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 do not meet quality standards. Many are simply the wrong size.

Unfortunately, you may need 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 from the front and rear rocker boxes. Each cover will be secured by six Allen screws. Loosen them just a fraction of a turn to remove the initial load. Now loosen the Allen screws gradually. That helps avoid distortion. Once you’ve turned the Allen Screws a time or two, you can quickly relax and 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 like steel. Use some type of plastic or aluminum scraper. Loosen the valve adjusting nuts on the rocker arms. This takes the load off the rockers and valve gear. DO NOT skip this step. You just need to make sure the rocker arms are loose. Do not remove the adjusting nuts from the valve.

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

But how will you know if a valve is open? Simple. Look at the ends of all the valves where they meet the rocker adjusters. If the rocker adjuster pushes one of the valves all the way, it is open. You may want to start the engine several times to familiarize yourself with the movement of the valve. It will become clear when you look at it for a moment.

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 shaft.

To use it, follow these simple steps. First loosen and remove the domed nuts on the right side that secure the rocker box oil feed tubes. There will be one for the front rocker box and one for the rear. The pipes are joined in a tee joint which in turn is connected to a rubber pipe. Remove the copper washer from under each dome nut. Remove the oil feed tube. Remove the copper washers behind the rocker arm oil feed tube. There should be one on each rocker.

Now temporarily replace the dome head nuts, but not the copper washers. Save the washers for reassembly later. The idea behind this stage is to get the rocker box oil feeds out of the way. It is not necessary to adjust the domed head nuts. 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, with a leather mallet or rubber mallet, tap one of the domed head nuts. This will drive the rocker shaft through the rocker box from the right side of the engine to the left side. Do not hit the shaft more than a few millimeters. The idea is to slide it in far enough to expose the o-ring and the o-ring groove on the other end of the shaft. Keep tapping gently until you can see the o-ring. Do this on the front and rear rocker boxes. You may have to loosen the domed nuts a turn or two to facilitate this. It will be clear to you when you are working on it.

Next, take a small pointed or hooked tool. Gently lift the old o-ring off each shaft. They will now be flattened instead of rounded. You can remove and throw them away. Clean the end of the rocker shaft with a rag and check that nothing is damaged. If the shaft is damaged, it will have to be replaced. If the axle is apparently in good condition, grease it with a little fresh motor oil. Then grease one of the O-rings. Gently roll the O-ring over the end of the shaft and make sure it fits snugly into its groove. Do this with both rockers.

Now you need that special tool mentioned above. Lubricate the interior with new engine oil and slide it over the O-ring and rocker arm shaft. Be careful not to snag the O-ring. Next, remove the dome head nut at the end of the shaft and temporarily replace the rocker arm oil feed. Replace the dome head nut and gently tighten while holding the special tool at the other end of the shaft. The idea is to slowly pass the shaft and the O-ring through the small conical tube. This helps prevent the O-ring from snagging when it is put 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 tube is used here only as a spacer. You’ll see why fast enough. Some owners try to hit the axle again from the left side. You should avoid this. Just use the above method. It is slower and more controlled.

Next, anneal 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 bare heat until it turns a cherry red color, then immediately drop into cold water. Copper can be safely annealed in this way. Ferrous metals must cool slowly.

Remove the oil feed tube from the rocker arm and place a copper washer on each axle. Replace the oil feed tube. Place another copper washer on each feed tube. Carefully replace the dome head nuts and tighten. This will push the rocker shaft with more force into the rocker housing. Do not over-tighten.

Reset the valve clearances. Eight thousandths of an inch for the inlet side and six thousandths of an inch for the exhaust. It is unusual to have the intake rocker clearance larger than the exhaust, but this is correct for the T140 engine due to its camshaft profiles. Replace the rocker box gaskets. A joint sealant stain 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 fully warm up. If, after a few days or weeks, the axles are not leaking, they are probably repaired. But if one or both are still leaking, you will have to try again. It should always work, but it doesn’t. These are old bikes and in need of a bit of a fuss.

If you have persistent problems here, you can try removing both rocker boxes and stripping them completely. Then you can try to machine a very small 45 degree (or more or less) bevel on the left side of each rocker box where the shaft goes. This bezel can help prevent the O-rings from snagging when reassembling. A millimeter or two will suffice.

Always buy the best o-rings you can from a reliable source. Don’t bother testing any kind of resin or sealant around the o-rings. Rarely, if ever, does it work. Remember that the shafts do not actually rotate. Instead, the rocker arms inside the rocker arms rotate around the shaft. The object of the exercise is to slide these new O-rings gently into the rocker boxes. Look at the problem in terms of engineering. Never force anything in this area. You shouldn’t need it. Be slow and methodical. If you want to check the spindle housing for wear, start the engine and place a gloved finger against each spindle in turn (left side then right side).

If the shafts bounce up and down, or move at all, 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 rocker and use the special tool with plenty of oil.