Fixing Leaking Triumph T140 Rockerbox Spindle O-Rings

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Triumph T140 Bonneville rockerbox spindles are prone to leakage, especially on high mileage bikes. Simply replacing the tiny rockerbox O-rings often isn’t enough. Many O-rings on the market are sub-standard in terms of quality. Many are quite simply the wrong size.

Unfortunately, you might need to try a number of O-rings before you find a pair that work. But they are cheap to buy and easy to replace. Fixing them just seems more complicated than it is. Here’s how to do it.

With the engine off and cold, loosen and remove the aluminium inspection covers from the front and rear rockerboxes. Each cover will be secured by six Allen screws. Loosen them just a fraction of a turn to take off the initial load. Now undo the Allen screws incrementally. That helps avoid distortion. Once you’ve turned the Allen screws once of twice, you can relax and remove them quickly.

There should be a paper gasket under each inspection cover. Remove this gasket carefully by scraping and peeling gently. Do not use a hard scraper such as steel. Use a plastic or aluminium scraper of some kind. Loosen the valve adjuster nuts on the rockers. This takes the load off the rocker spindles and the valve gear. Do NOT omit this step. You need only ensure that the rockers are loose. Don’t remove the valve adjuster nuts.

If, however, you see that one of the valves is in the fully open position, gently rotate the engine (using the kick starter) to close the valve. This operation also helps take the load off the rocker gear and the rocker spindle. To facilitate closing a valve, you might prefer to remove the spark plugs to relieve cylinder pressure. The engine will turn over 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 one of the valves is pushed all the way down by the rocker adjuster, it’s open. You might want to turn the engine over a few times to familiarise yourself 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 one inch (25mm) long designed to fit snugly over the rocker spindle.

To use it, follow these simple steps. First loosen and remove the right hand dome headed nuts that secure the rockerbox oil feed pipes. There will be one for the front rockerbox, and one for the rear. The pipes are joined at a T junction which is in turn connected to a rubber pipe. Remove the copper washer from under each dome nut. Remove the oil feed pipe. Remove the copper washers behind the rocker oil feed pipe. There should be one on each rocker spindle.

Now temporarily replace the dome headed nuts, but not the copper washers. Save the washers for later reassembly. The idea behind this stage is to get the rockerbox oil feeds out of the way. The dome headed nuts do not need to be fitted tightly. They are replaced only to protect the threads on the end of the rocker box spindles. Wind them on just a couple of turns.

Next, using a hide mallet or rubber mallet, gently tap one of the dome headed nuts. This will drive the rocker spindle across the rocker box from the right side of the engine to the left side. Don’t tap the spindle more than a few millimetres. The idea is to slide it across just 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 might have to loosen the dome headed nuts a turn or two to facilitate this. It will be clear when you’re working on it.

Next, take a small tool with a point or hook. Gently lever off the old O-ring from each spindle. They will now be flattened rather than rounded. You can just pull these off and throw them away. Wipe the end of the rocker spindle with a rag and check that nothing is damaged. If the spindle is damaged, it will have to be replaced. If the spindle is in apparently good condition, oil it with some fresh engine oil. Then oil one of the O-rings. Gently roll the O-ring over the end of the spindle and ensure that it’s a snug fit in its groove. Do this with both rocker spindles.

Now you need that special tool mentioned earlier. Oil it on the inside with fresh engine oil and slide it over the O-ring and rocker spindle. Take care not to snag the O-ring. Next, remove the dome headed nut on the end of the spindle and temporarily replace the rocker oil feed. Replace the dome headed nut and gently tighten whilst holding the special tool on the other end of the spindle. The idea is to slowly draw the spindle and the O-ring through the small tapered tube. This helps stop the O-ring from snagging as it’s pulled 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 and try again. Remember to use plenty of oil. The oil feed pipe is used here purely as a spacer. You’ll see why quickly enough. Some owners try to tap the spindle back through from the left side. You should avoid this. Just use the method above. It’s 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 feed pipe on the right hand side of the engine. To anneal, hold each washer in a naked flame until it’s cherry red, then drop immediately into cold water. Copper can be safely annealed this way. Ferrous metals need to be cooled slowly.

Remove the rocker oil feed pipe and fit a copper washer onto each spindle. Replace the oil feed pipe. Put another copper washer on each feed pipe. Carefully replace the dome headed nuts and tighten. This will pull the rocker spindle even more tightly into the rocker box. Don’t 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’s unusual to have the inlet rocker gap 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 gasket 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, ensure that the oil pressure light goes out, inspect for leaks, and ride the bike to warm thoroughly. If, after a period of a few days or weeks, the spindles aren’t leaking, they are probably fixed. But if one or both still leak, you’ll have to try again. It ought to work every time, but it doesn’t. These are old bikes and need a little fussing.

If you have persistent issues here, you can try removing both rocker boxes and stripping completely. Then you might try machining a very small 45-degree bevel (or thereabouts) on the left side of each rocker box where the spindle is drawn in. This bevel can help prevent the O-rings from snagging on reassembly. Just a millimetre or two will suffice.

Always buy the best O-rings you can from a reputable source. Don’t bother trying any form of resin or sealant around the O-rings. It rarely, if ever, works. Remember that the spindles don’t actually spin. Instead, the rockers inside the rockerboxes pivot around the spindle. The object of the exercise is to feed those new O-rings gently into the rocker boxes. Look at the problem in engineering terms. Never force anything in this area. It shouldn’t need it. Just be slow and methodical. If you want to check the spindle housing for wear, start the engine and put a gloved finger against each spindle in turn (left hand side, and then right hand side).

If the spindles are bouncing up and down, or moving at all, they need replacing. Sometimes this movement is clearly visible. If you prefer, you can completely remove each rockerbox and replace the O-rings on the bench. But it isn’t necessary if you first remove the valve/rocker-load and use the special tool with plenty of oil.

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