By Sandy Roca

The super rare Citation 500 was actually a Horex in disguise

The year 1958 was a good one for horses, both the flesh and the iron kind. A thoroughbred colt named Citation won the Triple Crown and a brand spanking new motorcycle model  — with the same name — from Germany hit the American market. Full page magazine ads from the importer — Berliner Motors Corp – heralded the Zundapp Citation 500 as the “motorcycle  you’ve been waiting for.”

But the real story of the ill fated Zundapp Citation really commences a few years earlier. Because in fact the motorcycle was not a Zundapp but was actually manufactured by Horex Werke K.G .

Over a half century ago, in 1955, a family-owned motorcycle manufacturer in Germany named Horex introduces a new line of parallel vertical twin cylinder 400 cc motorcycle models with overhead camshaft  (OHC). This is not some bolt out of the blue, for the company has a long history of developing OHC twin cylinder engines.

Horex was best known for its post war single cylinder Regina model but they had produced several prewar vertical twins with overhead camshafts and used these designs for racing as well. Horex design genius Hermann Reeb was apparently the brains behind these innovative and advanced machines. In the early 1950s. Horex  protoypes a modernized 500cc vertical twin, but for one reason or another it is shelved.

That all changes with this new 1955 offering which appears to be an update of that prior prototype.  Horex calls it the Imperator, and with good reason. Because roughly translated, that name equates to Emporer. And technically, this bike is arguably a leader for its time. The legacy of Reeb can be clearly seen.

It has full width alloy brake hubs front and rear, a transmission case in unit with the engine and a nifty parallel twin cylinder powerplant with aluminum hemispherical heads. The crankshaft is supported by three main roller and ball bearings and uses an  innovative Hirth coupling in the center. The impressive feature list continues: enclosed rear chain, helical gear primary drive, wet multiplate clutch, chain driven overhead cam, rocker arms with hard chrome faced followers, and gear driven oil pump. The ignition is timed by a single breaker point on the crankshaft end firing both cylinders via dual coils in a clever cost-effective “wasted spark” arrangement. The breaker cam is controlled by spring loaded bob weights, for automatic ignition advance. It is interesting to note that many of these engineering niceties are readily copied by other manufacturers and passed off as technical innovations, years hence.

Hopes are high that this new model will be a world beater. Horex exports the bike to the USA through its sole agent and importer Foreign Motorcycles Corp. FMC begins delivering Imperators to its nationwide dealer network and places advertising in the premier print media of its day, Cycle Magazine. A 1956 road test positively proclaims,  “My sugar  is so refined.”

But by 1957 all is not happy in the Hamburg home of Horex. It becomes apparent that sales are disappointing. The competition for the Imperator is trim and muscular British motorcycles like Triumph, BSA, Royal Enfield and Norton. By comparison the 400cc engine of the Imperator is significantly smaller than the 500, 650 or 750 cc of the Brit iron. At a hefty 432 pounds wet, it weighs more than its English bike brethren, as well. The $895 price tag is an issue, too. The cc limitation is a clever ploy to sell bikes in Germany which has government-mandated usurious insurance rates for motorcycles over 400 cc. This policy is a carry-over from the early postwar era, when European economies were supposed to focus on rebuilding war-ravaged basic industries, not consumer durables. But these subtleties generally escape appreciation by American buyers, who are not exactly turning out in droves at Horex showrooms. Also so not helping is the Imperator’s styling. Its oversized black fenders and a standard dorky leading link front “Earles” fork seemed inspired by Art Deco design. Which is cool retro now. But not then.

So, Horex makes a decision that ultimately dooms the company. They enlarge the engine, add a eye popping bright blue green paint scheme, install higher “ western” handlebars and rename the model the trendy moniker of Citation after the winner of horse racing’s Triple Crown in 1958. But they do not deliver this new and improved version of the Imperator to Foreign Motorcycles Corp. Instead, they apparently sign murky deals with Zundapp in Germany and with a direct distributing competitor to FMC in the US,  Berliner Motors.

Of course FMC has an exclusive to import the Imperator. Horex in German apparently thinks they can skirt that deal by calling the Citation a “Zundapp.” They arrange to put a Zundapp badge on the tank and Berliner introduces its new  “Zundapp Citation 500” to the US market in early 1958.

The Citation features a newly enlarged cylinder bore of 66 mm for an engine displacement of 452 cc, allowing the Madison Avenue types probably on retainer to Berliner to call it a full “500”. The cylinder heads have been reworked for more power with enlarged ports, bigger dual Bing carbs, a longer duration “R-3” cam and more durable cast iron exhaust port sleeves. The lubrication system now features a paper cartridge-type element for better oil filtration. The crankshaft is revised for greater rigidity.  And numerous details inside the engine improve lubrication. Better  looking telescopic forks are offered as an option. But make no mistake, the bike is purely Horex. Intrepid motorcycle mechanics quickly find that engine internals provide rich support for this thesis, being liberally covered with the traditional Horex-crown forging marks. Berliner’s advertising claim of 100 MPH “guaranteed” top speed is backed up by a 1956 Cycle magazine road test of the smaller engined Imperator, which stated 96 MPH in fourth gear.

It is hoped that sales of the Citation, (taking advantage of Berliner’s expanded dealer network, distribution muscle, and advertising campaigns) will begin to head in a direction more appealing to Horex headquarters back in Germany. A road test in American Motorcyclist lauds the machine for its “clean design, sparkling performance and quality finish.”

The performance improvements work, to say the least. Well-known motorcycle drag race mechanic John Gregory retunes a Citation with megaphone exhausts and richer jetting and the machine rips off a quarter mile in under 13 seconds at over 100 mph. After a sprocket and spark plug change, it is timed at over 125 mph on road race track.  These impressive numbers are more like a 650cc Triumph than a little 452cc Citation.

But the color change, extra power and detail improvements do little to cure the Citation’s styling, and hence marketing, woes. The American market seems be more interested in trim looks than ultimate speed or engineering prowess. Furthermore, big legal trouble begins brewing in the USA.

It apparently does not take long after the Citation’s splashy launch for Mark Stern, president and owner of FMC, to march into court to enforce FMC exclusive contract to import Horex Imperator motorcycles … including this new blue thinly veiled Citation model. We surmise that Berliner also has some kind of a contract from Horex. The result is: the court bans any further importing of this star crossed model by everybody. After an initial import of about 200 Citations Berliner and FMC are allowed to sell additional stock on hand — but that is it. There are no winners. Horex is now stuck with warehouses in Germany full of Citations that are unsellable by Berliner in the U. S. AND unsellable in Germany because of insurance rates. Horex suddenly faces an even greater menace. Domestic sales in Germany slump due to a recession. Now saddled with the loss of a major market for its flagship model and huge unsold inventory, it can go no further. Horex ceases production in 1960. All in all, less than 400 Citations make it into American owners hands.

To this day, there remain many unanswered questions. Why did Horex think they could get away with such a flimsy stratagem to skirt FMC’s apparently exclusive agreement? Why did Berliner and Zundapp enter as a willing partners in the scheme that was certain to generate controversy at best? And why were all parties unable to come to a compromise that would have at least saved the source of machines — and potential profits — for all?

The lengthy passage of time eventually heals the wounds. FMC ekes out an existence for some years selling Horex parts and importing brands with forgettable names like Pannonia, Danuvia and Zanella. Berliner moves on to distribute the likes of Norton/Matchless, Ducati, Moto Guzzi. In time, saner heads prevail over the German insurance situation, rates ameliorate, and the warehoused Citations are sold off as  “Imperator 450S” models to willing German buyers. Berliner’s substantial parts inventory goes to Domiracer in Ohio. Horex owners clubs keep interest in the marque alive and well even to this day, aided by a few machine shops in the business of making parts.

Occasionally a scheme to revive Horex surfaces. The most notable of these is an effort by noted motorcycle enthusiast, former racer and magazine publisher Floyd Clymer in the mid-1960’s. Clymer, flush with cash from his sale of Cycle magazine to Ziff Davis and perhaps somewhat departed from his senses, enters into a deal of some sort with Friedl Munch, former chief Horex engineer who had purchased the original tooling and the Horex name. Clymer and Munch build at least one running prototype of a new and improved “Horex” under the name Indian. These are interesting machines indeed, with chassis and running gear by Tartarini, Marzocchi and Grimeca of Italy and engine improvements that include 600 cc alloy cylinders, alternator, magnesium castings, and larger dry clutch. But  Clymer is striken suddenly with a heart attack and deceases. Production plans come to naught, and the effort dies with Clymer.

Today, the Horex name lives on, its corporate identity having been acquired by a new motorcycle manufacturer specializing in low-production cafe racers. But in mute testimony to the appeal of the Citation, it remains a much sought-after rarity by German buyers worldwide as the last and best effort from Horex.

The next chapter in this epoch begins in 1965. The Citation is now largely forgotten, an obscure orphan from a defunct company. A recent high school grad is envious of his friend’s neat little single cylinder motorbikes. They range in size from Honda 90 to a Jawa 125. His parents won’t buy him one. But they say he can buy one himself if he earns the money. He works two jobs for most of the summer of ’65. And determined to out-cc his friends, he starts searching for the biggest bike $125 can buy.

That odyssey lands him one August night in the basement of Corey’s Cycle, Upper Saddle River, NJ. What confronts him is a $125 basket case motorcycle with twin cylinders and not 200, not 300, but 452 cc of displacement. Horex or Zundapp, who cares. Sold.

Eventually this youth discovers the rarity he has adopted, finds even rarer parts, and rebuilds it. In his travels he quickly learns that if he encounters another rare example of Citation in disrepair, do not quibble and purchase it promptly as a parts source. He finds a wrecked Citation at the Sports Spot in Wayne  NJ and buys it for– you guessed it — another $125.

And that’s where the motorcycle you see at the start of this blog comes in. It arrives at Pete Andrews Motorcycles in Boston, Massachusetts some time in the mid 1960s in need of repair. The repair apparently does not go well . One thing leads to another, and it ends up stashed in Andrews’ dark and dank basement, forgotten, partially disassembled, missing many parts — including the all important cylinder head — and presumably lost to motorcycledom forever.

The youth in question comes to the Boston area for college. He dutifully commences studies … and a search of all shops in the area for derelict Citations. He discovers the forlorn Citation carcass at Andrews. But the basement is a nasty swamp of cobwebs, rats, dirt and foul smelling god-only-knows what. And Andrews, pack rat that he was, is not interested in selling. Crusty old Pete is not exactly an easy mark to cajole.

Eventually Pete goes on to some Bike Hoarders Heaven in the sky and his mammoth cache of old parts, bikes, and what-have-you falls to his widow Alice. A sale finally takes place. What is left of the Citation is plucked and picked out of that subterranean Haedes, hauled up a greasy stairs, pushed out in to the light of day and taken home.

Other things in that youth’s life take precedence and so the Citation hulk leads a quiet life in yet another basement. Decades pass and the youth is no longer young. But he has not forgotten the Citation. And slowly but surely he collects the missing parts for the bike. From Louisiana comes a cylinder head. From Chicago, a new old stock seat and very rare telescopic fork conversion. England and Germany contribute many small parts and a pristine gastank. Plus of course the nation known as Ebay participates.

Eventually he crosses path with a one Ken Owen in Tennessee,  who is a fellow Citation owner, machinist, mechanic and gunsmith par excellence . He quickly determines that Mr. Owen is perhaps the only person outside of Germany capable of expertly rebuilding the Citation whether it be Horex, or Zundapp, or whatever.

He mocks up his collection of parts, sends it Owen bound, and before long, the Owen magic has it results as seen on these pages. Whether it be Zundapp or Horex, this bike has successfully returned to the real world after a perilous journey and very long wait.

Citation fixes and tips

For its time, the 452cc vertical parallel twin had precious few design flaws. A copious supply of oil via the hollow overhead cam to rocker arm follower faces made valve stem sealing paramount. On the exhaust side the original neoprene o-rings cooked after a few thousand miles and no longer sealed adequately . This caused a white smoky exhaust that mimicked the symptom of worn valve guides. Installation of high temperature viton ring seals cures this problem. It is also possible to remachine the upper end of the valve guides in situ and install modern stem seals.

The oil pressure to the lower end and camshaft relies on the good condition of crankshaft main oil seals located in the center bearing section. On machines stored for lengthy periods, the seals deteriorate and good pressure is lost. Rebuilders must attend to these seals to safeguard cam and rod bearing  condition. The German firm Prazisionswerkzeuge Buren
( ) still carries out the rebuilding of crankshafts. Prices and delivery vary with condition and modifications undertaken.

The overhead cam is driven via chain at half crankshaft speed by means of two sprockets driven in turn by a reduction gear set in the center of the crankshaft. The correct juxtaposition of the upper and lower sprockets is critical for valve timing, but is difficult to orient once the head and cylinder are assembled, due to the narrow cam chain tunnel. Applying a dab of white paint to the lower sprocket location at the correct TDC position before assembly greatly aids visibility.

The Citation cylinder head is aluminum, but the valve / tappet adjust covers are cast iron. The different coefficient of thermal expansion between these two materials means the covers tend to loosen and  leak oil. Machining and installing aluminum covers results in a cleaner engine.

Most machines were equipped with Earles type leading link forks that incorporated hydraulic spring shock absorbers . There was a very rare telescopic fork option that simplified appearance. However, this front end did not have true hydraulic damping, relying merely on internal sliding-part friction. Ammon Technik ( in Germany offers a fork conversion that adds true hydraulic damping for better handling.

Due to its rarity, finding parts for the Citation is a challenge. However it is essentially a Horex Imperator and thus most parts interchange.  Imperator parts are available in Germany from several sources, most notably Ammon Technik ( There are also sources for Citation and Imperator parts on both American and German Ebay.

Both the Citation and the Imperator had chromium plated fuel tanks, a very fetching feature when new, but a vexing problem for restoration. It is very difficult and expensive to bring the metal of an aging and corroded gas tank back to condition pristine enough to take a mirror finish for plating.  However, tanks carefully refinished with filler can be smoothed and then covered with a mirror wrap film. The result does a very good job of mimicking chrome plating.

The crankcase halves and the side covers of these engines are retained by oval slot headed screws and bolts. It is difficult to properly torque these fasteners for good strength and oil tightness. Remachining these castings with a piloted counterbore tool in each bolt hole allows the use of socket head cap strength cap screws that are much easier to torque (18 foot pounds for case bolts and 7 foot pounds for side cover rim screws.)

About the author

Sandy Roca is a retired motorcycle journalist. The very first motorcycle he bought was a Zundapp Citation. He rebuilt it in 1969 and still owns it. He is currently building a Wolfgang Kayser 500cc Citation special.

Engine: 452cc aircooled SOHC parallel vertical twin cylinder, alloy heads, cast iron cylinders, Mahle pistons, 66mm bore x 66mm stroke, 8.0:1 compression ratio, 34hp @ 6600 rpm, 42hp @ 8000 rpm with open megaphones

Top speed: 100mph

Carburetion: Two Bing 27 mm

Transmission: 4 speed, constant mesh, wet multiplate clutch, helical gear primary drive , directional reversing, enclosed rear chain

Electrics: 6 volt permanent magnet Bosch generator, crankshaft mounted artmature

Frame/Wheelbase : Welded tubular dual cradle with brazed lugs/ 1380mm (54-1/4 inches)

Suspension: Earles front fork (standard); telescopic front fork (option); Boge spring shock units (rear adjustable)
Brakes: Full width drum hubs, 190 x 40 mm front and rear

Tires : 3.25 x18 in. front; 3.5 x 19 in. rear

Weight: (dry)  385 pounds (175 kg)

Seat height: 30.9 in. (785mm)

Fuel capacity: 4 gals  (15 liters)

Original/current price : $899.00 / $12,000+/-


The engine runs forward, same as the rear wheel, but the camshaft runs backward. The engine turns clockwise when viewed from the side that has the gear primary drive and ignition breaker points. The notch in the end of the camshaft, where one of the bearings is pressed on, goes to the right side of the engine,  the side that carries the points and has the primary drive.

The cam is at top dead center (TDC) when that notch and key way for the sprocket are facing straight up. The cam sprocket in the head will also show a tooth facing straight up at TDC.

But, on the lower sprocket a valley between teeth will be showing at TDC. There should also be adjacent or nearby a oil hole showing on the shank of the sprocket. It is possible to have a top dead center on the crankshaft with the sprocket showing a tooth straight up where that oil hole is not nearby or visible. That is a false TDC and the engine must be rotated 180 degrees so that the oil hole is showing and the valley is straight up. An engine that is mistimed  one hundred and eighty degrees out will still run but will produce very inadequate power and low speed.

The cam chain is 64 links, 2B20 type. The master link should be inserted from the left side of the engine to the right. Also the clip should be installed so that the round end faces the back of the motorcycle. This is because the cam chain turns in the opposite rotation of the crankshaft. Original type cam chains have a polished side to all the link edges. This side should face outward so that it engages the rub strips on either side of the cylinder.

The cam chain tension adjuster bolt  on the front of the cylinder should be screwed in until the central pin is flush with the end of the bolt. Note the two sealing washers.

There is an o-ring in a little cap on top of the valve guide. That is a valve stem seal. It is important to use a viton oil ring on all four of valves. The originals were just neoprene and they cooked and hardened up after a few thousand miles and the engine would put out a lot of white valve guide smoke.

Adequate lubrication to the camshaft relies upon good oil pressure from the bottom end. There are two oil seals in the center section of the engine crankshaft that maintain the oil pressure. If the seals are worn out, cracked, et cetera, there will be inadequate pressure and oil supply to the camshaft. Unfortunately to get to these two seals to replace them you have to take both flywheels off of both sides of the crank. I’ve never done that myself. There is a guy in Tennessee who does this in case you want to send that part of it out. Also in Germany the shop PRÄZISIONSWERK
BÜREN does this repair.

When the cylinder head is bolted to the top of the cylinders and the cam chain connected around the cam sprocket, it is much easier if the rocker arms have not been placed into the cylinder head. This allows the camshaft to rotate freely and easily for placement of the cam chain. Once that has been done, then the rocker arms and their shafts can be placed in the cylinder head. Using a spanner on the crankshaft nut the engine can be turned over so that each cam lobe is at its base circle and that makes insertion of the rocker arms easiest.

The rocker arm shafts use a 3mm x 21mm split pin at the inner end. Each rocker arm pair for each cylinder can be placed in loosely before installing shafts. Press the shafts from the inside of the head outwards, with the short end of the pin facing downward. It is also possible to pull the shafts from the inside out using a 6 mm long bolt and spacers. At the outer end, the shafts are held by a large head 6mm screw with a sealing washer underneath. The seal can be augmented with anaerobic gel gasket compound or heat resistant silicone RTV caulk.  The 6mm screw should not be over tightened because that can break the roll pin out of the shaft at the other end.  Tighten to less than 7 ft pounds and use high temp 246 loctite to secure the screw.

 Rocker arm end play is adjusted by 10 x 19mm shims. End play is  between 0.2 and 0.5 mm. Use the shims to center the rocker cam follower face on the cam lobe and the tappet adjustment screw over the center of the valve stem. It may be necessary to compromise the spacing between the stem and the cam.

There is a small hardened receiver button that goes between the bottom  of the adjustment screw and the valve stem tip. Be careful not to lose these.

Tappet clearance, engine cold is 0.05mm, inlet. 0.10mm, exhaust. (.002, .004 inch).

In addition to the usual bearings and seals, the following parts should be put into the crankcase sides before they are put back together: the slipper type cam chain tensioner to the front side of the chain and its  two little bushings, the shifter for the transmission, and the kick-start shaft that ends with a partial gear.

When putting the transmission with the trapdoor into the crankcase, the transmission should be in first gear and the shifter already in the crankcase should be in first gear.

All the transmission and engine bearings are looset tolerance   C3 clearance bearings. However the two clutch bearings are not C3. Grease all clutch bearings before assembly.

It is recommended to use oil seals made out of viton throughout the engine.

There is  little straight ball bearing on the end of the clutch push rod. This can be replaced with an angular contact bearing. The bearing number is 7000ACM.

There are several very small rubber bushings that fit in recesses in the center crank section of the engine and also in the oil pump. These are easily lost so keep a sharp eye out for them.

There is a thick shim on the end of the shifter drum shaft  where its fits into the transmission trap door (mounting plate). Also a smaller  second  shim on the end of the countershaft (that holds the drive sprocket) where it fits into the trap door. Finally, a third shim underneath (inside) the inner roller bearing race (NJ204 bearing) is placed on the mainshaft (that holds the clutch hub) towards its ending in the engine case. The objective to have all gears aligned once the transmission is installed in the engine case but also have that inner race running adequately and fully in the NJ204 bearing. The parts manual shows this third bearing in differing thickness to achieve that. With the transmission fully mounted on its plate and installed in the engine case, there should be only minimal endplay ( .2 to .5mm) on both shafts, even before the nut retaining each shaft to the sprocket or the clutch is attached.

The big nut on the right hand side of the crankshaft is a left hand thread.

The bolts that hold together the crankcase halves end in a slot head.  It is advisable emachine the cases to use socket head cap screws. This means then that the cap screws can then be torqued to 18 foot pounds.

In general, 10mm bolts torque to 18 foot pounds, 6mm screws to 7 foot pounds.

The engine should be installed and removed from the frame from the right-hand side of the frame. It is necessary to tilt the engine forward and then move it to the side out of the frame. Tilting the engine forward allows the engine mounting bosses to slide under the tabs on the frame where the engine is mounted. A simpler method is to complete the engine and then put it on its side. Then the bare frame can easily be lowered onto the engine, rotating it to clear the engine bosses, and then rotating it back.

The ignition is a wasted spark type. Because the breaker points cam runs at crankshaft speed there’s an extra spark in the exhaust cycle.

The rotating portion of the generator sits on a taper on the end of the crankshaft. To release it from the taper you put a pin (same size as the generator hole diameter) in the generator mounting hole and then screw the generator mounting bolt onto it. This will pop it off the taper.

Ignition point gap is .012 to 0.015 inch
Spark plug gap is. 0.024 inch
Spark plugs NGK B8 ES or B9 ES or B7 ES
Retarded ignition timing: breaker points separate at 0.5mm BTDC or 9 degrees BTDC
Total ignition advance is 40 degrees.

Engine oil capacity 3.0 liters, SAE 30.Do not screw the dipstick in when checking oil.