What causes a core plug to drop out?

[tom1985]

Alright everyone,

Finally got my Morris (it's a '68 2-door) back on the road after 6 months after fitting new seats, carpets, replacing the rear brake cylinders and getting around to getting it MOT'd. Didn't get far on my first run though. Just down the road from my house I noticed the temperature gauge (glad I fitted one now!) rising alarmingly fast, passing 100 and carrying on. I pulled over in the nearest available place to investigate. Turns out a round plug or cover, about the size of a 2p coin, had dropped out of the side of the engine (immediately behind the oil dipstick) and the engine had lost all of its water as a result. Got the moggie recovered safely home without too much fuss thankfully.

Having searched various posts on this forum I am now aware that the plug is called a 'core plug' or 'frost plug' and are there as a precaution against damaging the block if the water freezes. However, the winter hasn't been overly severe - certainly the car has survived worse without anything untoward happening - and the water contains anti-freeze too. Has anyone got any idea why this might have happened, and if there may be some more serious underlying problem with the engine? Would these pop out if the engine overheats? Or do the plugs just wear out sometimes? The only difference between this winter and previous ones is that the car hasn't been driven at all for the last 6 months (longest period out of use since I've had it); that said, it drove to and from the MOT garage without any issue.

Couple more questions - is any damage likely to have been done to the engine (if it was OK previously that is) by driving it without coolant - even though this was only for a few hundred yards? Secondly - assuming it is just a case of replacing the core plugs - do I need to buy anything else? Do they require a gasket or sealant of any kind? Is it wise to replace the others at the same time?

Thanks for your help - and I hope you're having better luck in your Minors today than I have! Cheers

[bmcecosse]

I guess it just had decided it's time had come - if there was antifreeze in the block, it won't have been the frost. Engine damage - depends how long/far you drove it before stopping......it certainly won't have done it any good ! When renewing - clean the location carefully - smear of something like red hermatite is often used -and bang the new plug in HARD, so it collapses in the middle. Then wipe away every trace of the incriminating hermatite...

[DAVIDMCCULLOUGH]

Ive had one or 2 just simply rust, even in my blue minor that get almost everyday use. Started as a trickle that left a rusty mark and lowering of the water level, then I noticed and changed it. At least it the easier one to get at behind the dipstick and no the one behind the oil pipe and distributor!

[welshrat]

Also had one or two fail from rust, core plugs are mild steel, usually start as a few pin holes then show as a major leak when hot. The only one I recall as the plug dropping out was a MK 1 escort which had no antifreeze and an -10 frost. Motor will probably be fine, knock a new plug in and see how it goes.

[mike.perry]

A friend witht a 1950 Series MM had the core plug a the back of the block blow out on the way to a Nat. Rally. We managed to get the car to a nearby garage which just happened to be opposite another Series MM owner. Normally it is an engine out to replace the back pore plug as there is very little room between the back of the engine and the bulkhead crossmember.
Anyway the garage owner disappeared into his workshop and came out with a core plug and a crowbar. He placed the plug in the hole and put the crrowbar against the crossmember, gave the coreplug a quick tweek and it was in. It stayed there with a slight drip for several months until the engine had to come out for another job

[simmitc]

About 30 years ago (golly, I'm suddenly feeling old!) I had a pin prick leak in the side core plug, which I stopped with a small self tapping screw. On another occasion, a factory (BMC) reconditioned engine ran for about 500 miles before the brand new core plug popped out. I too spotted it only because of a temperature gauge - with all water gone, there's nothing left to steam. That engine then went OK for 60,000 miles before I sold it, so it didn't seem to do any harm.

[tom1985]

Thanks for your replies guys. Helpful as always. As for fitting the new plug, what tool is best - hammer or mallet?

[bmcecosse]

You need a heavy hammer - and a metal punch (not sharp - obviously) to wallop the new plug in the middle ideally with one really hard blow, so it seats and the centre collapses inwards to wedge it in place. 'Wee taps' are definitely not the way!