One of my core plugs has become weepy over the last month. Engine has about 1000km on it and was built in summer. The core plugs are the convex brass ones i did use correct sealer on them as well. It doesn’t weep when the engine is warm, or even during the day. Not enough to drip, i can just see some fluid sitting. I wipe it and it doesn’t immediately re-appear.

It has been getting down to 5 &6 degrees overnight, and i see the fluid the day after I’ve been out. I figure the brass is shrinking just enough to let something by.

Question is - how worried should i be ? Obviously dont want it to progress. Should i wait for a coolish morning and give it a few more taps ? Or pull it out and start again. Anyone successfully corrected an issue like this ?

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1966 Mini Deluxe - “Gabby” 34 years under restoration and counting...
1970 MGBGT
1959 MGA

I am no expert by any means but if it was mine I would use the larger microadjuster I have nd give it a good hearty whack or three before any other course of action.

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Cheers Dave.

Hi, there is only one way. Remove it.
Those small motor dome plugs have to be flattened/stretched a reasonable amount to be secure. They can fall out, and if you miss any water/ steam then you may destroy your motor by overheating it. The temp gauge does not read when you run out of water.
INMHO, I would drill a hole in the middle of the welch plug, fit a large self tapper you can grab, and see how much effort it takes to pull it out. If its loose, then chances they all are.
The race guys 850/998 etc used to tap a thread either side of the welch plug, and screw a plate over it to be certain.
Its an easy job now.
cheers Lindsay siebler

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1967 EX works rally mini restoration virtually complete OBL45F
1965 Mk1 Cooper S
1970 Colt 1100F SS
2014 Mitsubishi Imiev fully electric daily driver
1948 Allard M coupe Olds 215 v8 project
1971 Twin cam Escort ex Uk championship winning rally car project

All mini blocks are getting older and there is often corrosion of the cast iron around the core plugs and the thermostat seat too. The corrosion may not be obvious until you see the leak residue.
Had the same with a rebuilt 1275 block. Removed the leaking plug and replaced with a cup type of plug with a coating of Stag gasket cement.
I assume the cup type plugs are available for the small blocks.
I have more faith in cup type as there is a larger surface area for it to seal against and you can see how far in it must go. The disc type needs a bit of practise to know how hard and where best to hit it when you don't do them very often.

Pretty sure the cups are 1275 and Cooper s only. Small bore are the convex ones.

I did notice the original ones in there were not brass which wouldn't have the shrink issue - but then again would corrode.

Like you say, probably a bit of corrosion/pitting on the mating surface would do it.

Ok - What IS the best method to flatten out then ? I started in the middle until it deformed enough to not move with my finger, then spiralled outwards until it looked relatively flat. Used the 3 bond sealant.

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1966 Mini Deluxe - “Gabby” 34 years under restoration and counting...
1970 MGBGT
1959 MGA

one decent thump in the middle of it - I've only built one small bore motor with lens type plugs, but the one I did had a plug pop out the first time it got hot, I put it back in and gave it a good bash and it's still there 8 years later

it sounds like you were a bit too gentle with it - the goal is to "invert" the curve

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did I tell you that I won a trophy?

Hi, I use two hammers. and you should wear safety goggles to do this. I have one with a flat round head of 25-30mm odd and hit that quite hard with a largish copper hammer. As Simon says, you need to see a good flattened out area. You can sort of feel the plug tighten up. I also put threebond sealer inside, just like you appear to do. I am sure there are a multitude of ways to achieve the same thing.
Cheers Lindsay Siebler

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1967 EX works rally mini restoration virtually complete OBL45F
1965 Mk1 Cooper S
1970 Colt 1100F SS
2014 Mitsubishi Imiev fully electric daily driver
1948 Allard M coupe Olds 215 v8 project
1971 Twin cam Escort ex Uk championship winning rally car project

I have used an O/S unit for Cooper S (cups) as the pitting around the surface was not going to withstand any OE replacement - with sealer. I had the cores re-sized to accept Honda cups.

Small bore dome OE plugs are 1 5/8" (41.275mm). If you go down the path of a re-size you could do a search for non-OE replacement units after a resize of cores. 'Kelpro' do a 1 13/16" / 46.03mm for example - doing a search for more appropriately sized unit could be an option. If you go down this route allow 10-15 thou in the core undersize to allow the (brass) plug / dome OD to interference fit - with sealer.

I have used Loctite 540, a thixotropic, anaerobic sealant (pricey), or Permatex No1. HTH

Well this is a timely thread - Minnie has now got the same incontinence problem

Motor was fully rebuilt and has done about 2,500 miles now. I first noticed a slight weep from the middle plug on the front after a run on a pretty warm day. Temp never got over about 95 as I was keeping a close eye on it. After that run, I noticed the green engine paint had come off as well as the red stag that was on the outside of the plug. Mainly around the edges of the plug. Other 2 are fine and paint still like new.

As it's not leaking much, I haven't worried about it but did a run of a few hours last weekend and while parked overnight, I noticed about maybe a tablespoon or so of coolant on the drip tray and some coolant also on the block below the centre plug.

The plug itself still has quite a noticeable "curve" shape to it, much like a new one I suppose (I didn't fit them so not sure).

I've read the info about giving the plug a good whack but before doing that, I'd like to check if:

1/. It would be best to pull the old one out and start again?

2/. If I try giving the current one a hit as described, what can go wrong? Can I hit it too hard or will I break whatever seal the stag has and the leak will be worse?

I've never played with these before so would be getting an experienced Mini bloke to put a new one in if that's the best option.

Thanks Rocky

I don't quite understand: if small bore engines use 1 5/16inch lens type plugs, that is the size of Cooper S cup-type plugs. So Cooper S cup plugs could be used on small bore blocks?

Also, is it possible that an anaerobic sealer won't work on a leak because there is a passage for air or water to track out along the corrosion path, limiting the anaerobic sealer to set. That it needs something to fill the gap and air dry.

The Cooper S block has no shoulder in the bore? the small bore block recess is probably too shallow for an S cup to work....
Edit
I'm going on memory here! I sold my last S block some years ago.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

No.. Cooper S blocks have a deep recess, small bore blocks have a shallow recess. Lens type plugs would work in an S block (not that anyone would do it) but cup type plugs won't go in a small bore block

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did I tell you that I won a trophy?