We got the crank installed to the spare block yesterday. Some photos:
Here’s something I have found on almost all Saab two-stroke engines I have dismantled:
A destroyed distributor drive gear.
So – what causes this? The gear that is on the distributor shaft is positioned slightly off-center compared to the drive gear on the crank. Because of this most of the wear happens on one side of the drive gear. Apparently it was common practise just to turn the gear around when it got too worn and started to give un-even spark. This does fix the problem short-term but ultimately will destroy the drive gear. Here’s why:
There is a small chamfer on the crank shaft – right where the drive gear fits. And on the drive gear there’s a similar – negative – chamfer. If the chamfers do not meet the drive gear will not fit snugly against the end of the drive shaft. This will allow for the drive gear to move as the crank vibrates and will ultimately cause the gear to wear down even faster.
So – above is the correct way to install the drive gear. Mate the chamfers!
Also – the little “hub-puller” in the Saab workshop tool box is there for a reason. The fit of the drive gear and the vibration damper wheel to the crank (and the locking wedge) should be so tight you really need the puller to remove them. If this package is so loose you can pull it out by hand – it is too loose.
I have seen two types of wear happening: The drive gear is quite soft material and if it is installed the wrong way around it will start vibrating. This will in turn cause the little groove for the locking wedge to enlarge and the gear will wear down even faster. The second type of damage I have seen is the wedge groove on the crank enlarge. This will of course cause the whole package to become unstable and your timing will be all-over the place.
I had both of the problems with ’66 engine for the white car. What I did was make a new – wider – wedge so it fits very snugly in the crank, drive gear and damper wheel grooves. As all these grooves were slightly different width I filed the wedge so it is actually wider on top and narrower at bottom – but also wider where it meets the drive gear and narrower at the damper wheel. The fit is so snug you really need the puller to remove the parts – and the timing stays spot-on.
I am also testing a better way to use a slightly worn gear by turning it around. Of course the “normal” way of doing it doesn’t work (as I have described above) so I tried to find a better way. On the white car I turned the gear around and shimmed it out a few millimeters to find a better placement against the distributor gear. Here’s pic to illustrate:
None of the drawings is in correct scale or proportion! They just illustrate the basic principle.
I will report on how these modifications work as I clock more miles on the car.