gas tanks and cap

/5 and /6 BMW motorcycle gas tank and cap

by Duane Ausherman

This page covers models from 1970 thru 1976.  Models covered are R50/5, R60/5, R75/5, R60/6, R75/6, R90/6 and R90S.

The timeline for /5 and /6 BMW motorcycle gas tanks.

The /5 series first came out in late 69 to be the 1970 model.  It was offered only with a 6.3-gallon tank.  It was considered by all of the motorcycle magazines to be very ugly.

To provide an alternative, BMW came up with a new design.   In 1972 BMW came out with a smaller tank of only 4 1/2 gallon capacity.  It had chrome panels on the sides and has become known as the “Toaster Tank.” It was very unpopular in those days.  Now the magazines and riders both made fun of it.  Quickly, the older “ugly” tank became far more popular.

We wondered why the cavity in the underside of the “toaster tank” was so different from the larger tank.  It wasn’t until mid 73 that we found out why.  It was a space for a, soon to be added, master cylinder to actuate the disc brake of the next year.  As far back as 72, BMW knew that a hydraulically operated disc brake was in the future.

The large tank was still available as an option, but very hard to get in spite of the reluctance in the acceptance of the “chrome tank” that was the butt of many jokes.  Butler and Smith, the importer, didn’t order nearly enough bikes with the large tank.  They also “couldn’t get” but a trickle of large tanks as spare parts, or so they said.  How come I was able to get them by the dozen directly from Germany from various connections through the factory?  Even the Germans also couldn’t understand why anyone would want the small tank.  They had plenty of large tanks and were happy to arrange for me to get them through an “underground” source, which I never understood how it happened, but I wasn’t going to ask too many questions.  They were very happy to learn that not all Americans were “small tank” crazy.  As a result, my dealership had dozens of new small tanks, replaced with large tanks, sitting in storage.  Nobody wanted them back in the day, now they are popular and worth $$.

When Europeans wanted a different tank from the stock tank, it was always a larger one, not a smaller one.  Several companies, most notably Heinrich and Hoske, offered tanks up to 10 gallons.

In mid 73 we noticed that the large tanks had the wide cut out for the master cylinder too, same as the small tank.  That reduced the capacity to about 6.1 gallons.  The two are totally interchangeable, as is the small one.

The tanks in 73 have a very confusing history.  There is no rule for the tank in 73 because there were 4 tanks that came out on new bikes.   Some started coming with the chrome panels and pin stripping.  Later they came with no pin stripping.  Then some came with no chrome.  A few came with the highly desired large tank.

To compound the issue is the fact that we dealers could swap a tank in minutes.  One could change the color set in less than an hour and so our customers could come in and have us build a bike to suit.  What went out the door for tank and color had little relationship to what BMW assembled.  We tried to have at least a half dozen color sets in stock as spare parts to meet the demand.

The Europeans hated the chrome as much as the tiny capacity.  Knowing this, BMW had rubber knee pads made to go over the mount for the chrome panel.  One could remove the emblem and carefully slide the chrome off and glue on a rubber pad.  That one small change completely altered the personality of the bike.

I ordered the knee pads by the dozens and we never had enough to satisfy the demand.  We hardly sold another bike with the chrome panels.  Nearly all of our customers with the chrome came in to get the rubber pads installed, or buy a large tank.  My dealership was the only one in the country with the rubber knee pads.  The other dealers were livid for losing new sales to our shop.  We offered new bikes with the large tank, or rubber pads for those that liked the small tank, but not the chrome.  You should have seen the two large stacks of chrome panels.  I hated them so much that when I sold my shop, I only took two sets with me and left dozens.

I was quite shocked to get a call one day from a friend at Butler and Smith.  He wanted to know the part number of the rubber knee pads.  I asked why and he informed me that they wanted to order them and had no part number.  I also asked why he called and not someone else.  His answer was that they knew that we were friends and there was a better chance that I would give him the info.  They were right, as he was the only decent person working there at that time.  Besides, why stonewall them, BMW surely must know the part number.  My “inside” person at the factory never got discovered.


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The front view of the cutout

This shows my hand in the enlarged area for the master cylinder.  It is about 5″ wide (125 mm) at this point.  A tank without this area has the tunnel going through the underside at about 1/2 this width.  Someone, please send me this measured dimension of the narrow tunnel and I will add it here.

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The Bottom view of the master cylinder cutout

This shows another view of the cutout for the master cylinder.  At the bottom center of the picture is the welded center line of the tank.  This width would stay the same through where my hand is positioned on the old tank.

The newer tank with the cutout for the master cylinder will fit just fine on the older models.  The older tanks won’t fit on the models with the master cylinder.

The BMW motorcycle /5 and /6 gas tank cap

When the /5 model came out in 1970, it had only the large 6.3-gallon tank.  The cap hinged from the front and fastened at the rear.  This apparently led to some injuries from riders going over the handlebars.  Guys, your imagination will suffice.  In 1972, BMW came out with the hinge at the rear of the cap.  That configuration presented less of a danger.   That meant that the large tank came in 3 varieties.  1.  Front hinge gas cap for 70-71.  2.  Rear hinge gas cap from 72 on.  3.  The large vacant area for the later master cylinder.  Very few of these were seen in the USA.


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Can you imagine going over the handlebars on that bike? You could have a “serious injury” before you even landed.  Isn’t that tank just plain beautiful?  Photo supplied by Stephen in Seattle, thanks.

In 1974 with the advent of the /6 models, BMW chromed the gas tank cap.  Prior to that it was just aluminum and didn’t shine up well.  In 74 the “S” model had a different shape tank.  In 1977 the /7 tank was the “S” design of shape and a different cap.  This is the one to have, as it is really beautiful.

The cap has a gasket under it to seal fuel.  A rivet is pressed into the cap to hold the parts in.  It is hard to remove.  Some gently grind flats on the sides so that pliers can get a good hold on it and rotate while pulling it out.  Some owners tap the hole and install a screw.  That makes it easy to repair in the future.  I suggest using Locktite on that screw.  If it falls into the tank, it is hard to remove.

The gas cap is mounted with a hinge pin.  It is 1.380″ (35 mm) in length and .155″ (4 mm) in diameter.  At one end are teeth that are .003″ (.08 mm) larger in diameter than the main shank.  It is these teeth that holds the pin in the cap.  The cap can be removed by gently tapping the pin out.  The holes in the two sides of the cap aren’t the same size.  One is a loose fit and the other is an interference fit for the teeth of the pin.  It is better if the pin is tapped out so that the teeth don’t go through the cap, but come directly out.  If the interference hole is too large for a tight fit, use a drop of Locktite to secure it.

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BMW motorcycle gas cap pin

One can see that the teeth are slightly larger than the main shank of the pin.  The pin need not be fully removed when removing the cap.

Prevent Rust

Rust is a major threat to these old motorcycle gas tanks.  One very simple way to prevent rust is to consider adding a cup of rubbing alcohol to your tank in the wet season, especially if your bike sits out in the rain.   Rubbing alcohol is very cheap, much cheaper than some products sold for this issue.  One can easily buy it just about anywhere too.  I have never heard of anyone commenting on any negative impact of alcohol in the tank coating. has a superb collection of photos of large tanks