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Gearbox selector plunger

Submitted by jack_holgate on Fri, 14/02/2020
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Light Twins

Should the spring in the beveled gearbox plunger that screws into the bottom of the motor/gearbox be strong enough to allow you to properly fit it into the slots in the selector shaft before you begin to screw the threaded part into the case during assembly?  Or will the bevel slip into place no matter how its installed once the shaft begins to turn?

Jack

Submitted by dan_field on Sat, 15/02/2020

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Good question! Funnily enough I was talking to Andy about this at the Bristol show, he was saying it was difficult  putting the gearbox together with the plunger in place, I said I thought I put mine together with it undone, although the engine was in place ...... but I can’t remember how I lined it up, or kept it in place, but I did! Can you see it through the inner cover once it’s installed to check it’s located correctly? 

Dan

Submitted by jack_holgate on Sat, 15/02/2020

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Dan,

That's a good question too.  Since I have my box all back together I can't see beyond the end plate assembly.  I can see the large splines on the end of the selector shaft from the plunger hole under the motor.  So I know how to insert the plunger to obtain the right orientation.  But once I start turning the part that threads into the bottom casing I don't know if the plunger is remaining in place.  And since my spring doesn't extend out far enough into the plunger, I can't use spring pressure to slip the plunger in place and hold it there while I begin to engage the threads - if that makes sense.

I thought about getting a new plunger if I could find out if the new springs in them are usually long enough to hold the plunger snuggly against the shaft splines.  Thus my original question.

There's also been some discussion in earlier threads about whether or not the plunger listed by the Navigator/Electra part number is actually the same as what we have - i.e. is the end of the plunger round or beveled like the later Navigator/Electra ones.

Jack

Hi Jack,

The correct plunger for the later lightweight gearbox selector is shaped like a ridged roof. Two sloping faces to a central ridge which engages in V-shaped slots on the selector mechanism.

I've never had a problem in assembling the plunger, spring and housing into the assembled gearbox. The plunger seems to find it's proper alignment very easily.

There is an oil hole in one of the sloping faces and this can be fitted in either plunger orientation.

I've seen later gearboxes fitted with a pointed plunger, which I believe is used on earlier gearbox. 

Better to use the proper part.

When draining or changing the gearbox oil the plunger housing can be a lot easier to remove than the proper drain plug, which has a very short hexagon and poor access.

I always put a bit of gasket compound on the threads before screwing the housing into casing.

Peter

Submitted by jack_holgate on Sun, 16/02/2020

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Thanks Peter.  Thats reassuring.   I feel like I was able to fit mine properly.  But I've had so many problems getting my gearbox right, I wanted to make sure the bevel wasn't moving out of place when I began to tightened the threads.

Jack

Submitted by andy_sochanik on Tue, 18/02/2020

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Yes, agree with Peter.

The earlier (pre-'63) gearbox had a pointed plunger & spring that had to be put in first, when assembling the box (if the primary chaincase was in situ). It was thus 'fighting' you as you tried to assemble the selector mechanism. (That was the discussion with Dan).

When designing the later (Electra-type) gearbox, one change they made was to relocate the plunger to the bottom of the gearbox. Thus you can assemble the box peacefully, & once its all in place you can screw in the 'roof' shaped plunger & spring. Happy days!

Submitted by jack_holgate on Tue, 18/02/2020

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I may be over thinking this and worrying about a problem that doesn't exist, but how do you keep the bevel from moving out of place between the shaft splines once the keeper part is turned during its threading into the case?  Is the spring supposed to be long enough to make the bevel come in contact with the shaft before the threading starts and thus hold it there?

 


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