Oil tank tab broken again

Haven't posted in a while... been busy ridin'.

The oil tank on my 74 850 keeps breaking at the rear mounting tab. Over the years it's broken a few times and I've brazed it, welded it, and epoxied / JB Welded and it still kept breaking. This season I replaced the tank with a perfect used one and after only a few hundred miles this one broke too. The tab breaks right where it's welded to the tank.

I have the correct rubber mounts at both ends, and the correct grommet for the bung in the bottom. I'm going to have to weld this thing again and would surely like to make it last a while. Is there something that I'm missing here?

TIA

Send it to CNW. If you get it to them by the 1st you should be able to get it back by the end of the month.
http://coloradonortonworks.com/part-cat ... /#post-295

strange that your tab keeps breaking at the same point

do you still have the bottom oil tank secured the stock way?

if so, that could be the reason for the stress being placed on that upper tab

I removed my bottom securing bolt 17 years ago and have had no further problems with the two upper mounts, just a thought?

Another endoresment for CNW's oil tank make over. Matt will fix it once and for all.

JD

Snorton74 said:

Send it to CNW. If you get it to them by the 1st you should be able to get it back by the end of the month.
http://coloradonortonworks.com/part-cat ... /#post-295

Click to expand...

+ 1 on the CNW mod.

Beef it up by welding/brazing a strap between it and the filler tower. Mine went every 1000 miles +/- and even with 50% wider and thicker material it always failed. A good used tank went within 600 miles and that had probably survived on another Commando for many years, assuming it had been in use! The vibration seems to vary and mine has a high frequency vibe I cannot lose even with squared off ISO's, softish rubbers etc. Of course it could be low frequency vibes doing it.........who knows. Pic is of first attempt so not pretty but has held up now for at least 10k miles.

I am not advising for or against CNW's mod, especially since I am an Atlas guy and know nothing of the mod, but here are some things to consider, as they may impact CNW's or your own DIY fix.

Brazing puts copper/tin into the steel surface and follow-up welding is not recommended. This could explain why your welds failed. Maybe others expert in welding can advise how it might be done.

My Atlas tank tab failed as yours, and I brazed it....wrong! When I decided to do it right, my welder had to cut out the area with the brazing residue, then welded a cover plate over a wider area, upon which we welded on a new tab of heavier gauge. Body work was of course necessary. The welder found spidery cracks under the old tab which grew when he applied heat....only recourse was to cut out a wide area.

If you DIY: I am assuming the Cdo tab has a 90 degree bend in it...if so, fabricate a new tab of heavier gauge with at least a 1/8 inch radius at the bend. Do not weld on the side with the radius...this will effectively make it a sharp corner where stresses accumulate and eventually cause fracture. Any cracks or holes in the tank that will be covered by the tab, must of course, be welded and ground flush.

Slick

I had mine break three times. Each time I took it to a local machine shop. His explanation for the reoccurring breakage was most likely the type of alloy metal that was used. When heated from re-welding it weakened the metal and each time the fracture occurred just above the weld. He told me it would happen but I told him to reuse the original tab. The last time it happened he removed the old tab, made one of a heavier and different type of metal and so far so good. Goes along with the DIY suggestion.

Scott

CNW removes bottom mount and reinforces the bottom or so it looks. The tank then hangs from the top two and can move about. I think this is the cure.
Make sure you use real squiggy mounts.
You can send him your tank for an exchange, at least I did and it was fast. Matt is all about service and you can
ring him and get quick response. Matt is often above my budget level but if you use him you get full service including
problems later.

Onder said:

CNW removes bottom mount and reinforces the bottom or so it looks. The tank then hangs from the top two and can move about. I think this is the cure.
Make sure you use real squiggy mounts.
You can send him your tank for an exchange, at least I did and it was fast. Matt is all about service and you can
ring him and get quick response. Matt is often above my budget level but if you use him you get full service including
problems later.

Click to expand...

Well it does hang, but the bottom sits snugly on high density foam that is supplied in the kit, so there is little or no 'pull' on the top brackets, the rear one he strengthens anyway.

Certainly the cNW mod is excellent but you can do it yourself pretty easily - just cut off the boss from the bottom of the tank, build up the area under the tank with rubber so the tank sits on that and replace the standard oil tank vibration mounts with the larger sized ones used for the mufflers.

Mine has been that way for years in Mexico riding over copious cobblestones and I promise if anything will cause parts to break/fall off, cobblestones is it!

My oil tank tab broke off my Commando as well. So I fabricated a new tab with the sides flanged upward thinking that would stiffen it so it would never break again. That idea lasted about 200 miles before breaking off. So I welded the tab back on to the tank and then fabricated and welded an additional tab that I fitted to the top of the tank's flanged seam, then to the back of the mounting tab and then to the side of the filler neck. This mod has held up beautifully with several thousand miles on it. Here are a couple of pictures of my modification. It is kind of hard to see, so I supplied two pictures:

Do we guys with Mk3s have to worry about this too? I've never done anything special and mine still appears fine.
Jaydee

I have one thing to say...CNW mod!!!!

On the CNW mod... it's not just new tabs and elimination of the bottom mount. Matt boils the tack clean...no bit of gunk, grime or oil remains. The exterior is (sand) blasted and the tank recieves a perfect powder coat. The tube that feeds the chain luber is sealed shut. So what you get back is a better than new part, and If I say so myself, it is well worth the modest cost and I think that the time and processes involved, Matt may just be loosing a few coins for this service. Hats off and Kudos to Matt.

JD

Gidday mate. I would humbly like to give a quick report based on my experience.

had my fastback since 1977. had not the proper tank. I then fitted a fastback one. However, instead of using the rubber mounts, I replaced them with solid nylon discs of 1" to 1 1/4" outside diameter and of measured thickness..drilled in the centre to take the bolt. this was entirely because I had no factory rubber mounts available and had to use what was on hand.

THE IMPORTANT BIT.. alignment of front and rear tabs. the biggest reason for failure using standard mounts, especially at he rear is the tabs are not in line with their respective holes and not parrallel with their mounting plates in 2 planes.. this stresses the rubber mounts and tabs. the main cause is the rubber flexing and transmitting this vibration to the tabs. under constant reversing stress, the tab fatigues at the place of maximum stress concentration, where the weld is. !! what I found what works is to by whatever method first make sure the tabs are parrallel to the front and rear plate and the holes are in line. then carefully measure the distance from each tab to the plate and make some nylon discs of the measured thickness. Use the standard bottom rubber mounting grommet. to conclude, if the tank cant move, is in line and not under mounting stress, the tabs will not break.

As I get stuff all vibration at highway speed, rigid mounting is not an issue to me. bad isolastics may also contribute to tab failure. as more vibration from the frame is transmitted to the moving tab, combined with misalignment, guess what. so the advice is check iso how bad is the vibes as a possible contributor, carefully align, then solid mount. This has worked for many years now. has never been given a second thought.

However please check out other opinions. I would encourage you here to always seek other opinions..Hope to hear from the engineers out there myself on this. anyway hope this is of helpful information. Bradley

Bradley says some good stuff above.

Vibration flexes metal back and forth until it breaks, just like bending a tab until it breaks.

Slick

Get a 69/70 central type oil tank.

ludwig said:

Stop welding tabs .
Make a clamp around the filler neck :

Click to expand...

Yet again, Ludwig comes up with a seemingly obvious, yet brilliant solution!

That really is a simple solution.

He doesn't say much, but when he does, I've learnt that it is usually worth listening to!

If the lower bung was buggered up I would certainly entertain the CNW mod. But that one is OK and there's a new grommet in there. I agree with B.Rad in that the front and rear mounts aren't aligned, and that there's stress on the rubber isolators. I have had the rear isolator fail on me also, where the rubber pulled away from the metal stud.

ludwig said:

Stop welding tabs .
Make a clamp around the filler neck :

Click to expand...

Outstanding. I will consider either that or something similar to PeterJoe's bracket weld. Either one should be doable and maybe not even have to take the tank off the bike again.

Thanks muchly, gents.