the block if you are not careful.

One thought on “The XPAG Core Plug: a Temporary Solution

  1. Jim Mink says:

    Why not carry several expanding plugs of the proper size? They are available at most auto parts stores, are cheap, and can be installed with a standard wrench.

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