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Norton SOHC Tunnel Repair

Probably most talked about issue with the Norton cambox, is the tunnel that the cams run in. This tunnel is designed to keep most of the oil away from the seals making their job considerably easier. Over the years the rocker pads wear and the rocker arms hit the tunnel breaking it up. Whether you’re lucky enough to have a cambox with a tunnel that is still intact, or you have one that has been repaired, it is important to keep an eye on the pad wear. The first sign that the pads are wearing quickly is a rapidly expanding tappet clearance. If every time you check this, you find it is increasing but you can’t find any wear on the valve end its worth checking the end inside the cambox. If however you didn’t catch it in time, or you have bought an engine that’s already damaged, the tunnel needs replacing. Here is how it can be done:

Before starting, measure the inside diameter of the original tunnel so the new one can be made to the same dimensions!

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Bore out the old tunnel

Set the cambox up on the milling machine and use a boring bar as pictured to remove the old tunnel and make it deeper by about 0.100’’. (Other people use a lathe to do the same job. Either way works just fine).

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Make the new tunnel

The tunnel needs to be turned to a cup shape as shown in the image and with the dimensions as below: 

a) The inside diameter the same as the original tunnel

b) The outside needs to be a tight sliding fit in the your bored out cambox

c) The overall height of the tunnel must bring the end back up to flush with the gasket face. You may want to consider leaving it slightly proud to begin with and then skim the whole gasket face to further help with an oil tight joint.

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d) The end must be 0.100’’ thick to return the tunnel to the same depth as it was before boring

e) There also needs to be a hole in the end to allow the camshaft to pass through, this must be the same diameter as the existing hole in the cambox

f) You will need two countersunk holes in the end to take the two screws that hold the tunnel in place. However these should be machined in the next stage. 

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Fit the new tunnel in the cambox

Your new tunnel should be a snug sliding fit in the cambox shell. You should be able to slide it into place using your fingers. Now drill two 2BA (M5 is fine) tapping size holes through the end of the new tunnel and into the original cambox. Ideally the closer to the outside of the tunnel you get the better but remember you need room to put the countersink in the end of the tunnel. Once these holes have been marked remove the tunnel again, tap the holes in the cambox to 2BA and open the holes in the tunnel out to clearance and countersink them to suit your screws. 

With the tunnel inserted you may need to re-drill and thread the two holes below the tunnel used to hold the end cover and bearing housing on. 

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Drill oil drain holes in the bottom corner of the tunnel

Your new tunnel will have blocked these off so it is important at this stage to drill the tunnel to allow the oil to drain as intended. The easiest way to do this is set the cambox up on the milling machine minus the tunnel and line a drill up with the existing hole. Then, with the cambox left in place, screw the tunnel in and drill the holes. 

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Machine holes for the cam followers

With the tunnel secured in place with the two screws use the rocker arms to mark where the two holes need to be in the top of the tunnel. Once this is done remove the tunnel again and use a ½’’ slot drill to drill the top of the tunnel. This part is fiddly and time consuming to get right.

You can elongate and widen the holes as required later but remember; the smaller they are the less oil can get out past them!

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Assemble rocker box with camshaft, cams and rockers

Assemble the rocker box and check that the cam followers make contact with the cams throughout the rotation and do not make contact with the tunnel at any point. It is not uncommon at this stage to need to thin down the tunnel slightly around the holes in the top as shown in the image. 

Finally, once you are happy with your work remove the two countersunk screws and after ensuring everything is clean (especially the oilways, easy to miss!) re-fit with some Loctite to ensure a strong and long lasting fit.

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