Repair Leaking Triumph T140 Rocker Box Spindle O-Rings

The rocker box shafts on the Triumph T140 Bonneville are prone to leaking, especially on high mileage bikes. Simply replacing the small rocker box o-rings is often not enough. Many o-rings on the market are sub-standard in terms of quality. 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 is 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 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 prevent distortion. Once you’ve turned the Allen screws a time 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 shafts and valve gear. DO NOT skip this step. You just need to make sure that the rockers are loose. Do not remove 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 to relieve the load on the rocker gear and rocker shaft. To make it easier to close a valve, you may want to remove the spark plugs to relieve cylinder pressure. The engine 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 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 vintage Triumph dealers. Inexpensive 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 right dome nuts that secure the rocker box oil feed tubes. There will be one for the front rocker box and one for the rear. The tubes are joined in a T-joint which in turn is connected to a rubber tube. Remove the copper washer under each cap nut. Remove the oil feed tube. Remove the copper washers behind the rocker arm oil feed tube. There should be one on each rocker shaft.

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

Then, using a leather mallet or rubber mallet, lightly tap on one of the dome 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 spindle more than a few millimeters. The idea is to slide it in far enough to expose the o-ring and the o-ring groove at the other end of the spindle. Keep tapping until you can see the o-ring. Do this on the front and rear rocker boxes. You may need to loosen the dome head nuts a turn or two to facilitate this. 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 shaft with a rag and check for damage. If the shaft is damaged, it will need to be replaced. If the shaft appears to be in good condition, lubricate it with a little fresh motor 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 shafts.

Now you need that special tool mentioned above. Lubricate the interior with fresh engine oil and slide it over the o-ring and rocker shaft. Be careful not to snag the o-ring. Then, remove the dome head nut at the end of the spindle 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 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 reinserted 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 quickly 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 oil feed tube from the rocker arm and place a copper washer on each spindle. 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 more strongly into the rocker box. Don’t overtighten.

Restore valve clearances. Eight thousandths of an inch for the intake side and six thousandths of an inch for the exhaust. It is unusual for the intake rocker gap to be larger than the exhaust, but this is correct for the T140 engine due to its camshaft profiles. Replace the 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 off, inspect for leaks, and ride the bike to warm it 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 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 more or less) chamfer on the left side of each rocker box where the spindle enters. This bevel can help prevent the 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 trying 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 rockers inside the rocker boxes rotate around the axis. The object of the exercise is to gently push those new o-rings into the rocker boxes. Look at the problem in engineering terms. Never force anything in this area. You shouldn’t need it. Just be slow and methodical. If you want to check spindle housing wear, start the motor and place a gloved finger against each spindle in turn (left side then right side).

If the axles bounce up and down, or wobble, 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 bank. But it is not necessary if you first remove the valve/charge from the rocker arm and use the special tool with plenty of oil.