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MG MGB Technical - Freeze plugs

I installed a rebuilt engine about 3 years ago and have put 32,000 miles on it, mostly highway. This is an engine that I bought already built and I am getting ready to rebuild the engine that came out of the car. I would like to get another 30 - 50K out of the engine in the car before having to remove it.

Aside from serious oil leaking at first, the replacement engine has been fine.
The engine had been carelessly sealed so after replacing the timing cover seal and properly replacing the timing cover and rocker cover gaskets, the engine is mostly dry. It is clear that the rebuilder was not meticulous about sealing.

In early January after a series of nights below freezing, I noticed antifreeze leaks at two freeze plugs. The leaks became progressively worse until a dose of radiator stop leak powder stemmed the flow for the most part. Last weekend, I made a 600 mile roundtrip and there was antifreeze on the garage floor pan the next day after the engine completely cooled off.

One of the plugs is accessible, the one over the distributor. The other leaker is at the rear of the block and would not be accessible without removing the engine. The question is whether or not one of these freeze plugs could blow out, especially when the weather gets really hot. This would be a disaster a long way from home. Some of the trips are 5,000 miles or so. Also, is it worth replacing the freeze plug the sits above the distributor? If so, how difficult is this? I have a replacement which is the cup shaped plug, not the disc. I suspect that the mechanic was not meticulous about installing the freeze plugs either.

I find it strange that it took almost three years for them to start leaking. The coolant mix is heavy on the glycol, maybe it should be diluted a bit with tap water (with all of its calcium and other minerals).
Glenn Mallory

Flat or cup? Flat can pop out if you get detonation, cup much less likely.

More care is needed with fitting flat, you need a drift that covers most of the surface to drive it near flat to generate the greatest pressure against the sides of the hole. A smaller drift or ball-pein hammer can dent the middle inwards before the outer part has flattened enough to create the seal.

If you can get the rear one out a replacement can be fitted in-situ with 'The Betson Bolt' - loads of references to this found with Google.

If you had the correct ratio of antifreeze then it shouldn't have frozen and shouldn't have damaged the plugs. So maybe just down to sealing, and they can corrode. I had that with a cup-type fortunately the middle one so easy to replace in-situ.
paulh4

I used to use tap water, before moving to an area with harder water. I now use the water that I save from the dehumidifier in the garage.
Dave O'Neill 2

Core plugs leak, sometimes after bad installation or due to corrosion.

If you had a correct anti-freeze mixture, the cold would not have caused leakage. Neither would warm weather running as coolant pressure is controlled and limited by the pressure cap.
Your block may not accept the cup type Core plugs as the counter-bore on the recess needs to be deeper than that required for disc plugs.

Disc core plugs are slightly convex when installed and the seal is made by attempting to reduce, I,e., flatten, the curve.

You might try repeating this "flattening" process which could improve the water tightness of the plug.
Allan Reeling

Yes, the rear plug can/will blow out. Yes, you can get to it without pulling the motor. There is a hole in the rear plate that aligns with this plug.

Mine blew out during the initial break-in run.

With metal pugs, use a magnet on a stick to get it aligned.

After you get it installed, put in a Betson Bolt.

This is a bolt that you put through the back plate hole with a nut on the inside. It puts pressure on the plug to keep it from blowing out.

I lathed out a threaded piece that fits into the rear plate hole.






Bruce Cunha

Thank you for information about the Betson Bolt solution.

Since both of the plugs are still in place, I am wondering if installation of the Betson bolt against the existing plug at the rear of the block might solve that problem. Otherwise, extracting the existing plug looks challenging as would having it pop out at speed thousands of miles from home. Maybe carefully tightening a bolt against the current offender would offer at least offer insurance against spontaneous rupture?
Glenn Mallory

I think it would take a considerable time for the plug to corrode, if the system has been filled with antifreeze, so it’s more likely that the plug wasn’t fitted properly. Personally, I would try the Betson bolt against the existing plug first.
Dave O'Neill 2

I am almost certain that the plugs were not properly fitted. I have maintained the cooling system and the antifreeze has been replaced with no signs of corrosion in the system otherwise.
Glenn Mallory

in checking to see how difficult the installation of the Betson bolt might be, I discovered that there is already one in place. Would the rebuilder have put one in just in case? The bolt is drawn up firmly against the transmission side of the backing plate so a longer one wound be required to further dimple the freeze disc. At least I am reassured that the leaking plug has no place to go.

Note the leaking plug over the distributor. With the Stop-Leak in the cooling system, there is currently no dripping. Maybe I should replace the disc over the distributor for peace of mind.


Glenn Mallory

Difficult to tell from the photo, but it looks like there is a dimple in the middle of the plug, so it may have been over-centred.
Dave O'Neill 2

The disc over the distributor was probably hit too hard. But I didn't expect to find the "Betson bolt" pressing against the disc at the rear of the block. I can't believe that the rebuilder took a preventive measure to make sure the plug didn't come loose. It must have already been on the backing plate in response to a previous failure prior to the rebuilding of the engine..
Glenn Mallory

Maybe easier to leave it in :o)

If that one over the distributor is dented inwards (as looks) then the wrong tool was used, i.e. a ball-pein hammer or similar-sized drift. It needs something that is nearly as big as the plug. Even though it has to be hit harder to flatten it it's much harder to go too far.
paulh4

Does the Betson Bolt over the rear plug look OK as insurance against a blowout? It is visible in the image, although not prominent. I have no idea why it would be there on a rebuilt engine that had never been run.

Regarding the tool used on the discs, I can only imagine that it was a hammer and not according to the proper procedure (I have found other signs of sloppy work on this engine).
Glenn Mallory

Glenn. Cris Betson's idea of using a bolt to provide additional support to the rear core plug is simply to provide a little extra insurance on a core plug that is very difficult for most of us to replace. It, in no way, was ever intended to make up for a sloppy installation. Many cars, including every factory built engine, got by without such a device and provided years, often decades, of fine service without problems. But, after we had discuss the matter here, many years ago, and Chris had posted his idea of providing this form of extra insurance, many of us adopted the idea and use it on our own builds. Similar to "strapping" the core plugs as the factory did on some of their racing/rally engines with the added advantage that the bolt is physically touching the core plug and may work better than the factory solution. It, also, means that you do not have to drill and tap the block as you would with the strapped form of core plug support.

Les
Les Bengtson

Thank you Les.

I checked the bolt through the backing plate and it is making firm contact with the plug in the rear of the block. Coolant has been seeping down the block but in minor quantities, especially with StopLeak added.

I will replace the plug over the distributor. Hopefully the cupped replacement that I obtained will go in firmly. Should I use Locktite around the edges as a lubricant and sealant?

Then if there is a minor leak from the rear plug, I know that it cannot blow out. I will be able to live with that until the engine is sufficiently worn to drop in the one I am currently building.
Glenn Mallory

From MGA Guru:

"Chris Betson kindly advised that:
The new core plug should be edge coated with a Loctite permanent sealant, not silicone.
Place the core in a clean and dry recess in the block Put the bolt in the backing plate and tighten to just touch the plug.
Tighten the bolt at least another full turn, depending on the pitch of the thread, so the center of the plug is depressed level with the outer edge
The bolt can be secured with Loctite."

The implication of the final sentence is that the bolt was left in position.

Chris Betson was using it with the dished type, for the cup-type if there isn't one there already I'd say you will need to use a 'spreader' that fits inside the cup as it will need to be pressed in all the way with the bolt, you want the cup-type to remain flat once fitted.
paulh4

I once had a block which had cup type core plugs fitted. It was a factory reconditioned gold or silver seal engine. I believe the core recesses were deepened to accept cup plugs as part of the reconditioning work. Before you create more problems, once the leaking core plug is removed, check the depth of the recess. Cups need to be pressed in all the way. The core plugs I have are 5/16" deep!!
Allan Reeling

My Gold Seal engine is the same, and I think it was Peter Burgess that told me those blocks were bored deeper to take the cup-type, although Nigel Brown has said his has the dished type, close to the surface.

Even the depth of the cup-type varies, mine are flush, but this unknown engine seems to have them recessed by about 1/8".

As with anything else concerning these cars examples vary!

paulh4

To the best of my knowledge, only the dish type plugs were used from the MG factory on their new engines. In my experience, when properly installed, the factory style core plugs hold up quite well in use.

Some time ago, someone recommended the use of silicone sealant on the core plugs, I tried this and the result was four blown plugs on initial start up. A through cleaning of the recesses and new plugs corrected the problem. Silicone is some lubricant properties and simply allowed the plugs to blow out as pressure built up.

Another suggestion was to use some J-B weld on the mating surface of the block and core plug, flatten the core plug properly, and put more J-B weld around the outer area of the plug where it mates to the block. This system is John Twist's current recommendation when we discussed this topic recently.

Loctite is inferior for this type of application unless you go with one of the sleeve lock products, the normal "red" and "blue" thread locking compounds do not work as well and are not intended for such things as holding core plugs in place. The sleeve locking versions will do a better job than the thread lock, but are not really intended for such use.

Les
Les Bengtson

This thread was discussed between 14/02/2021 and 18/02/2021

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