Jumping out of reverse.

[rob needs]

Whats involved in fixing this problem,mine suffer's from it.Always gives me a scare when it pops out.

[Keith Clements]

First read all there is to know about gearboxes here.
https://jowettnet/forum/download/file.p ... &mode=view

[Mike Allfrey]

G'dday From The Sunny But Windy South,

I was about to ask the same question. it may be a bit long-winded, but be patient and read on!

My Jupiter had a tendency to jump out of second, first and reverse, mostly on the over run.

When I installed the re-built engine, I tackled the gearbox's oil leaks. At my last count there were 59 possible leak points!!

One of them was at the front where the layshaft protrudes through into the clutch housing. The layshaft was a loose fit in the housing, to the extent that it slid out on the work bench. That caused some juggling!

It was then that I discovered that the bore on the clutch cover for the shaft is considerably larger than the shaft diameter. For all these forty-odd years I had believed that the layshaft had formed an aligning dowel - not so. I purchased a 7/8" diameter cup type core plug and pressed it into the clutch housing so that the concave edge was flush with the clutch housing gearbox face. Once pressed in, the core plug became a neat fit for the layshaft protrusion. I had noticed that the bore in the clutch housing was not a fine finish hole, and therefore may not have been accurate.

After installing the engine and gearbox assembly, the car has lost its habit of jumping out of reverse and first gears. It will still jump out of second gear if persuaded though. In normal driving it stays in.

It is a well known fact that if the three shafts that make up the drive train - first motion, second motion and third motion shafts - are not parallel to each other, or not perfectly aligned with each other - gears will jump out and no amount of 7 BA washers under springs will help one iota.

I think I have unwittingly re-aligned the layshaft with the other two shafts and, Jowett Cars Limited, in my case, did drill the hole in the clutch cover in exactly the right place. I think the core plug was reasonably accurate.

One of the prime causes of gear jump-out in any vehicle gearbox, is wear at the spigot bearing at the front of the third motion shaft (main shaft in Jowett language). Any movement here will affect the angle of the gear teeth in relation to those on the layshaft. As ever and according to that great thinker, Murphy, this angle allows the gears to climb apart - instead of climbing further into mesh. This is one we can not win!

Now, getting back to the reason for having a look in Jowett Talk, here is my question:

One of our club members has a PB Javelin with only four forward speeds at present. We drove the car for a 464 round trip on Sunday with no reverse gear at all! Fortunately we were able to park at the Historic Winton race meeting, in such a way that we did not have to reverse out.

Now, the gear lever travels all the way into the reverse position. Upon releasing the clutch pedal, the gears just gnash and the lever returns promptly to the neutral position. I have not yet checked what is happening at the gear change shaft. I have been assured that the nut securing the lever is tight. - that is all so far.

Any clues out there as to what could have caused this sudden phenomenom?

Helpful response will be appreciated and acknowledged.

Regards,

Mike Allfrey.

[PAUL BEAUMONT]

Mike, I know nothing about the Javelin g-box, so what follows may be complete twaddle!, but he problem you describe sounds exactly the same as we had with my father's CC Bradford gearbox sometime shortly after Noah sailed!
In the CC gearbox one end of the lay shaft that the reverse gear cluster sits on, is carried in a cast boss inside the gearbox. Somehow this split in half leaving a "C" shaped location for the shaft. When reverse was engaged this end of the shaft moved until the gears did not mesh, with the accompanying music! Once you disengaged reverse presumably the shaft settled down once more in its "C" shaped nest....... until that is, the day that it got bored and made a bid for freedom throught the side of the gearbox.
Hopefully it is something much more simple!!

[Keith Clements]

Has happened to me on a rally, the set bolt securing the reverse gear shaft had worked loose causing the shaft to come out and spill oil all over the desert, and let the sand in. From memory the reverse actuating lever had also come out so did funny things to the reverse gear when the gearchange was used,
Amy worked wonders in French to get some dubious oil to fill the box from some Berbers . Made it back to Spain/Portugal when the sand finally took its toll.