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Which gear oil level is correct?
#795798 01/16/20 10:10 pm
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Recently I did some work on my gearbox and refilled the box with what I thought was the correct level. I looked up the amount first. It appears that it was enough to fill it to level lists as #1 below.
However, there is another level plug shown as #2. I can't find any documentation that verifies the correct level. Does anyone know?
From just a logical perspective, you might think the lower one supercedes the upper one because both bosses exist and only the lower one is drilled and tapped. But I'd just as soon be re-assured that my assumption is correct before I burn up my gearbox.
Also, the excess fluid leaked out through my speed cable almost immediately making me suspect that the factory did this for specifically this reason.
[Linked Image from i.ibb.co]
Thanks in advance.
Bill

Last edited by HawaiianTiger; 01/16/20 10:11 pm.

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1985 Honda Nighthawk 650
1957 Thunderbird/T110 "Black Tiger"
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Re: Which gear oil level is correct?
HawaiianTiger #795800 01/16/20 10:15 pm
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The level plug in the back of the inner cover was too high when the frames went to swing arm and the tilt of the box changed.


Amateur Loctite enthusiast.
Re: Which gear oil level is correct?
triton thrasher #795801 01/16/20 10:20 pm
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thumbsup
I feel better now grin
Cheers,
Bill


Bikes
1974 Commando
1985 Honda Nighthawk 650
1957 Thunderbird/T110 "Black Tiger"
Antique Fans: Loads of Emersons (Two six wingers) plus gyros and orbiters.
Re: Which gear oil level is correct?
HawaiianTiger #795802 01/16/20 10:26 pm
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Mine only has the rear plug. I can just see the oil level through the (pre-Slickshift) filler hole, so I keep it 1/3 of the way up the visible end of the layshaft.


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Re: Which gear oil level is correct?
HawaiianTiger #795808 01/16/20 11:34 pm
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Very topical.
I am in the process of trying to figure out the drain and level plug positions on a 1939 T100 box.
The parts list shows two plugs and two washers.
Assuming not much changed during the war when I look at the diagram for the 1951 box it also shows two plugs and two washers.
One appears from the exploded diagram to be on the rear face of the main gearbox casting whereas the other is at the rear face of the inner cover.
Problem is that I cant seem to find where the holes are for these plugs on the box I have.
Thinking about it in the dentists chair today I thought that maybe I would put a bit of oil into the box and see where it came out!
Any thoughts would be welcome.

Re: Which gear oil level is correct?
HawaiianTiger #795818 01/17/20 2:01 am
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Bill, I believe this is the "documentation" you seek.
[Linked Image from pic20.picturetrail.com]

Re: Which gear oil level is correct?
HawaiianTiger #795822 01/17/20 2:28 am
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...hi; is like TT is saying for the swing arm models as the bulletin confirms .
For the rigid frames is with the other plug; in fact in my 1948 500 I only have that one plug.
I use 90 not thin like the bulletin mention.

Re: Which gear oil level is correct?
HawaiianTiger #795870 01/17/20 6:18 pm
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As far as I know, the essential structure of the pre-unit gearbox remained the same from the earliest Speedtwin to the 1962 650's, and that the only changes were to superficial external casting features. Indeed almost all of the essential internal engineering remained largely unchanged right up to the end of 750 twin production (such that the 5-speed clusters can be fitted to the pre-unit box) and even much of the Trident/Rocket3 is much the same.

I believe the pre-unit drain plug has always been located in the lower central casting, just above the bottom mounting, though I think that the pre-slickshift type box (pre-56) had the drain to the rear, whereas it is on the front with the 56-on slickshift type (presumably for the same different leaning reasons as the level plug change).

The main case went through one major change (other than the minor change noted above) between rigid frame and swingarm frame versions, in that the upper mounting was moved from the top-front of the casting (for the rigid) to top-centre of the casting (for the swingarm).
This happened at the same time as the chaicases went from long (for rigid) to short (for swingarm).

All the other changes were to the inner and outer covers, and some of these changes I would strongly suspect were made pre-slickshift. Some of the changes quite possibly happened during the brief overlap between rigid and swingarm frames for various models, so both bases would be covered.

The inner cover for the rigid (and the early swingarm) gearbox had a cast boss for the clutch cable, the cable pulling a vertical lever pivoted in the outer cover, which has an angled fill plug.
When the molds for the rigid version of the inner cover were made, I don't imagine their crystal ball showed them that the gearbox would be mounted at a different angle in years to come, so the only level boss was at the high rear position.
When the swingarm bike was introduced, I can imagine the existing mold being modified in a small way (it is simple because it is removing material from the mold, not adding to it) to produce the lower boss.
The cover could then be machined to suit either application at that era. This soon became irrelevant, as the models became entirely swingarm, but the old rigid boss remained in the casting till the end of the pre-units (though not drilled). It's like an evlutionary vestige of pre-history.

This arrangement persisted until the slickshift superceded it. The ouer cover was then changed considerably, to place the clutch lever moving horizontally on the top, the end of the cable now held in a special drawbolt, the filler moved to the top of the inner cover, and the distinctive chrome cover with 2 screws on the outer cover.
I don't know how many of these actually had the slickshift knob inside it. Was it meant for lazy riders or for sporting ones? It wasn't present in my 6T gearbox when I got it in 79, so I've not had the luxury of seeing how it worked.

It is hard to see how the speedo drive could ever be oil tight in the long term, but challenging it with high oil level is an invitation to a puddle or oil getting up into the speedo head.

Re: Which gear oil level is correct?
Oil Changer #795871 01/17/20 6:44 pm
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Originally Posted by Oil Changer
Bill, I believe this is the "documentation" you seek.
[Linked Image from pic20.picturetrail.com]


Although they wrote that wrong, confusing the driven speedo gear with the driving one.


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Re: Which gear oil level is correct?
HawaiianTiger #795879 01/17/20 8:00 pm
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I think the error in the bulletin seems to stem from the author being under the illusion that the speedo spindle (hence the driven gear) is above the driving gear on the end of the layshaft.

Of course, the driven gear (speedo spindle) is at the lowest point of the inner cover, with the layshaft drive gear above it.

That bulletin must have caused much confusion, as the higher driving gear cannot be "just covered" without the driven gear being considerably submerged (to the depth of the diameter of the driving gear).
The oil level would then be approx the top of the layshaft.

I agree with TT that this is much too high. Only the lower parts of the layshaft gears need be submerged for oil to be splashed everywhere needed. So as TT said, about a third of the way up the layshaft (or as I think the bulletin meant to say, just covering the driven gear) is quite appropriate as an initial fill level.
If you imagine the delve of the layshaft gears into the gearbox sump, the level can fall quite a lot before they cease picking up oil for splashing about, well below the level of the bottom of the driven gear.


Moderated by  John Healy 

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