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MG MGB Technical - Core Plugs / Freeze Plugs

I think my '68B has it out for me some days.

Yesterday was one of the first semi-top down days we've had. I got the B out and ran it for about 20 miles - everything was fine. About a mile from home I accelerated and ran the engine up to about 4,500 RPM in 1st. Then I hear a pop, saw a puff of smoke, and the engine ran rough for a few seconds. It evened out, but I could smell hot engine coolant. I've blown the rear core plug or freeze plug depending where you are in the world, and emptied the coolant onto the distributor, alternator and the rest of the electronics on the engine. The block is an 18GV block that was re-built 6,000 miles ago - new crank, pistons, the whole deal. The head was re-built this past summer and has about 1,000 miles on it - hardened valve seats, new valves etc.

My questions are:
1) What might have caused this failure - is it something bigger than I think right now?
2) What do I need to do to replace the core plug? It isn't covered in any of the manuals I have except a fleeting reference in Lindsay Porter's book.
3) Should I replace both of them now, or is the second unlikely to fail?
4) Is there anything I need to worry about with coolant on the dizzy, alt, and some of the wiring harness. I've dried it as best I could already.

Thanks in advance guys!

Rob Hardy
Rob Hardy

The worst thing that could have happened is you tanked the motor running it without coolant. Let's assume you stopped immediately and all is well there!

Also from your description is seems like you popped te #3 plug (between cylinders 3 and 4) as the rear plug is literally on the rear of the engine... There are also plugs on the head so I must assume you really mean the block.

But you may mean the head as there are three plugs on the right side of the engine (ironically the "dry" side) and one on the rear, whereas the head has only one on the rear... er, now I've confused myself!

In any event, it is very easy to replace if you have access to it (the one on the back of the block is a stinker as it is behind the rear cover plate and requires an engine R&R and un-mating of the transmission to get to).

Just clean up the area with a wire brush, de-rust with whatever you like that leaves a thin coating that can be removed or no coating at all... like a very vigourous wire-brushing or a cup-brush in your drill.
Use gasket compound in the margin of the hole (for the purpose, I use a 3-m product that is goopy and sickly yellow that I can't recall the name of at the moment). Place the welch plug in the hole concave side down (domed part "out"). Now comes the tricky part, using a drift, apply a couple meaty smacks with your BFH in the "won't go" size (translation, use a big-arse mallet). This will "flatten-out" the dome-shaped plug and expand it into the hole. Dimpling the plug is OK but don't get excessive, the other plugs on the block will provide a general guide... or maybe not as one has already popped out.

Oh, use a fairly large diameter punch (about 1/2" and flat) so you don't punch clean through!

Why'd it happen??? Well, what did your temp gauge look like? If you weren't overheating (and the rad cap should blow first) then I'd assume poor installation of the plug in the first place. It is remotely possible that your rad cap is bad but then a hose should have popped first.

Good luck.
M.J. Anderson


had this problem last year 30 km (20 mls) after rebuilt. My engine blew out the rearmost plug just over the flywheel having been falty installed at the machine shop.
Don't care about the dizzy or generator, check the head gasket. In my car No. 2 and 3 exhaust valves locked up in the guides due to overheating and bent the pushrods, leading to damage of the cam bearings.
Do not hope the same happened to your engine, but check carefully for these points and a new head gesked should be on your list too.
Refitting of the plug is very forward following the advices M.J. made but keep in mind not to punch too often as the plug will become unlock then!

Good luck


Thanks guys - I think I understand now.

To clarify a few questions, it is the plug directly above the distributor between cylinders 3 & 4.

The temp guage when it happend was a little below normal, and the car hadn't been hot all day.

Nothing tragic happened to the engine - it was a 45F day, by the time I got home the gauge was about 1/3 above normal, but not pegged. Nothing siezed, no valve noise beyond the normal (noisy valves are happy valves).

What I'm curious about is why the entire plug came out - no leaking, this thing just blew out.

I'll look for the 3M goo, over on the gas tank sending unit thread one poster mentioned the Toyota FIPG (form in place gasket) sealant - any thoughts?

Also, when I spoke with Moss today to order the parts, they said that the proper installation is simply to pound it in place, no gasket or sealant material - any thoughts on that?

I never thought I would get to use "the proper implement" - the 6 pound fist maul (Big Ar*e Hammer for the Brits out there)- on my MG engine.

Thanks again for your help - I knew someone here would know what needs to be done.


Rob H

Some English mechanics I know drill a series of 3 holes and tap for a #8 or #10 screw adjacent to the core plug hole. Using a large diameter washer, this traps the plug in place. Won't prevent it leaking, but wiill prvent a catastrphic blowout.

I've seen this done on MGB engines prepped by Bob Yarwood up in Oregon before he retired, and on a Weslake Ford V6 used in a Capri
greg fast

I had the same thing happen to my engine with about 100 miles on the rebuild. The plug right above the distributor went out. I took the old plug into Kragen and they had a plug that uses an acorn nut to tighten it that was the correct size.

I cleaned out the hole and put a little locktight on the new plug. then cranked the nut down. It has held ever sense. I also bought more of this type of plug for the future. It appears to work very well.

Cris DeYoung

I wonder. If these plugs were not replaced at rebuild, could a chemical block bath be a factor? Either where they were secured with a gasket/sealent material or where buildup of sludge behind the plugs may have been providing a form of 'seal' against leaking of failure in the previous use?
Roger T


Don't use a drift - you'll only dent the plug!

Go here for the routine -

Chris at Octarine Services

You will need to copy & paste that link!
Chris at Octarine Services

Rob. Went through the same thing about 18 months ago. Seems like Rover put out some core plugs which were slightly smaller than they should have been. The advice I received was to throughly clean the seat for the plugs (done before installing the bad ones), install the new plugs and use a little J-B Weld epoxy when installing the new plugs. This seems to have held up well. However, I pulled the engine to do it. The fact that core plug had blown out disturbed me. As you know, there are four of them. Three on the right hand side of the engine and one in the rear of the engine. It was the one in the rear of the engine that scared me. If it blew, it would be a bear to replace. Thus, I removed all of the core plugs, replaced them using a thin bead of J-B Weld before hammering them in place, then, put a bead of J-B Weld on the outside edge where the block and core plug met. This seems to have worked well over time.

The core plug that Cris mentions is probably the one I used to keep the car on the road until I had time to pull the engine. It was purchased as Checker and is marked:

Exp. Plug, Copper

I keep one in my car "just in case".

Greg metions the physical restraint of the plugs. Some of the period publications mention the use of a strap across the plugs to prevent them from being blown loose. This system, as described, would not keep them from leaking, however. I am thinking, on my next rebuild, of making a similar set up using some 1/8" thick by 1/2" steel bar stock. It would be secured, at the top and bottom of the bosses, by 6-32 button headed cap screws epoxied in place. In the center, I would drill a hole and tap it to take an 8-32 set screw. After the core plug is hammered into place, the strap would be installed. Then, the set screw would be tightned against the center of the core plug. A nut would be tightened over the projecting end of the set screw and more J-B Weld placed upon it. All of which could be removed by heating the parts with a propane torch, but should hold up under normal engine operating temperatures. Even if something should happen that would cause the core plugs to lose their seal, the strap and set screw should restrict the movement sufficiently that most of the water should remain within the cooling system.

As to any other form of sealant on the core plugs. Someone, once, recommended the use of silicone sealer on the core plugs when installing them. Tried that. Made for a rather dramatic evening. The J-B Weld recommendation I have tested and found to be of use. Other forms of sealants have not worked well for me.

Les Bengtson

Hmmm. I thought I used a drift. Besides, it is very hard to hold the plug with your fingers with the car in the engine and smack it with the mallet! Perhaps the goober-schmuckum will hold it.

Insofar as JB-weld is concerned... Um, I've used that in other applications and you had better prepare for your machinist to get rather irritated with you when it comes time for another rebuild! It is permanent!

Thanks for the advice guys.

I'll look for the core plugs with the screw in them, and as Les suggested keep one in the car, along with the other survival gear (fuel pump,throttle cable, oil, tools...).

With the engine in the car, I need to use something in addition to the hammer to set the plug - there's no good way to get at it directly. I'll probably use some 1 1/4 round bar stock I have as a hammer extension.

I'm also glad to hear that this has happened to others and doesn't indicate a bigger problem - other than the potential damage from overheating.

The parts are on their way from Moss, and with any luck I'll get it back on the road this weekend.

Thanks again!

Rob Hardy

m. Epoxy starts to loose its strength at about 500 deg F. A propane torch is sufficient to generate that temperature in a localized area.

As to the use of a "drift", I guess it depends on how you define the term. I use a round section of steel, 1.25" in diameter and 6" long to place against the core plug and strike with the hammer. It gives me more control than trying to use a hammer alone.

Not sure I would try to install the plugs with the engine installed. I used the copper expanding plugs until I could pull the engine and they held up well for the time (several months) they were installed. My biggest concern was the plug at the rear of the block which would be a bear to replace in situ. Since the car was my daughter's, I elected to pull the engine and install new plugs, secured with J-B Weld, in all the holes. The system has held up well over many thousands of miles, including long road trips. Were it my own car, I might just have left the expanding plugs in place and driven the car, confident that, should a plug blow, I would realize it in time to prevent engine damage. However, after my experiment with the silicone, having three core plugs blow out while doing the initial engine break in (good learning experience--do not believe all you read even if it seems to make sense), I do not trust the plugs after the first one blows out and would not recommend that anyone do so.

The core plug nearest the dizzy seems to be the first one to blow out when there is a problem. Thus, a good indicator.

Les Bengtson

The simple/quick/cheap do it anywhere fix to a blown block frost plug is to put in a block heater. Clean hole, insert block heater and tighten the machine screw to pull it tight against the bar.
Kelvin Hawkins

Not being the best predictable "aim and hit" with a hammer, I take one smaller ball peen hammer and hit it with a bigger hammer to dimple the plug.

But my '73 has one core hole that just won't hold a standard plug. I found a bronze expanding-type with a stud and nut that has been in for years (evidently, like the one Les used).

Wayne Pearson

I think what I'm going to do at this point is:

1) Replace the blown plug with a new one from Moss with the engine in the car since I can get at it fairly well.
2) Hope for the best, but have an expanding plug with me when I'm out and about in the B
3) Have all four done when I have the tranny re-built next year. That will complete the mechanical overhaul of the B.

Thanks again - especially for the JB Weld tip - I can see many many uses of that stuff.


Rob H


Hammers have hardened faces which are relatively brittle. Hitting two together is known to cause one of them to shatter, throwing sharp steel splinters about. Doing this is a definite NO NO!


Larry Hallanger

Larry is correct, at least that was what I was taught in a machining course many years ago. That is why such things as center punches are made with a hardened steel (tool steel) forward end and a mild steel rear section with the two being welded together prior to heat treatment. Same thing with axes and hatchets.

Les Bengtson

A round blunt bit installed in an air chisel can be used to install these in the car.
John H

Okay,to everyone out there...We all know what freeze plugs look like, a debate....what is the correct side to face inward on a freeze plug???My friend (that has been a mech.for 28 years said the machine shop installed mine backwards when i had my block hot tanked for my rebuild.They installed them like this ( FACING INWARD: and he states they should be installed like this )facing inward to the engine!! sooooo who is right on this issue?YOU HAVE to love my diagrams,Right?I dont want mine to blow when i fire this machine up.Thanks,Rich
Rich Osterhout

Rich. The domed section faces outwards and you beat on the center to firmly seat the core plug into the block, which has a lip to cause the plug to expand outwards and seal.

Many cars use a cup style plug which has the curved section facing the outside of the engine and the domed section facing the inside of the engine. The MG is not one of these, however.

Les Bengtson

I experimented with several plugs, removing them after fitting with a small hole drilled in the centre and a slide hammer to see how well they could be retained by expanding with a drift or hammer and various sealers.

Due to their design expanding these plugs doesn't hold them very tightly. A sealer must be used as the block edges may be pitted and the pattern core plugs are not especially well made. Using a soft (non hardening) gasket paste merely lubricates the process and they slide out very easily (silicone is the worst).

Clean the block and use JB Weld 300C epoxy as a sealer/retainer just as you would gasket paste. After expanding smooth a neat fillet around the edge which will help retain the plug.

A hammer alone is imprecise and tricky if the block is in the car. Use a flat-faced drift of mild steel, alumin or brass a 1/4" smaller diam than the plug. This ensures the plug is expanded from the centre outwards. Keep it square and hit it till the top of the dome flattens 3/4" diam on top. You'll feel it start to go after just bouncing as you make the first strikes. If you don't flatten it the plug just springs back rather than expanding to the block.

Just a thought about the temperature reading for future reference - if the sender in the head had no coolant around it wouldn't the temperatures have been much higher than indicated?
Steve Postins

Thanks to all,and to you Les,I will imform my mech. friend that they(machine shop) installed them the right way.Thanks,Rich....( domeside out ...RIGHT?????
Rich Osterhout

<< domeside out ...RIGHT????? >>

Yes - as you look at it the centre of the plug will be nearer to you.


Rich,dont confuse me now with this "center" stuff.<<<< this is (< domed,and this"(" faces outward.... IGot it
Rich Osterhout


"(the one on the back of the block is a stinker as it is behind the rear cover plate and requires an engine R&R and un-mating of the transmission to get to)."

Not so. My dad blew this one out of his '63. When I saw the hole in the rear cover plate, I told him we could rig something to replace it without pulling the engine. I searched the archives, sure enough someone had "been ther, done that" and beat me to it.

FYI: Check the archives before you pull the engine to replace the core plug in the back of the engine.
Carl Floyd

Rich,dont confuse me now with this "center" stuff.<<<< this is (< domed,and this"(" faces outward.... IGot it

Yeah, if you say so he he :)

Funny, all my core plugs are cup-shaped, both V8 and roadster.

For a gizmo to aid in installing the rear core plug do a Google search on "Betson Bolt" (include the quotes). I think Chris denies inventing it, but he described it and the name has stuck.
Paul Hunt 2

This thread was discussed between 27/03/2006 and 30/03/2006

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