Cylinder head welsh plug replacement
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Thread: Cylinder head welsh plug replacement

  1. #1
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    Default Cylinder head welsh plug replacement

    When replacing the welsh plug behind the water pump on 505sr is it important to use a steel plug as opposed to a brass or alloy one, or doesn't it matter ? How should it be held in ? Any thing else to watch out for while I'm at it ? Thanks in advance..............Wayne

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    Fellow Frogger! Meggsy's Avatar
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    The replacement welsh plug should be the same4 metal as the adjacent material - the welsh plugs is usually retained by the friction from knocking it in - it is a good practice to use something like duralac https://www.whitworths.com.au/main_i...AbsolutePage=1. In the past I have put a couple of indents with a centre punch around the outside to ensure a better fit say one each 60 degrees. The need to use the same material is due to electrolytic action that accelerates corrosion - Id buy the original plug if I( were you.
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    Fellow Frogger! andrepug's Avatar
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    Haven't seen a welsh plug mentioned for ages so did a quick google on the name - click
    History of the Welch Plug

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    If you use coolant in the future, you should reduce any potential for corrosion. From memory, the original welsh plugs are galvanized or zinc coated.
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    1000+ Posts Beano's Avatar
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    My 505SR does not have a welsh plug behind the water pump. My '83 GR did, but I believe they were phased out sometime in '84. Is yours an '84 model SR ???

    Even though it's dissimilar metal, brass should be ok if you use corrosion inhibitor.
    I always fix em in with screws, as often the hole in which the plug is retained is corroded and I don't trust this to retain a plug anymore. Don't even think about using ordinary old self-tappers. They break off very easily (and rust). Stainless steel screws are the go (they are REALLY hard to break), and you need to drill the holes in the head a little wider than you would if you were putting the screws into sheet metal. This is because you are screwing into solid metal. Usually (as you know), the drill bit has to be the same size as the central part of the screw...disregarding the threaded part. But if you do that, the screw will be very difficult (if not impossible) to put in. Try it and see.....there is no harm done by drilling too narrow a hole at first.

    You can do it easily if you just take out the radiator. The cylinder head does not have to come off. Apart from that, the question is WHERE to drill :
    The way I do it, is that the plug is held in by 3 screws. The holes are not drilled through the plug, but just outside it's edge. You cannot drill THROUGH the plug, because the screws need to be a little way outside the large hole (the one you are causing to be filled by the welsh plug). They are too close to the edge of the hole if you drill THROUGH the plug.

    Two screws on the left side go into one of the columns (in the head) through which one of the head bolts go, and another through thinner metal. Use 4 if you like. I use 3. Have a good look and you'll work it out. They are almost equidistant, but not quite. Enough so they stop the plug coming out though. The screw bodies stop the plug moving sideways, and the heads stop it coming OUTwards.
    Oh...and I use a tiny bit of grease on the screw. It'll be difficult to screw into solid metal, so use a screwdriver which fits the screw head very well.

    The screw heads should not stick up so far that the pump impeller hits em. After you do it, put the the pump on there (with gasket or without). You can always grind or flatten the tops....but I've never had to. I've fitted about 6 welsh plugs like this.
    Oh...and they only need to be short screws. About 1cm. You're going through solid metal mostly, apart from one of em. If they're too long, they will hit the cylinder head bolts !
    Last edited by Beano; 26th August 2014 at 08:04 PM.
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    Quote Originally Posted by Beano View Post
    My 505SR does not have a welsh plug behind the water pump. My '83 GR did, but I believe they were phased out sometime in '84. Is yours an '84 model SR ???

    Brass should be ok if you use corrosion inhibitor.
    I always fix em in with screws. Don't even think about using ordinary old self-tappers. They break off very easily (and rust). Stainless steel screws are the go (they are REALLY hard to break), and you need to drill the holes in the head a little wider than you would if you were putting the screws into sheet metal. This is because you are screwing into solid metal. Usually (as you know), the drill bit has to be the same size as the central part of the screw...disregarding the threaded part. But if you do that, the screw will be very difficult (if not impossible) to put in. Try it and see.....there is no harm done by drilling too narrow a hole at first.

    Apart from that, there is WHERE to drill.
    The way I do it, is that the plug is held in by the screw heads only. That is to say, holes are not drilled through the plug, but just outside. Three should suffice. Two into one of the columns (in the head) through which one of the head bolts go, and another through thinner metal. Use 4 if you like. I use 3. Have a good look and you'll work it out. Oh...and I use a tiny bit of grease on the screw.

    The screw heads should not stick up so far that the pump impeller hits em. Put the the pump on there (with gasket or without). You can always grind or flatten the tops....but I've never had to. I've fitted about 6 welsh plugs like this.
    Oh...and they only need to be short screws. About 1cm. You're going through solid metal mostly, apart from one of em. If they're too long, they will hit the cylinder head bolts !
    sounds too scary, luckily i don't have a welch plug either...

  7. #7
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    Welsh plugs and screws... :-(

    If it needs doing.... do it right

    In situ

    How To Replace Welch Plugs

    Or corrosion in the hole then re machining is necessary...screws...centre punch..why not just bash in a lump of red gum ...farmers bush mechanics of yester year??? Wood actually swells from water and works very well

    Welch Plug Installation, Expansion Plug Installation, Hubbard Plug Installation, Cupped Plug Installation, Freeze Plug Installation.

    HOLE PREPARATION

    The hole diameter should be larger than the minor outside diameter but smaller than the major outside diameter - .003 larger than the minor diameter is a good guideline.
    100 micro inches or smoother is a good finish guideline for the hole.
    The hole should be round within .002 T.I.R.


    INSTALLATION

    Plugs should be driven from the inside bottom of the plug, not from the rim of the cup.

    If a sealant is not used, lubricating the plug and hole with oil or grease before insertion minimizes scoring / galling and resulting leaks.

    Plugs should be driven squarely into the hole.
    A bottoming shoulder on the installation tool controls the depth of insertion and plug alignment.

    After installation, the top of the cup should be at least .035 past the end of the lead in radius of the hole to be sealed.
    The radius on the end of the installation tool should not interfere with the radius on the inside bottom of the cup it should be large enough to provide adequate clearance. A clearance chamfer works as well.

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    I have always been under the impression that the Peugeot domed plugs were an expansion plug rather than welsh plug (normally having a cupped appearance).

    They are deemed expansion because to fit the dome of the plug is has pressure applied to expand the plug.

    While there all kind of niceties in installing into the ideal, brand new situation this is often impossible with a partially corroded Peugeot XN cylinder head.

    Generally the old plug will drop out or be able to levered out through a rust hole . Use a self tapper and a claw hammer if the plug is still firm.

    Clean up the the aluminium recess shiny clean (as best a possible anyway) and lay in 6 mm diameter bead of a nice sticky silicon based sealant. I use a Wurth brand orange silicon (forget the name)

    Position the plug in place and tap the centre of the plug with 50x50 wooden block - it may get minor dent , don't worry too much.

    Let it all go off for 24 hrs and check the plug tightness , it it seems loose stick a couple of stainless self tapper in - make sure they miss the pump impeller.

    An easy job done with basic tools on hand without too much effort.

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    Combination (of all of above) repair carried out 23 years ago! (Crap photo). I used screws as in pic. (Looks like a couple of them were not SS).

    Worked for me.
    Attached Thumbnails Attached Thumbnails Cylinder head welsh plug replacement-dsc00391.jpg   Cylinder head welsh plug replacement-dsc01083.jpg  
    Present fleet:-
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    I always used a socket of a size that just cleared the ID of the plug.

    Tap the socket with a hammer a few times and the plug went in nice and square.

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  11. #11
    1000+ Posts Beano's Avatar
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    Quote Originally Posted by robmac View Post

    While there all kind of niceties in installing into the ideal, brand new situation this is often impossible with a partially corroded Peugeot XN cylinder head.
    Indeed

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    Don't use a brass welsh plug if it's an alloy head. It will corrode the head so fast it's not funny. The aluminium head becomes the sacrificial anode.

    Don't use screws.

    Use Stag paste that you get from your local autoparts shop and tap it in using a socket that is a slightly loose fit in the plug so that the socket doesn't jam in there when fitted but still supports it enough to drive it in true.

  13. #13
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    Quote Originally Posted by Bluey View Post
    Don't use a brass welsh plug if it's an alloy head. It will corrode the head so fast it's not funny. The aluminium head becomes the sacrificial anode.

    Don't use screws.

    Use Stag paste that you get from your local autoparts shop and tap it in using a socket that is a slightly loose fit in the plug so that the socket doesn't jam in there when fitted but still supports it enough to drive it in true.
    Unless 30% of the retaining land is corroded away. Then the pressure plug fits like a cock-in-a-bucket.
    Remember many of these heads are 40 years plus old now and corroded as anything.

    And up 8mm thinner than they started off.

    Stag is a non hardening plumbers and mechanical services screw thread sealant/ jointing paste. Excellent for gas, water and steam etc etc pipe threads. But does not have many adhesive properties.

    I'd suggest something far more tenacious. Anyone who has a plug come adrift , seize the water pump, stuff the magnetic clutch and then hole the radiator will employ belt and braces and stick a couple of screws in if the fit of the pressure plug is at all suspect.
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    Quote Originally Posted by Bluey View Post
    Don't use a brass welsh plug if it's an alloy head. It will corrode the head so fast it's not funny. The aluminium head becomes the sacrificial anode.

    "Don't use screws". And here I was thinking that 23 years (so far) wasn't a bad fix!

    Use Stag paste that you get from your local autoparts shop and tap it in using a socket that is a slightly loose fit in the plug so that the socket doesn't jam in there when fitted but still supports it enough to drive it in true.

    Why glue it when you can screw it?
    cjl likes this.
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  15. #15
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    Default Repair the hole

    A corroded hole can be properly repaired with an adjustable parallel hand reamer

    It will give you a nice round hole of the correct dimension

    Attachment 60095

    Hire one if required

    Also check out metal corrosion table

    http://www.corrosionist.com/Corros1.gif

    Stainless screws cause more Galvanic corrosion with Aluminium than Brass
    Last edited by driven; 28th August 2014 at 11:07 AM.

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    Some confusion exists as to the description of the "plug" behind the water pump.
    It is a slightly convex type not the cup type "frost plug" as found on cast iron blocks ie early Holdens.

    As has been mentioned the plugs have usually disintegrated along with corrosion of the alloy surround.
    The method of securing the new plug with stainless self tappers has proven successful along with a sealant* [Stag].
    The screws should be strategically placed so that there is no interference with the head stud 'ole.

    Bear in mind that it is not necessary for the plug to seal 100%. There being coolant on both sides of the plug.

    *I can't for the life of me recall the product name that my "Head Guy" used to secure the plug on my 504 head. It was one of those that can be drilled and tapped readily. It will come to me
    I knew it would come "Devcon".

    I have fitted an earth wire from the thermo housing to the body to overcome the dreaded stray electric current that has been espoused as being the cause of the alloy corrosion.
    I looked at the earth wire as being one of those 'nothing to lose' ideas.
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  17. #17
    1000+ Posts BIGRR's Avatar
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    Quote Originally Posted by driven View Post
    A corroded hole can be properly repaired with an adjustable parallel hand reamer

    It will give you a nice round hole of the correct dimension

    Attachment 60095

    Hire one if required

    Also check out metal corrosion table

    http://www.corrosionist.com/Corros1.gif

    Stainless screws cause more Galvanic corrosion with Aluminium than Brass

    The welch plug hole in the head is counter bored about 3mm deep and around 50mm dia.

    I would not hesitate in using the small stainless steel screws again, 3 or 4 screw heads' surface area is considered very, very small compared to that of the aluminium. (metal corrosion is a black art!)

    If the plug, refitted into a corroded head is not fixed in positively (mechanically) I would recommend keeping a spare water pump in the boot.

    Good Luck



    Present fleet:-
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    Peugeot 73' 504 Ti from new
    Peugeot 08' 407 Hdi Coupe from new

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    Peugeot 92' 205 Gti
    Renault 72' 16TS from new
    Renault 69' 10
    Renault 71' 10s
    Renault 68' 10 from new

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  18. #18
    1000+ Posts robmac's Avatar
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    Quote Originally Posted by driven View Post
    A corroded hole can be properly repaired with an adjustable parallel hand reamer

    It will give you a nice round hole of the correct dimension

    Attachment 60095



    Hire one if required

    Also check out metal corrosion table

    http://www.corrosionist.com/Corros1.gif

    Stainless screws cause more Galvanic corrosion with Aluminium than Brass
    With due respect, It's absolutely obvious you never have never done the job the OP is requesting information about.

    The pressure plug in question is around 50mm diameter and sits into a machined recess. Try to find an adjustable reamer 50mm diameter and use it to restore the recess sits into and that is partially corroded away.

    The stainless screws galvanic corrosion is least of your worries, consider all the steel water outlets pressed in the head casting as well as multiple brass sensor screwed into the head and water pump. Two 2.5 mm stainless self tapping screws about 8mm log simply don't enter in the galvanic corrosion equation nor cause an issue. And it eliminates the (expensive) possibility of the plug falling out and jamming the pump impeller.

    I would hope this post is about helping 504 owners to carry out the repair task effectively.
    Last edited by robmac; 28th August 2014 at 12:30 PM.
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  19. #19
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    An interesting debate. Aluminium (and it's alloys) are more anodic to most other metals, thus a galvanized steel plug would have been used originally. I'm not familiar with the situation, but if the brass is directly in contact with the aluminium, I'm not sure any amount of inhibitor will help your cause. The electrons will flow regardless. In any case, I'd use Threebond 1211. You can also use it on your zits and in place of windex.

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  20. #20
    1000+ Posts robmac's Avatar
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    Quote Originally Posted by PeterT View Post
    , but if the brass is directly in contact with the aluminium, I'm not sure any amount of inhibitor will help your cause. The electrons will flow regardless. In any case, I'd use Threebond 1211. You can also use it on your zits and in place of windex.
    What you is may be all very true ,Ollie.
    Fact: Peugeot has installed brass temperature sensors into alpax aluminium cylinders heads and steel oil pressure switches into aluminium oil filter bodies since the release of the Peugeot 203 and all RWD models thereafter. By my estimation that is a period of around 40 years.

    In that time I've never seen heavy corrosion on either of the above items threads.

    So I'm inclined to suggest yours is at theoretical concern and in practice the galvanic voltage doesn't matter at all.

    You can tell us your experience with the FWD engines but I would think it's the same.

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    Quote Originally Posted by robmac View Post

    You can tell us your experience with the FWD engines but I would think it's the same.
    Yes, they have brass fittings in the head and the even more anodic aluminium block corrodes like mad behind #1 liner.

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    1000+ Posts robmac's Avatar
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    Quote Originally Posted by PeterT View Post
    Yes, they have brass fittings in the head and the even more anodic aluminium block corrodes like mad behind #1 liner.
    Ah! real Pugs have cast iron blocks - so I've never seen what you experience. Fair enough.
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