Why does the oil tank bracket break?

maylar · Jul 24, 2018

The rear oil tank bracket on my 74 850 has broken off multiple times since I've owned it. It's been welded, reinforced, epoxied, and still breaks. I replaced the tank with one from my buddy's parts stash and that one too has broken. Twice.

Why? What are the stresses that cause that piece of metal to fracture? My buddy's 72 uses the factory rubber mounts like I do and his has never broken. What is it that's different from one bike to another that puts these things under stress sometimes and other times not?

Yes, I know I can send it to CNW for a permanent solution - but I want to understand what the underlying problem is with the factory mounting.

Bernhard · Jul 24, 2018

We need to know what, if any spacers you use between bracket and frame, also if your rubber doughnuts are old, split, incorrectly adjusted causing higher vibrations!

rvich · Jul 24, 2018

I'm guessing this is going to turn into a balance factor conversation. Obviously the '72 750 is shaking in a more harmonious manner in regards to its oil tank. Rubber only goes so far in damping vibration. Are both bikes set up identically other than the obvious difference of one being a 750? If you want to solve this, a close examination of any differences between the two bikes including riding styles and miles clocked needs to be taken into consideration.

Torontonian · Jul 24, 2018

Yes the rubbers petrify with age and heat. The one underneath that is a pain to get to gets cooked solid by the heat. I filled a Much larger circumference one in there and it works. Others remove the long thin bolt and cushing rubber altogether and fit a support pad that can handle the heat. Consider yourself lucky the brazed support area for the long bolt threaded insert did not crack yet and spew out oil. Again that rubber goes hard and causes this. The other support rubbers should be new not old and aligned up to relieve side stresses on them or devise better rubber support ?

texasSlick · Jul 24, 2018

Vibration is a killer.

Slick

acadian · Jul 24, 2018

After breaking a bracket years ago I cut off the bottom mounting lug and took up the space with a rubber pad (a'la CNW mod), no issues in 10 years since

1up3down · Jul 24, 2018

Decades ago I got so tired of buying new rear bracket things that I gave up and stuck a piece of high density foam between the frame and tank back there and it has been just fine ever since.

maylar · Jul 24, 2018

The mountings are stock configuration: grommet with a spacer in the bottom mount and stock rubber mounts front and back. I know I've replaced the rubber parts in the past though I can't say how long ago. Neither are cracked, though the grommet is pretty hard now. I'm willing to replace them again and try another round of welding repair but I don't have any real confidence that it'll last. Maybe add a brace between the back of that bracket and the tank.. dunno.

My buddy's 72 is a combat interstate, so sure the vibration profile is different than mine. It just seems odd that he's had no issues with his in 20 years and mine breaks repeatedly.

And like 1up3down, I stuff a wad of foam rubber between the tank neck and the frame... but I'd sure like to fix this permanently.

kommando · Jul 24, 2018

My 850 has got through 3 oil tanks as the bottom threaded insert splits on the outside and the oil escapes, I have just welded a plate over the top and it sits on a pad. The upper brackets never were an issue but I beefed the mounts up in diameter when I put the pad on the bottom.

N0rt0nelectr@ · Jul 24, 2018

acadian said:
After breaking a bracket years ago I cut off the bottom mounting lug and took up the space with a rubber pad (a'la CNW mod), no issues in 10 years since

I did the same. When my oil tank lower mount was being removed they found a crack in it. Had that all repaired and a pad put under the tank like you said and now many years later no problems.

Onder · Jul 24, 2018

CNW does repair. They dont use the bottom mount just a pad on the inner side .
I bought one and put in new upper rubber mounts and no worries in four years.

MexicoMike · Jul 24, 2018

FWIW...certainly the CNW repair is superb but back in '06 I modded my oil tank in essentially the same - though less elegant - way that they do it. The thrust of the issue is to remove the single point of stress on the tank and that was a "known" issue back when Commandos were being produced. I got the "mod method" from the Norton dealer in Memphis, TN, where I bought my '71 back in the day. They recommended this "mod" on their new bikes, which they performed if you wanted them to do it. Basically the mod was simply to cut off the boss on the tank and install a rubber pad underneath the tank so the tank essentially rested on the pad and also replace the original upper rubber mounts with the larger exhaust-size rubber mounts.

My Commando has had that mod for nearly 10 years and has been operating on VERY rough roads...

So, whether you choose to do it yourself or have CNW do it, it is certainly a worthwhile mod.

maylar · Jul 25, 2018

The muffler mount / headsteady rubbers are different length than the oil tank rubbers. Anybody have a source for beefier ones that fit properly?

austinnorton · Jul 25, 2018

I cut and rewelded the front top arm to fit the muffler rubbers on my MkIII. The OEM rubbers are pretty dorky and wear out quickly. I think the firmer mount up top reduces load at the bottom but the rubber pad at bottom sounds good. When I had mine apart I had a minor leak and pressure tested the tank and found a couple of small cracks in the area around the bottom boss surrounding the braze. I decided to Tig braze it up and found the metal was incredibly thin. After finishing and pressure testing I reinforced the entire bottom of the tank with a layer of JB Weld for good measure.

SteveA · Jul 25, 2018

maylar said:
.............I stuff a wad of foam rubber between the tank neck and the frame... but I'd sure like to fix this permanently.

Maybe it is just an attitude thing, if you can accept the rubber pad is a permanent fix, it is a permanent fix!

MexicoMike · Jul 25, 2018

maylar said:
The muffler mount / headsteady rubbers are different length than the oil tank rubbers. Anybody have a source for beefier ones that fit properly?

I used the exhaust mount rubbers - maybe I had to modify them but if so, I don't recall what it was and it couldn't have been much of a mod - maybe shortening the threaded portion or enlarging the hole where it mounts. I don't remember and I'm in the UK and the bike is in Mexico so I can't walk out to the garage and look at the mount to "refresh" my memory!

rvich · Jul 25, 2018

I have to admit I thought we were working on the why of this and not the how. But regardless, if you beef up the rubber would that not potentially stress the metal more? I've been thinking about moving to springs to hold the tank in tension, but have yet to experiment with it.

MexicoMike · Jul 25, 2018

The most important thing, as per the CNW mod, (and the mod per the dealer in the early 70's) is to fully support the bottom of oil tank. The upper mounts just keep the tank from shifting around.

maylar · Jul 25, 2018

rvich said:
I have to admit I thought we were working on the why of this and not the how. But regardless, if you beef up the rubber would that not potentially stress the metal more? I've been thinking about moving to springs to hold the tank in tension, but have yet to experiment with it.

Yeah, my original question was why. But other than "vibration" I don't think we'll be able to figure that out. So, without a solid answer to that the next best thing is to figure out how to keep it from breaking in the future. And my thoughts about the rubbers are the same as yours - it seems like stiffer would put more stress on the tank brackets.

I just did the bracket weld repair a week ago and after only 150 miles it's broken again. Granted, I'm not the world's best welder but I reinforced it with another piece of steel and it still broke. Tank has to come out again to see if it broke in a different spot or not. Still unsure how I will proceed.

Fast Eddie · Jul 25, 2018

Maylar, without some close up pics etc I don’t see how anyone can help you with your ‘why’?

Except to repeat: vibration + stress = fractures.

I’d hazard a guess that there is something with the mounting of your tank that’s causing increased stress.

If so, it could just be that these discussions about how are also linked to your why!

Why does the oil tank bracket break?

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