Norton SOHC Cambox Tunnel Repair
- Andy Marks
- Mar 17
- 4 min read
This guide explains how to repair the damaged camshaft tunnel in a Norton SOHC cambox, a common issue on these engines. The article shows the full machining process required to restore the tunnel, including boring the cambox, turning a replacement tunnel in the lathe, drilling oil drain holes and reassembling the cambox correctly.
The Common Problem with Norton SOHC Tunnels
One of the most common problems with the Norton SOHC cambox is damage to the tunnel that the camshafts run in. This tunnel helps keep oil away from the camshaft seals, making their job considerably easier. Over time the rocker pads wear and the rocker arms begin to strike the tunnel, eventually breaking it up. If you are fortunate enough to have a cambox where the tunnel is still intact, or one that has previously been repaired, it is important to monitor rocker pad wear carefully.
Diagnosing Worn SOHC Rocker Pads
One of the first signs of worn, or wearing rocker pads, is a rapidly increasing tappet clearance. If the clearance continues to grow each time you check it but no wear can be found at the valve end, it is worth checking the cam follower end inside the cambox.
If the damage has already occurred, the tunnel will need to be replaced.
Bore the Cambox to Remove the Old Tunnel
Before starting, measure the inside diameter of the original tunnel so the new one can be made to the same dimensions!
Firstly, we set the cambox up on the milling machine and use a boring bar, as pictured, to remove what's left of the old tunnel. We also make it deeper by 0.100’’.
NB: Other people use a lathe to do this part of the job. Either way works just fine.
Turn a New Tunnel in the Lathe
Next we turn a new tunnel to screw into place. It needs to be cup shaped as per the image and use 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 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.
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.
Fit the New Tunnel into the Cambox
Once we have done this the tunnel is a snug sliding fit in the cambox shell and can be slid into place using your fingers.
Once in place we 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.
This marks the holes in both places so we can remove the tunnel again, tap the holes in the cambox to 2BA and open the holes in the tunnel out to clearance before countersinking them to suit the screws.
NB: 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
Drill Oil Drain Holes in the Bottom Corner of the Tunnel
With the new tunnel screwed into place the oil drain holes are completely blocked off. So our next job is to drill the tunnel to allow the oil to drain as intended.
The easiest way to do this is to 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, we can screw the tunnel in and drill the holes.
Machine New Holes for the Cam Followers
With the tunnel secured in place using the two screws, we use the rocker arms to mark where the two holes need to be in the top of the tunnel.
Once this is done we 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.
NB: You can elongate and widen the holes as required later but remember; the smaller they are the less oil can get out past them!
Assemble Cambox with Camshaft, Cams and Rockers
Before finishing the job, we 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.
Final Check and Assembly
Finally, once we are happy with our work, we remove the two countersunk screws and, after ensuring everything is clean (especially the oilways, easy to miss!), we re-fit with some Loctite to ensure a strong and long lasting fit.
With the repair completed the cambox can be reinstalled and the bike can be used again without oil pouring out of the cambox. Well, that's the aim, we can't promise an oil tight cammy Norton but we can promise that this will give you a better chance of achieving it!
Return to:
Comments