Leaking Gearbox Outer Cover Screw

I went to change the engine oil today on my 1974 850 as the final winter maintenance task and discovered gear oil on the floor under the gearbox. I traced it back and it appeared to be coming from the lowest gearbox outer cover screw and not from the gasket between the outer and inner covers. When I checked, the screw was actually loose. The screws are the original slotted screws. I tightened the screw but am wondering if it is unusual for the gear oil to leak back through the screw hole as opposed to through the joint between the inner and outer gearbox covers, and if there is anything I should do to the screw to prevent leaks and/or keep the screw from loosening.

I would de-grease the holes and screws, then apply blue LocTite.

Slick

A set of new SS bolts for your gearbox would be helpful as well with above tip ...

WEM said:

I tightened the screw but am wondering if it is unusual for the gear oil to leak back through the screw hole as opposed to through the joint between the inner and outer gearbox covers

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On my own motorcycle I have come to find it not unusual to leak oil from any place oil is being contained. Find, fix and refill until the rebuild.

I would not say your situation is unusual at all.

Replace them with allen head screws and put a bit of sealer or Loctite on the threads before putting them in, its not unusual they leak from the screws, I replaced my slot head screws with allen head screws over 43 years ago.

Ashley

Technically the slotted screws are "fillister" type. Learned this the hard way when I tried to replace them. I like the way they look, although the allen heads are better as long as you are careful not to overtighten them.

Mmmm. I run the original slotted screws and not a drop. I wouldn't put loctite on slotted screws.... you might not ever get them back out. The gasket prevents the fluid from reaching the screw. You might want to try spray coating the gasket.

Are you sure it's not the kickershaft seal?

Gearbox oil is usually UV light sensitive.
Buy a $10 UV flashlight at Walmart, and that will tell you exactly the source of the leak.

I have occasionally seen the "paper" gaskets dissolving into goo/mush. Then the oil leaks out.

elefantrider said:

Mmmm. I run the original slotted screws and not a drop. I wouldn't put loctite on slotted screws.... you might not ever get them back out. The gasket prevents the fluid from reaching the screw. You might want to try spray coating the gasket.

Are you sure it's not the kickershaft seal?

Gearbox oil is usually UV light sensitive.
Buy a $10 UV flashlight at Walmart, and that will tell you exactly the source of the leak.

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Mine's leaking from kickstart shaft, replacing the shift lever seal while i'm in there. Putting in x-ring seals.

Mine has slow seep from the two bolt openings on front side of casing for the cam plate and selector knuckle pivot. When I rebuilt box last year I fitted new bushing in those openings and followed Mick Hemmings guidance on leaving some space towards outside of opening for o-rings to seat. But seems theres not much contact btwn o-rings and the washer under the bolt heads...so GB oil seeps. I managed to fiddle another additional o-ring over, then under the washers to give more force....which lessened the seep but still have some. MH never actually stated how far below surface to place the bushings. With bolts fully done up, there is a significant gap under the washers to the outside of casing.

Tornado said:

Mine has slow seep from the two bolt openings on front side of casing for the cam plate and selector knuckle pivot. When I rebuilt box last year I fitted new bushing in those openings and followed Mick Hemmings guidance on leaving some space towards outside of opening for o-rings to seat. But seems theres not much contact btwn o-rings and the washer under the bolt heads...so GB oil seeps. I managed to fiddle another additional o-ring over, then under the washers to give more force....which lessened the seep but still have some. MH never actually stated how far below surface to place the bushings. With bolts fully done up, there is a significant gap under the washers to the outside of casing.

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Same here.

ashman said:

Replace them with allen head screws and put a bit of sealer or Loctite on the threads before putting them in, its not unusual they leak from the screws, I replaced my slot head screws with allen head screws over 43 years ago.

Ashley

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If you go with the socket head cap screws, which I did both on the gear box and the timing cover, you will find most cap screws readily available are stainless steel. If stainless, it is a good idea to apply anti seize compound which mitigates Galvanic corrosion between dis-similar metals.A handy chart is presented here:

https://www.fastenal.com/content/feds/pdf/Article - Corrosion.pdf

Or, you can apply blue LocTite which will prevent corrosion and loosening. Fasteners are easily removed using Blue LocTite.

Stainless socket head cap screw sets are available for timing covers from:

http://www.britishfasteners.com

And while you are preventing Galvanic corrosion, do not forget the most likely place for such corrosion to occur ... at the contact junction between the fastener head and the part being clamped. To cover this I turned down 1/4" aluminum washers to an OD of 0.410 - 0.412. Thanks to member robs ss for the tip that hard aluminum washers are available from:

https://www.aircraftspruce.com/catalog/hapages/flatwashers-alum.php

Slick

Tornado said:

Mine has slow seep from the two bolt openings on front side of casing for the cam plate and selector knuckle pivot.

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NPeteN said:

Same here.

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A nip of teflon tape on half the thread length from just under the bolt head.

gortnipper said:

A nip of teflon tape on half the thread length from just under the bolt head.

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I think the issue is more about the washer under the bolt head not being firmly in contact with the rubber o-ring sitting the groove made by having the bushing pressed in a bit too much. If the bolt would thread further into the shaft then there would be more sealing pressure on the o-ring. Or if the bushing was not so far in the casing, making the groove thinner, might also help.