Broken Oil Tank Tab

My '75 Commando broke the rear oil tank mounting tab - again. I got nearly 20,000 miles out of the last repair, so I guess I shouldn't complain too loudly. However, it is annoying. Does anyone have a sure-fire remedy to prevent recurrence?

Thanks,

Jason

This one is on my to-do list:

http://coloradonortonworks.com/catalog/mods.asp

Jason -

I agree with Brian. Send it to Colorado Norton Works and have them do the mod. I did it a few months ago.

Keith Kelly

Thank you Brian and Keith!

I have the same problem with each repair lasting 6-8000 miles. Last yr I bought an ebay tank with all original unrepaired brackets. It went 800 miles and fractured after probably tens of thousands of miles on the original bike! Now, since first failure many years ago I make sure there is no stress on the brackets and the tank just sits in place and gets the nuts tightened without forcing the rubbers out of place. So the rear rubber doesn't break but the bracket always cracks above the weld. I even beefed this up to 1" wide x 0.060" thick. That lasted longest. What I did was weld a brace between the bracket and filler tower which to my mind stops the brkt bending with low and high frequency secondary vibes (back and forwards). It transfers the loads into the tower in compression and tension rather than bending. See if this link to pic works, welding is c**p but strong enough. Two braces would be better but doesn't leave much room for tightening the nut.
http://www.photoworks.com/servlet/GetTh ... &svr=web13
The bottom has the reinforcement and that's a must though I never had trouble here. The CNW mod is professional but if you are breaking brackets and not rubbers I can't see a tougher rubber solving that issue and if the mount is beefed up like they say what about the stress on the tank? The forces are then transferred to the weld and the tank is pretty thin there.
Have asked myself why I have a continuing problem when the tank is installed 'properly'. Is it the way I ride? These days the speeds are up so the heavy low frequency shakes are minimised but I also break c****y Norvil fairing clamps quite often. Maybe it's just my setup.

Keith,

Thanks for the link, but it didn't work. I'm with you 100% on the stiffer rubber and beefer mount; all this will do is transfer stress to the tank.

I'm thinking about a vertical rubber hanger of some sort bolted to the tank instead of the steel strap welded to the rear of the tank. The thinking here is that the strap will be more compliant to fore and aft vibration. Notably, the long, spindly front mounting bracket never fails. This, it seems, is because the bending stress is spread out over a greater area?

If anything, I'm thinking that a more compliant rubber mount may be better for the rear bracket. However, dealing with vibration is tricky business.

Also looser Isos, at the risk of increased amplitude, seem to decrease vibration frequency. And high frequency seems to be a major contributor to the problem here.

See if this works......
http://www.photoworks.com/servlet/GetTh ... &svr=web40
I forgot to make it sharing! doh!
I guess a rubber strap would work but you really need to restrain the tank sideways as well as fore and aft. The best idea would be a moulded rubber snubber between the frame and tank. I recall when I bought my bike 10 yrs ago the tank was wedged in place with carpet offcuts!! No rear brkt and no bottom mount either.
I figure the front survives since the nasty low or high Hz vibration is in a fore and aft plane (crank at 90 before and 90 after TDC) which puts that bracket in tension/compression. The rear can only break if it bends I think but I might be wrong. I borrowed a hand held SKF vibra pen a few years ago and front to back vibes were nearly 50% more than up and down levels though I don't remember the values.

Phil at Fairspares in San Jose Ca fixed mine. He deletes the problematic bottom mounting point and the tank is mounted on larger rubber studs (ala muffler mounts) I have had zero problems since this fix and highly reccomend Fairspares to solve this age old Command problem!

I'm running CNW-reworked tanks on both my Nortons. 6000 miles on the 750 since the rebuild and no problems to date. Rear mount was broken on my 850's tank when I got the bike, just above the weld as mentioned above.

Certainly CNW isn't the only option for tank rework, but it was attractively priced and super convenient for me. I sent Matt my old tank and three days later I had the reworked one in hand, ready to install. Kind of a no-brainer for me.

Debby

on the CNW site they talk about brazing the oil tank. Can anyone tell me why they braze as oppposed to just welding? It's a steel tank, right?

swensosc said:

on the CNW site they talk about brazing the oil tank. Can anyone tell me why they braze as oppposed to just welding? It's a steel tank, right?

Click to expand...

I would think brazing would be less susceptible to vibration fatigue, i.e., cracking. I just got done welding a plate on the bottom of my tank. We'll see what happens.

that was another question I had: why weld a plate on the bottom rather than just weld up the bolt hole?

Brazing thin metal, such as that found on oil and gas tanks, is often easier than welding. A quality "tig" welded joint is superior to a brazed joint.

swensosc said:

that was another question I had: why weld a plate on the bottom rather than just weld up the bolt hole?

Click to expand...

I figured the idea was to stiffen the bottom of the tank and the attachment point of the stand-off (bolt hole). I fabricated a 3/32" plate slightly smaller than the bottom of the tank. I then drilled a hole slightly bigger than the stand-off. I mig welded the plate to the tank and welded completely around the stand-off. Haven't got the bike back together yet.

I also like the idea some have used of isolating the tank from the frame.

Should have used Photobucket, now set up and easy for a PC thicko like me....
The weld and finish is not half as bad as this closeup looks!!