Core plugs ... how to ensure they don't leak

[Martin S]

Removed the two cylinder head core plugs yesterday adjacent to the spark plugs (not the one behind the exhaust manifold). The plugs were 'thick' ones and even though I tapped on one side of the inner cup they pushed through rather than pivoted. Not an issue ... just grabbed em and pulled em through with grips.

On inspection ... 1. a previous owner (PO) used silicone as sealant ... and plenty of it ... risking detritus circulating around system and 2. the holes have one or two vertical scores; I might have caused one ... the PO must have tapped the plug home with a screwdriver or something on the rim causing a high spot that scored the hole as it pushed it through.

So ... how to ensure a good seal when putting new plugs in? Forum rummaging identifies a number of robust candidates ... Hylomar, Wellseal, thread locking compound, RTV ... but what about something like Quiksteel (epoxy metal putty stuff)?

What's current 'best-thinking'?

Thank you in advance.

[Phil S]

Alway used hylomar blue gasket sealant, and a correct size socket to tap them in or if you have access to a lathe turn up a correct type of tool with a lip to stop the plug going in to deep

[dasadrew]

What's not to like about Loctite 648? "Designed for the bonding of cylindrical metal parts"

0900766b800b9992.pdf

I wouldn't use Hylomar - I find it has a stealth ability to act as a lubricant, but each to his own.

[richardthestag]

I have used wellseal until I had a series of failures which turned out to be due to the core plug and not the application of wellseal.

I subsequently bought this stuff https://enginecoreplugs.com/product/...red-70ml-tube/ Be warned it is very messy stuff to work with, plastic gloves are essential.

I use something very similar to that Drew mentioned above to locate bearings in gearbox casings, good for anti fret properties especially with the different expansion properties of the different metals.

Some folk use an epoxy metal glue, effective but I will bet the next person to try remove those will not be thankful.

Try to fit them with an impact socket that is just less than a snug fit inside the new coreplug, unless you want the socket to become a semi permanent attachment to your engine. Knocking them in square is the tricky bit, especially if the engine is still in situ. End up with the outer edge of the coreplug just below the outside surface.


P.S.
Rover v8 story and the reason that I found Dirko HT. I fitted a complete set of core plugs to the Rover v8 in my Range Rover and suffered a couple of the little bastads exiting at max velocity during driving.

I refitted, pressurised the system and no leaks, couldn't figure it out. Then after the second failure I decided that rather than pumping pressure into the system I would run the engine up to temperature.

No leaks until the engine started to warm up and then each one started to weep.

Full debacle is here; https://youtu.be/T16JgtQRp34

I swear, if you don't like the occasional swear word then please avoid.

In a nutshell the coreplugs under the same part number came in different sizes depending on who you bought them from.

[Seaking]

I used dirko ht grey on my inlet manifold. when I removed manifold to fit water pump bung. Not one leak in 11 years. As Richard says messy stuff but works for me.

[singapore stag]

Don't forget.... they are there for a reason and should'nt be bonded in rock solid or they won't do there job if needed. Never had to replace Core plugs, but which do you fellows use, Brass plugs? Or other?

However, IMO only...,Tigerseal will stop any leaks, dead cert! But getting the plugs out at a later time will be a pain.

[richardthestag]

Never had to replace Core plugs, but which do you fellows use, Brass plugs? Or other?

However, IMO only...,Tigerseal will stop any leaks, dead cert! But getting the plugs out at a later time will be a pain.

they are usually bare mild steel, I guess because they can deform to fit the hole so to speak. Some are zinc coated. They often rot from the inside, usually because there is a world of silt piling up behind the lower edge.

Backflush and use a decent corrosion inhibit and they should last longer.

Dirko HT doersn't leak with core plugs that are the correct size

[Martin S]

Thank you everyone for what is always a solid and wonderful bunch of people helping each other out ... and helping people like me avoid those potholes in the road to loving our classics. Happy Christmas everyone.

[jbuckl]

approximately 50-100 microns interference is needed…..for 25mm- 50mm diameter plugs.
up to 200 microns will work.
if the bore is undamaged, no sealant is needed.
wellseal will work for small (up to 100 micron deep) scratches if the fit is tight enough.

if plugs are pulled out at 90 degrees to the bore axis they will scratch the bore. The scratches should be addressed.

I watched Richard’s pain with poor fitting parts.

number of sold parts is a zero indicator of parts quantity (!)

core plug fit can be adjusted by swaging the plug before fitting….within reason.

pretty certain I’ve had to make metric plugs fit imperial holes when the parts have been incorrectly supplied…. But only found out when it’s a bit late!

[DJT]

ISTR that core plugs used to be made with a convex centre so they were fitted into the hole, then a sharp tap in the centre splayed them out to seal.

[dasadrew]

ISTR that core plugs used to be made with a convex centre so they were fitted into the hole, then a sharp tap in the centre splayed them out to seal.

I know of two types, used in different locations. The "cup" type with a sidewall, and the more-or-less flat convex type. The second type I've seen on gearboxes and the like, the first type on engines, where a tap in the middle would probably loosen them IMO.

[mole42]

ISTR that core plugs used to be made with a convex centre so they were fitted into the hole, then a sharp tap in the centre splayed them out to seal.

ISTR that as well - my early days were building rally Minis (A-series engines) - but I'm pretty sure the cup-type core plugs had a convex centre, so they were pushed into the hole and then a light tap with a drift and a hammer in the centre would 'lock' them in. No sealant, no leaks.

[richardthestag]

but the hole would need a step on the inside to support that type. cup plugs are more common on tv8

[marshman]

There are two main or common types. "Cupped" as per the ones used in the Stag heads and "Domed" (also called Welch Plugs) like the one on the top of the engine block.

I am fairly certain that the cupped ones are used in alloy (aluminium) castings and the domed or Welch type are used in cast iron blocks and heads. I assume that this is because the domed ones need to sit against an lip or ridge and because the are designed to expand when the dome is "flattened" I assume that this would not form a good seal against a relatively "soft" aluminium casting, hence the "cup" type which has a much larger contact area with the casting to form a seal.

Having said that you do see smaller diameter Welch plugs in alloy castings such as carburettors, some inlet manifolds (not stags) and on the rear face of Armstrong lever arm shock absorbers. But in those applications they are not sealing against the large temperature variations or pressures of the cooling system.

For what it is worth my goto for sealing the cup type as used in the Stag is a PU sealant such as Tiger Seal. Sticks and seals well - good gap filling properties, remains slightly flexible and doesn't stick so well that it makes them hard to remove in the future and not so "sqiudgy" when applied that it is likely to get into places you don't want it. I also use Tigre seal on the inlet manifold gaskets for the same reasons.

[DJT]

Something like this:


IMG_1618.jpg