Tech Torque: Essential scooter repairs with Stu Owen
No need to chew up your sprocket components in fitting; Stu explains all.
During Lambretta transmission maintenance there comes a time when the front sprocket will need removing. This may be because it is being changing for different gearing options or just a regular check to see that there is no wear on the cush-drive, especially on high powered engines. Sometimes there can be a problem when it comes to bolting it back together and getting the alignment exactly right… and this can lead to all sorts of problems and cause unnecessary damage. Ignoring the oil throw washer that sits behind it, there are six components that make up the Lambretta front sprocket and cush-drive set up. In order of fitting first, they are: the cush-drive sleeve, front sprocket, sliding dog, tension spring, top plate/cap and finally the retaining bolt.
Firstly the sleeve must be fitted to the end of the crank, making sure the oil thrower washer is placed directly underneath it to stop any loose debris getting into the main crank bearing. When fitting the sleeve it should slide tightly on to the splines on the end of the crankshaft. Unfortunately somewhere in the past, the splines may have been subject to being hit hard with a hammer to remove the crank and damaged or slightly rounded off at the ends. If this is the case, don’t try to bash the sleeve on to the end of the crank as this may cause more damage. Take a needle file and clean the splines up to make certain of as good a fit as possible. If everything is okay then the sleeve should fit without any force being required.
Next in line is the front sprocket which should slide straight on to the sleeve. If you are replacing the sprocket, make sure the replacement is a good one. Some cheaper aftermarket ones can suffer from poor machining and will not spin on the sleeve due to there being high spots or because they are just slightly out of shape. The sprocket should be a good fit and spin freely but at the same time without any play. Next is the sliding dog which should have the same fit as the front sprocket. This is followed up by the tension spring which is usually okay but may have lost some of its tension over time. To test this, if you place the top plate/cap it will not quite locate on to the splines of the sleeve and will sit around 1mm-2mm above it. If it does locate on to the splines this suggests the spring has lost tension and will need replacing.
You should now have all five items of the cush-drive in place ready for the retaining bolt to be tightened, holding everything in place securely. Here comes the problem though, as due to its large washer-shaped head it is not possible to see whether the top plate is correctly in line with the splines on the sleeve. Even if you have aligned the top plate correctly, as you tighten up the bolt it can move out of line but you can’t tell as the bolt
obstructs your view. This is compounded further as you can continue to tighten the bolt up, not realising that the sleeve and cap are out of line. This does two things, firstly it damages the sprocket cap by stepping the edges which renders it scrap. More alarming and dangerous it that the side case will seem like it fits flush to the casing but the front cush-drive will be pressing up against the casing. That might not be noticeable at first but when tightened down the pressure on the side case can cause it to split.
There is a simple method to make sure this doesn’t happen so that you can easily get the top plate to align correctly when tightening the bolt. Sit the raised edge of one of the six sleeve splines in the twelve o’clock position and place the sprocket cap in line to fit exactly. Using an instant marker pen, preferably red in colour, put a mark on the outer surface of the sprocket cap also in the twelve o’clock position.
One thing to note is when the crank is held solid with a con-rod holding tool, the position of both the sleeve and the sprocket cap may have moved slightly. This doesn’t matter as both will still be in line with each other. As the bolt is tightened you can see by the mark you made on the cap and if it has moved out of position. If it has, realign the mark in the correct position and continue to tighten. As a check, if you gently tap the edge of the sprocket cap with the round end of the hammer it should bounce from the rebound off the tension spring. If it is out of line there will be a dull thud because the cap will be solid against the sleeve. After several more turns of the bolt, it will go tight. At this point make sure it is secure but don’t go overboard with tightening as if the bolt snaps it is almost impossible to get it back out of the crank, making it scrap.
There are several things to note when undoing or tightening the sprocket bolt. You should use a con-rod holding tool to keep the crank in a fixed position for both procedures. The problem is this means that the top end will need to be completely removed. Doing this is a pain if you are just doing maintenance within the transmission. There are several methods of holding the crank in a fixed position, all of which put stress on other components so should be avoided. One option that does get suggested is to use a flywheel holding tool and to hold the crank in a fixed position from the flywheel. Under no circumstances use this method as all the force goes through the crank pin and could force the crank out of line, in effect twisting it.
Lastly, the retaining bolt has a 14mm hex head on it. Buy a six-sided hex head socket to use specifically for this job. Using a standard socket can easily round the edges of the bolt off. This can be a problem when undoing the bolt with the socket slipping off. Also because the bolt head is quite shallow it is best to grind the rounded socket end off so it sits deeper on to the head. Using these simple methods and ideas will make front sprocket fitting and servicing much easier and prevent any damage occurring.