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MG MGB Technical - freeze plug blowout

i have completely rebuilt the engine 73b all new parts block cleaned new therm. watwe pump hoses and rad. cap. i lost 1 freeze plug center right. then 1 mo. later lost the right rear one over the dist. is there too much pressure or what, thanks les
l.f. frank

Hi Les

It could be you didnt quite fit em right, more likely detonation is knocking them out. We tend to strap our plugs in with ally bar and tiny cap head screws.

Peter
P Burgess

I just replaced mine as they were weeping. Was no danger of them blowing out though as they were a bugger to remove. I broke the tip of a screwdriver and a metal punch trying to prise them out after puncturing them with a cold chisel.

I replaced them with new ones and used JB Weld as sealant around the edges from a tip found in the archives. I made sure they were well seated in. I hammer them using a small steel block I have about 3/4 of an inch diameter sitting on top of the plug then hit that with a BFH. I whack them till the middle is at the same level as the edges. Been for one drive and they seem good now.

I would doubt the cooling pressure would push them out? That should only be about 15PSI or so? Must be limited by the radiator cap. Didn't know detonation could though!

Simon
Simon Jansen

This usually happens because the plugs haven't been expanded enough in the center on installation. I use sealant on the outer rim of the plugs and hammer the center with a steel drift. Never had one leak or blow out. Your cooling system can't get the pressure up high enough to blow them out. The radiator or heater core should burst long before the plugs should come out. RAY
rjm RAY

Hi Simon

Even more prone to detonation shock wave removing the plugs with a 1950.

You have to remember that vast amounts of energy are instantaneously produced when detonation occurs, you are basically trying to contain an explosion in the cylinder, the shock wave travels through the water and the path of least resistance is the core plug. Under 'normal' conditions the flame front propogates at around 2.5 meters per second producing a steady pressure rise. Detonation is instant ! Just think of the pictures of Fuel/Air explosives demo'd by armed forces and you know what you are contending with.

Peter
P Burgess

Setting the dish core plugs is best achieved using an ordinary hammer, hitting the plug centrally several times - this creates the usual "dimple" effect seen on the plugs - the centre of the dimple should be level with the outside rim of the plug. Using a drift in the centre of the disc will not spread the plug sufficiently. It helps to use a little Loctite 270 or 271 as an additional sealant - I wouldn't use silicone sold for gasket making.

Gold seal engines usually have the plug seats bored out and are then fitted with cup core plugs these are fitted in a slightly different way as too much central impact will loosen the plug, not seal it. I use the Loctite again and then drive in with a stepped drift that fits inside the cup but rest on the rim.
Chris at Octarine Services

Hi Peter, that all makes sense. You're right about the power of an explosion. It always annoys me when people talk about internal combustion engines working though 'an explosion' in the cylinder pushing the piston down when it's a controller burning. Like you say, explosions in an engine are bad!
Simon Jansen

Peter,

I've always set them with a hammer as Chris describes.

But, for future reference, I would like to know where you drill and tap for the screws. Can you offer a pic or a more detailed description?

Charley
C R Huff

Hi Charley

Nothing clever I am afraid. The bar is approc 1/2" by 1/8" by the width across the flat part next to the plugs. we use 4 mm cap head set screws to retain the bars. Pack the gap between the plug and the bar. We use locktite or pipe sealant on the threads. The front bar is vertical and to the front of the coreplug centerline (fouls alternator bracket if otherway or in middle). The other plugs are strapped horizontally. I will see if I can get a reasonable pic and post it. The MG factory did this as a mod on some of the comp MGA Motors.


Simon

We have seen this happen with our own eyes on a couple of 1950 engines, makes you jump when the engine is underload on the rollers and blows a coreplug out!

Peter
P Burgess

"Gold seal engines usually have the plug seats bored out and are then fitted with cup core plugs"

I'd always wondered why mine were different to the ones everyone else described.
PaulH Solihull

Thanks Peter,

But, I'm still not clear about if and where you are drilling and tapping the block for the cap screws. Are you drilling and tapping into the boss around the perimeter of core plug, and if so, are you drilling a blind hole and not all the way through?

Charley
C R Huff

Hi Charley

Drilling right through on the flatish areas next to the plugs.

Hi Chris

I am not sure the cup-type plugs weren't standard fit on later blocks ...info seems hazy? Gold seal early ones werent done this way, seems like very late blcoks only.

Peter
P Burgess

Thanks again, Peter. It's a good tip that I have never thought of, but I have thought of how bad it would be if one fell out.

Charley
C R Huff

Hi Peter,

I have never seen a late engine with cup plugs and I have a late 79 car myself.

On the other hand, all the gold seals I have seen have them and not the disc type.

It maybe that they changed over to cup plugs at some stage and since the gold seals continued in production after 1980 then they would have been fitted with them - perhaps earlier gold seals had the disc type.

I always assumed the cup type were fitted because they couldn't guarantee a good seal on a possibly corroded seat?

Another one of those interesting facets of the B series!
Chris at Octarine Services

I attach a (not very good) picture of strapped core plugs.

Peter

P Burgess

Thanks for the clarifying pic.

Charley
C R Huff

Reading this thread reminded me of what happened to my B 20 years ago, I had taken the engine out and had the block bored, crank ground, balanced etc. I reassembled it with a stage 2 head.
The Machine shop which had a very good reputation for producing performance engines had renewed the core plugs. My wife as was then, took it out soon after, with strict instructions not to rev it, hammer it and so on.
When she returned,( I just happened to be on the drive ) I could tell the engine was rather hot. Opening the bonnet revealed a core plug had blown out, how long she had driven it I don't know. The machine shop boss came out and replaced the plug saying the engine paint didn't look burnt, and it would probably be ok, sure enough it's still going strong.
c cummins

This thread was discussed between 19/10/2010 and 29/10/2010

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