Triumph T140 Rocker Box Spindle O-Ring Repair Leaks

Triumph T140 Bonneville rocker box shafts are prone to leaks, especially on high mileage bikes. Simply replacing the small rocker box o-rings is often not enough. Many o-rings on the market are below standard in terms of quality. Many are simply the wrong size.

Unfortunately, you may have to try several o-rings before you find a pair that works. But they are cheap to buy and easy to replace. Fixing them seems more complicated than it is. Here’s how to do it.

With the engine off and cold, loosen and remove the aluminum inspection covers on the front and rear rocker boxes. Each cover will be secured with six Allen screws. Loosen them just a fraction of a turn to remove the initial load. Now unscrew the Allen screws gradually. That helps avoid distortion. Once you’ve turned the Allen screws a turn or two, you can relax and quickly remove them.

There should be a paper gasket under each inspection cover. Remove this gasket carefully by gently scraping and peeling. Do not use a hard scraper such as steel. Use a plastic or aluminum scraper of some kind. Loosen the valve adjusting nuts on the rocker arms. This takes the load off the rocker arm shafts and valve gear. DO NOT skip this step. You just need to make sure the rocker arms are loose. Do not remove the valve adjusting nuts.

However, if you see that one of the valves is in the fully open position, gently crank the engine (using the starter) to close the valve. This operation also helps relieve the load on the swing gear and swing shaft. To make it easier to close a valve, you may want to remove the spark plugs to relieve cylinder pressure. The motor will spin much easier this way.

But how will you know if a valve is open? Easy. Look at the ends of all the valves where they meet the rocker arm adjusters. If the rocker adjuster pushes one of the valves all the way down, it is open. You may want to crank the engine a few times to get familiar with the valve movement. It will become clear when you look at it for a while.

Next you will need a special tool available from good classic Triumph dealers. Cheap to buy (usually a few pounds or dollars), this tool is just a tapered tube about an inch (25mm) long designed to fit snugly over the rocker arm shaft.

To use it, follow these simple steps. First loosen and remove the right hand dome head nuts securing the rocker box oil feed tubes. There will be one for the front rocker box and one for the rear. The tubes are joined at a T-joint which is in turn connected to a rubber tube. Remove the copper washer under each acorn nut. Remove the oil feed tube. Remove the copper washers behind the rocker arm oil feed tube. There should be one on each rocker arm shaft.

Now temporarily replace the domed head nuts, but not the copper washers. Save the washers for reassembly later. The idea behind this stage is to remove the oil feeds from the rocker box. It is not necessary to tighten the domed head nuts tightly. They are replaced only to protect the threads on the end of the rocker box shafts. Roll them up in just a couple of turns.

Next, using a leather mallet or rubber mallet, gently tap on one of the domed head nuts. This will drive the rocker arm shaft through the rocker box from the right side of the engine to the left side. Do not hit the spindle more than a few millimeters. The idea is to slide it in enough to expose the o-ring and o-ring groove on the other end of the spindle. Keep tapping gently until you can see the o-ring. Do this on the front and rear rocker boxes. You may need to loosen the domed head nuts a turn or two to make this easier. It will become clear when you are working on it.

Next, take a small pointed or hooked tool. Carefully remove the old o-ring from each spindle. They will now be flattened instead of rounded. You can remove them and throw them away. Clean the end of the rocker arm shaft with a rag and check for any damage. If the shaft is damaged, it will need to be replaced. If the axle appears to be in good condition, lubricate it with a little new engine oil. Then lubricate one of the o-rings. Gently roll the O-ring over the end of the shaft, making sure it fits snugly into its groove. Do this with both rocker arm shafts.

Now you need that special tool mentioned above. Lubricate the inside with new engine oil and slide it over the o-ring and rocker arm shaft. Take care not to snag the O-ring. Next, remove the domed nut on the end of the spindle and temporarily replace the rocker arm oil feed. Replace the domed nut and gently tighten while holding the special tool on the other end of the spindle. The idea is to slowly pass the spindle and o-ring through the small tapered tube. This helps prevent the O-ring from snagging when it is pushed back into the rocker box.

Do this with both o-rings. Check as best you can that both o-rings are seated. If you cut or damage one, replace it and try again. Remember to use plenty of oil. The oil feed pipe is used here only as a spacer. You’ll see why soon enough. Some owners try to back out the spindle from the left side. You should avoid this. Just use the above method. It is slower and more controlled.

Next, screw on the copper washers. There should be four; one to fit behind and one to fit in front of the rocker arm feed tube on the right side of the engine. To anneal, hold each washer over a naked flame until cherry red, then immediately place in cold water. Copper can be safely annealed in this way. Ferrous metals must be cooled slowly.

Remove the rocker arm oil feed tube and fit a copper washer to each spindle. Replace oil feed tube. Put another copper washer on each feed tube. Carefully replace the domed head nuts and tighten. This will push the rocker arm shaft harder into the rocker box. Don’t over tighten.

Reset valve clearances. Eight thousandths of an inch for the inlet side and six thousandths of an inch for the exhaust. It is unusual for the intake rocker arm clearance to be larger than the exhaust, but this is correct for the T140 engine due to its camshaft profiles. Replace rocker box gaskets. A smear of joint sealant won’t hurt. Don’t overdo it. Tighten the rocker box inspection covers (incrementally, please).

Check that everything is as it was. Start the engine, make sure the oil pressure light goes out, check for leaks, and ride the bike to warm up completely. If, after a period of a few days or weeks, the spindles do not leak, they are probably repaired. But if one or both are still leaking, you’ll have to try again. It should always work, but it doesn’t. These are old bikes and they need some TLC.

If you have persistent problems here, you can try removing both rocker boxes and disassembling them completely. Then you can try to machine a very small 45 degree (or thereabouts) chamfer on the left side of each rocker box where the spindle enters. This bezel can help prevent o-rings from snagging when reassembling. Just a millimeter or two will suffice.

Always buy the best o-rings you can from a reputable source. Don’t bother testing any type of resin or sealant around the o-rings. It rarely, if ever, works. Remember that the axes do not actually rotate. Instead, the rocker arms inside the rocker boxes rotate around the shaft. The object of the exercise is to gently insert those new o-rings into the rocker boxes. Look at the problem in engineering terms. Never force anything into this area. You shouldn’t need it. Just be slow and methodical. If you want to check the spindle housing for wear, start the motor and place a gloved finger against each spindle in turn (left side then right side).

If the shafts bounce up and down, or move, they need to be replaced. Sometimes this movement is clearly visible. If you prefer, you can completely remove each rocker box and replace the o-rings on the bench. But it is not necessary if you first remove the valve/load from the rocker arm and use the special tool with plenty of oil.