The oil seal is not crucial. It stops to much oil getting out of the clutch slide, yet no oil in there means it will wear. I believe the clubs sell them. Back in the day we used to use old ATE brake slave cylinder dust caps, as some of their designs had round piston to shoe contact - BMW 600? A ready supply was available from your friendly local German Car mechanic.
Steven's prob is pretty much related to the need for an exploded diagram, as in the parts book, as his gears are in a set, allegedly (as we all know mixing used gearbox components is not a good idea, if it can be avoided). My Heinkel stuff is still in storage, but parts book is buyable, even if it is not on the net
Not sure I understand what Nick is asking in respect of a locking plate. The 175 had a locating bolt underneath, which was later dispensed with, leaving a blank on the later casings. The gear selection plate is not made to hold the car in gear, though I have seen it done.
Which gear is it jumping out of? If reverse or top then there is probably to much play in the gear cable. If a rod then excessive play in the engine mounts, which need to be solid Heinkel ones, not slotted Trojan ones. The cable change can even suffer if the mounts are really loose. This will see general loss of gears selected.
A naff primary drive will give the effect of jumping gearbox. Easy to see. The clutch chain gears can be little more than a row of bumps.
A missing collar shim on the clutch layshaft will have the gears wondering about. So will a missing integral mid layshaft cluster collar shim.
The two non sliding gears on bearings should be locked into position.
Sadly the most annoying reason, and in my experience the issue 50% of the time, is a bent selector. This can be for a number of reasons. Not fitted correctly. Bent while fitting. Bent in use because the gear cluster is an older set in a later case, or the other way round - I forget which way does not work. They are differing lengths. Or worse a mixed set of gears bunged together to make a gear cluster out of loose bits in a box. All to easy to have an odd component from an early engine throwing things out. Normally it is the selector mounted on the reverse shaft that gets bent. The shaft being to long and the gear thus leaning on the selector while in mesh, bending it.
Another 33% of the time the reason is that there is nothing actually wrong with the gearbox but the car is set up with loose everything, no real tick over and the guy driving it has little mechanical sympathy or panics when a gear change muffs and tries to beat the box. Being a constant mesh dog box you will never beat it. It changes in the time it takes to change and its up to you to adjust it so that your input is accurate and at low revs. Second is the gear that seems to be most problematic, and it works hard so you can find worn ones. But also it is the panic gear with a greater need for the engine to come to a slower speed to allow the dogs to mesh, so greater chance of pilot error.
Pretty sure Nick knows all that, even if he forgot he new it!
