Setting up the Selector Quadrant – to avoid jumping gears

My Messerschmitt arrived, as do most restoration projects, as a rusty hulk and a pile of bits. At least the engine was fitted, well ?just? actually. It was secured in place by a single bolt (without a nut!) and none of the services were connected. It seemed the lowest of the priorities, until the body and running gear were nearly completed. Of course I had the usual bedtime reading, the ?Der Grosse Karo tips? and the Club Engine Re-build Manual. These formed a useful knowledge base to be read and memorised, until I got to the point where I had a nice shiny nearly finished Messerschmitt and no engine! That was when I thought that I should start to delve within the oily bits.

Once dismantled (with the aid of a 2lb engineering hammer!) and inspected, it turned out to be a can of worms. Some bits were wonderful ? nearly new in fact. Others were simply destined for the dustbin. Crank, barrel, main bearings and dynastart were mint and perfect.

It had a nice new layshaft with perfect 1st gear and completely unworn dog clutches, the 1st and 4th constant mesh gears were really good (unworn), but the 2nd and 3rd gear fixed and sliding gears were a bit worn, but didn?t look too bad. The clutch plates and primary chain were consigned directly to the dustbin (from where I had to rescue one plate to make a clutch centre locking tool).

After completely dismantling the engine, all the gearbox parts and the engine casings should be thoroughly cleaned. This should reveal evidence of wear at the various contact points for the gears and dog clutch. I am not sure that I agree with all of the notes in the manual in this area, particularly the part about the shims (171) under the neutral selector! detent arm (170). My feeling here is that you should shim the detent ann first to ensure that you get the selector fork/quadrant (166) to give full engagement of both the 1st and 4th gear dog clutches (on gears 139 and 142) when the gearbox is assembled. If the engine has been operated with too few or too many shims fitted you will find a polish mark on either the face of the 1st constant mesh gear (139) or the sliding gear (138) where the quadrant has tried to force the flat faces of the one of the constant mesh and sliding gears into contact. Since the Quadrant has a fixed total sliding gear movement, you need to ensure that this is central and engages the dog clutches on these two constant mesh gears equally at the ends of its travel.I suspect that this is one of the primary causes of gearboxes jumping out of 1st or 4th with resultant 139 wear to the 138 dog clutches.

This adjustment is particularly important for top gear where the angle of the selector fork is large. A small movement in the arm can change the mesh of the 4th gear dogs significantly. This is one of the reasons why the end float on the shafts, particularly the output shaft, is important.

One trick seems to be to shim the output shaft to adjust the whole process further to aid centralisation of the whole assembly. I had excessive end float, about .Ol5inch (0.4mm) too much. I was able to position the output shaft shims to help the engagement of the 4th gear dog clutches in relation to the end movement position of the sliding gear. These are the most important consideration in avoiding jumping out of 4th gear. Ideally, I would prefer to Add see polish marks on BOTH the 1st here to constant mesh gear AND the 3rd gear correct face of the sliding gear!)

After cleaning, assemble all of the selector fork, detent and gears with the rollers in the timing side of the crankcase. Then check the 1st and 4th gear clearances between the sliding gear and the constant mesh gears. You may need to make a ?keeper plate? to hold the drive side shaft end at the appropriate centre to do this, but it will also allow you to see how sensitive the 4th gear mesh is to movement in the quadrant and it?s position. Refer to the diagram for details of how to add and subtract shims for the correct adjustment.

I believe this is the primary adjustment to the gearbox, if you can?t get the core 1st and 4th gear dog clutch engagement right, I would suggest that the rest is doomed! It should be done before completing any other assembly work. This is also a completely different way of looking at the eccentric shims under the neutral finder! detent arm assembly. I think that building gearboxes starts with alignment of gear/dog clutch mesh, not with getting the selector assembly to centre. Eccentric shims are available from MOC partsmart.