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Technical Information for Gunsmiths and for Classic Car Enthusiasts

MG Restoration

Making a Blanking Plate


for MGB Rubber Bumper Cylinder Heads


Les Bengtson


There are several reasons why one may wish to blank off the water choke take-off on the cylinder heads of cars equipped with the Zenith Stromberg carburetor.  The first is when a different carburetor is fitted. Both the Weber DGAV and the SU HS-4s are commonly adapted for use with the MGB engine.  The SU HIF-44 and Weber DCOE are also encountered at a significantly lower rate. None of these uses a water choke and the connection must be sealed if the cooling system is to be kept from leaking.


A second reason is the removal of the trouble prone water choke and it replacement with a manual choke on the Zenith Stromberg.  A number of people, based on the research of Barry Kindig, have begun to find that the Z-S carb is really a fairly good one, offering acceptable performance and economy (plus being part of the original pollution control equipment).  The primary problem with the carb is the water choke system.  They have found that replacing the water choke with a manual choke is far less expensive than converting to a different carburetor.


A third reason for wishing to plug the water choke take off is that one has replaced a cracked cylinder head on an earlier model engine with one from a later model car.  All of these reasons are valid.  Unfortunately, most of the solutions are not.


The standard solution to the problem of closing off the water choke take off on the cylinder head is to leave the existing hose in place and plug it. Sometimes, the hose is shortened and allowed to hang down behind the rear of the engine. It is less visible then, but is still a problem, perhaps a very expensive one, waiting to happen.  The insertion of a bolt, or in some cases a spark plug, into the hose, especially when a hose clamp is used to secure it in place, is a useful method of plugging a hose on a temporary basis.  It seals the end of the hose and does not allow the coolant, under 10-15 pounds of pressure, to leak out.  But, how often do people replace this hose?  We know that the radiator and heater hoses should be inspected regularly.  We know that they should be replaced every two years as a preventive measure. But, how many people remember to replace the hose left over from the water choke?  This is a great potential for a cooling system failure, with a possibility of engine damage.  So, if the plugging of the line is only a short term solution, what is the long term solution?


When I was faced with this problem, on my daughter, Theresa's, car, I decided to make a blanking plate to replace the hose connection on the head.  The hose connection is a piece of steel with a tube, bent at about a 90 degree angle, attached.  It is held on the cylinder head with two ¼" studs.  A paper gasket ensures that there is a good seal between the cylinder head and the hose connector.   It seemed to me that the easiest way to block off the hole in the cylinder head was to make a blanking plate out of scrap steel and use the original studs and nuts to hold it in place.  I did this, and it has worked well for over a year.


To make a blanking plate, you need the old hose connector off the cylinder head.  This will serve as a pattern for the new blanking plate.  The blanking plate is made from scrap steel that is about 1/8" thick  ( from .125" to about .180" will work).  The first step is to coat the scrap steel with some form of "layout" covering.  The clean steel can be colored with a magic marker, layout blue or given a quick coat of paint.  Next, the hose connector is placed on top of the scrap steel and the outline of the hose connector is marked. This can either be scribed, using a sharp steel scribe, or drawn with a pencil or a Sharpie marker.  Then, you cut the piece out.


The easiest way to cut the piece out is to use a hacksaw to remove most of the superfluous metal.   Cut as close to the lines as possible without cutting into the lines themselves.  You then are left with a rough blank.  It is in the general shape of the hose connector, but oversized and lacks the mounting holes. The next step is to bring the outside shape to the proper dimensions.


To shape the outside of the blanking plate requires either a grinder, a belt or disk sander or a file.   (Which ever method you use, wear safety glasses. Steel bits in the eyes are very uncomfortable and very expensive to correct.  I speak from personal experience when I say you do not want to find out how painful it can be.)  Grind, sand or file the blanking plate until you have just remove the scribed/drawn lines.  This represents the true size of the original hose connector.   When you are finished, you will have an oval-diamond shaped piece that duplicates the original almost exactly. It should be a close duplication, but need not be exact.  Comparing the two as you go along will help to make your first attempt a successful one.   Now, you have your blanking plate and only need to make the holes for the securing studs to have a completed blank.


The original hose connector is then laid over the new blanking plate and a pencil or scribe is used to outline the holes for the studs.  Remove the old hose connector and draw cross hairs on the blanking plate to locate the center of the circles you have just made.  I did this by eye and it worked out fine.  The holes in the original hose connector measured .283" while the studs are ¼" (.250"), so there is considerable tolerance built into the original piece.  I my case, I used a somewhat smaller hole of .261", using a letter G drill. You can also use a 9/32" drill (.281") which will duplicate the original holes.  Before drilling, you should center punch the centers of the holes or use a #2 center drill to start the hole.  If you center punch, use a 1/8" drill to drill a pilot hole.  If you use the center drill, run it down until the major part of the drill bit makes a hole for the larger drill to guide on.  Then, drill the holes to final size.  If you are lucky, they will be an exact fit on the cylinder head studs.  If not, determine where the hole needs to be enlarged and use a round or oval needle file  to file the hole to an exact fit.


With the blanking plate now fitting on the cylinder head, all that remains is a gasket.  You can either buy a gasket for the hose connector or make one.   All auto parts stores will have sheets or rolls of gasket material. Get the paper kind.  Then, lay your new blanking plate on top of a small section of  the gasket material and draw around it with a sharp pencil. Also, draw in the stud holes.  Use an Exacto knife or a pocket knife with a sharp blade to cut out the stud holes.  I find that pressing straight down with the knife in a series of overlapping cuts is the best way to do this.  Then, use a pair of scissors to cut around the outline of  the blanking plate.  You now have a complete blanking plate and a gasket that can be fitted to the cylinder head.  At this stage, I prefer to paint the blanking plate.


There are two reasons for painting the blanking plate. First, is to match the engine color (black). Second is so that the underside, where it is contact with the coolant, cannot rust.  You can use regular engine paint for this purpose.  Having  trained as a gunsmith, I am aware of some better coatings--ones that will resist heat and chemicals better than paint.  I used a coating called Gun-Kote because I had it available and it matched the engine color.  Brownell's, Inc. also sells a "baking lacquer" which does the same thing and is less expensive. Both are sprayed on, allowed to dry thoroughly and then baked in an oven to harden.  Any of these methods should work fine.


After the part has received its finish, I wire brush the studs to remove built up rust and crud.   If  the cylinder head is off the car, I use a wire brush in either a drill or a die grinder. If the cylinder head remains on the car, I use a wire brush by hand.  Then, a thin coat of gasket cement (non-hardening) is applied to the cylinder head and the underside of the blanking plate.  The gasket is fitted over the studs, the blanking plate is installed and the lock washers and nuts fitted. I use the blue Loc-tite on the studs to prevent loosening.  The engine is then run up to operating temperature and the new blanking plate checked for leaks.  There should be none if the surfaces were clean  before installation.  At this point, you can forget about the modification until the next time head work is required.  It should hold up indefinitely and you will no longer have that nagging worry about a burst hose.  The average time involve is about one hour and the cost less than a dollar. Quite a bargain for the piece of mind provided.


ADDENDUM


Since this piece was written, I have had the opportunity to remove the water choke take off studs from an 18V cylinder head. The studs are 0.770" long and are threaded ¼"-28 on both ends. The shorter threaded end, ¼" in length goes into the block.  The studs go directly into the water passages and, if removed, should be re-installed using some form of sealer such as Loc-tite or gasket cement.  The original nuts are plain nuts with lock washers. It should be possible to use a standard nylock nut. Providing extra security.  It would also be perfectly possible to replace the studs with two ¼"-28 grade 5 machine bolts of 3/8" to ½" length and use lock washers between the underside of the bolt head and the blanking plate.  This would make a slightly better looking installation.  Before re-installing the studs or using the bolts, run a ¼"-28 UNF taper tap into the holes to clean them up and ensure the threads are clean.


This monograph may be reproduced only for non-commercial use without other permission of the author. Reproduction for commercial use only by written permission.


Copyright © 2001 by Les Bengtson