One of the problems with the small bore head when a lot is machines off the face, is they leak or blow the head gasket out the ends. So, this is how to convert a small bore head to an 11 stud/bolt setup.
Just a note on machining the face of the head. I never take any more than 60″ – 80″ off the face of the head as it flexes and warps to much and you end up losing more than you gain with gasket seal.
Back to the 11 Stud/Bolt set-up.
I take an old Cooper ‘S’ head, sit it on top of the small bore head and locate it in place with drills down the stud holes. Now take 7/16″ drill bit and drill down through the end of the head at the no.4 end. Remove the Cooper ‘S’ head and take a larger drill bit or a counter sink tool and put a large chamfer on the top and bottom of the 7/16th hole you had just drilled. We will come come back to this hole to finish it off after we drill the other end.
Now, take the Cooper ‘S’ head and place it gasket face to gasket face onto the small bore head, then locate it with drill bits again. Now take a drill that fits the hole in the Cooper ‘S’ head at the no.1 end and drill into the face just enough to mark it. Remove the Cooper ‘S’ head and drill a 1/4″ hole through the gasket face only. NOT RIGHT THROUGH the head. We will hold this end down with 1/4″ capscrew underneath the thermostat in the water way.
Ok, now back to the no.4 end. We have to make a tube 7/16″ outside diameter and 5/16″ inside diameter to go down inside the hole we drilled.
Make the tube the same length as the thickness of the head as it has to be welded in. I brazed my tube in. To do this the head has to be pre-heated, so I just placed it on the BBQ with the lid down and warmed it up to 500-600 deg Fahrenheit or 260-315 deg Celsius.
The head will go a soft blue and with a very soft flame on an oxy torch, I filled the chamfer with brass with the tube in place. Turn the head over (It’s hot, so be careful) and repeat on the other side. NOTE. Use cast iron flux, not normal brazing flux. Once done, turn off the BBQ and put the lid back on and let the head cool down slowly.
After the head has cooled down, you can then machine both faces, top and bottom to clean them up.
Lets move on to the block.
Take the head and place it onto the block and locate it with studs or bolts, which ever you are using, and drill the block enough to mark the face. Remove the head and drill and tap the block with a 5/16″ UNC thread at the no.4 end and at the no.1 end drill and tap 1/4″ UNC thread.
When it is time to assemble the head to the block, you can use cap screws for both the new holes. The no.4 5/16″ cap screw can be tightened down to 15ft/lb, while the no.1 end I use a ball end allen key with a spanner on the end. This I tightened by feel, until the allen key starts to twist. because you are pulling down directly onto the gasket , you don’t need a lot of pressure.
There you have it. A small bore 11 stud/bolt head.
