In the operating instructions there is the beautiful bold heading "No fear of driving in" and I can only confirm that there is no reason at all to be afraid of running-in, nevertheless it can't hurt to make use of a few tricks in the process.
The new DKWs, so under all circumstances the RT 175, the RT 250/2 and presumably also from October still delivered RT 200s (for the latter I don't know for sure), are much easier to handle during running-in than the pieces delivered before October 1953. The reason for this is quite simple, that from this date on the mentioned engines no longer have the old, somewhat weak and only 15 mm thick piston pins, they have now received 18 mm thick ones! (Conversion of existing engines with 15 mm piston pins is only possible when the crankshaft is replaced, because a new connecting rod has to be installed!)
In addition, with this transition to a stronger piston pin, Auto Union has departed from an age-old belief: They no longer install the piston pins when the piston is warm and no longer with a mandrel and hammer as in the past, they now finally install the piston pins in a floating manner. So now you can press a piston pin into the room-warm piston with your fingers with moderate force, in extreme cases maybe not with oil but with tallow lubrication of the piston pin.
This floating bearing of the piston pin is primarily responsible for the fact that from October onwards there will be almost no more sticking during running-in on the new DKWs – this includes especially unfavorable conditions such as lack of fuel or totally wrong ignition settings. In the case of the previously manufactured DKWs with the tight piston pin seat, the piston pin seat is responsible for 99% of all sticking with an otherwise flawless engine. This is true in all those cases where, when removing the cylinder after a jam, you find the jamming points near the piston pin bosses. I have previously tried to deal with such clamps in the way I have always used with success by filing the clamp, but this has never really succeeded. After filing down 3 or 4 more jams, I used a very rough method from the early days of two-stroke reaming: I used a quick-setting reamer to carefully remove the piston pin bosses, i.E. Only a very fine shaving, until the oiled piston pin could be pushed through with finger pressure. This procedure is the horror of all piston makers, who of course take great pains to provide the piston pin bosses with a conceivably high surface quality by means of the fine boring process – this surface quality is made illusory by reaming. Nevertheless, by reaming, the piston can be made jam-free, so you have at least this advantage. This iron beard treatment with reaming out of the pin holes in the piston only applies to older engines in which the fits were actually too tight. However, if you have had the piston ground out and are supplied with a new piston, the first work to be done is to check the fit of the pin in the piston – it is not known whether word of the floating end installation has already reached the cylinder grinding shop!
If the pin in the piston works properly, the next important thing to check is the pin seat in the connecting rod eye: in new engines, the piston pin seat in the connecting rod eye is not a problem, but in freshly ground engines that have passed through the hands of any workshop, it is advisable to be suspicious: A sure instinct is not everyone's cup of tea, least of all that of a rural journeyman locksmith. In the connecting rod eye, the piston pin should be oiled and it should be possible to push it through with very light finger pressure, it should not wobble.
(I only write about the non-wobbling because I don't dare to write that the piston pin may wobble a little bit if necessary. I have had several DKWs with a piston pin wobbling violently in the connecting rod liner, they did not tick or otherwise make noise and one of them I was able to follow for a whole 20,000 km of running time without the air present increasing. But this is a single experience and I do not want to say that it is true in all cases.)
I am of the opinion that a tiny bit too much play of the piston pin in the connecting rod bushing does no harm, but it is indisputable that too little play will cause the piston pin to jam and then the piston will also jam. If you have had some serious jams, remove the piston under all circumstances and check the fit of the pin: the pin will inevitably show blue discoloration in the places where it was jammed in the connecting rod sleeve – so you will ream if necessary and then the jamming will also stop. (By the way, there are special clamping devices at DKW, with which even an apprentice can ream out a connecting rod liner very precisely.) In the case of ground-out engines or engines that have been in the hands of a village workshop at some time or other, mistrust of the connecting rod is also advisable from the outset: if the connecting rod has been bent for some reason, then of course the piston can never run smoothly and without jamming. When reassembling the stripped engine or when disassembling it after the first jamming, check for bent connecting rod by the well-known procedure:
Fig. 31: When the head is removed for the first time, the connecting rod is immediately checked for any distortion. When the piston is pressed against the cylinder wall in the direction of the piston pin, it must stick in both positions and must not spring back.
Fig. 32: The work of fitting the cylinder can be made much easier by immediately sawing a fork board onto which the piston can be placed on the connecting rod.
Fig. 33: The fork board makes it easier to put the cylinder back on; it is simply put on and the piston ring is pressed into the groove all around with a punch or a screwdriver; the cylinder then slides over the piston all by itself.
Fig. 34: Typical pinch point close to the pin boss, as is usually only caused by too tight a fit of the piston pin in the piston.
Picture 35: The same clamping point, already filed over with a new, sharp triangular file, the piston can be installed again.
Fig. 36: Once the slots in the cylinder have been freed from oil carbon, the edges of the slots are carefully deburred from the inside of the cylinder with the triangular scraper.
The cylinder is slipped onto the piston and pressed down onto the crankcase. The starter is kicked a few times so that the piston can pull the cylinder exactly into the position that it will take later during operation. Unfortunately, it is not possible to fit the cylinder head at this stage, so four tubes about 30 mm high have to be cut to fit over the tie rods and used to tighten the cylinder to the crankcase by means of the head nuts. These four nuts are of course only tightened after the engine has been run through several times as mentioned above. The piston comes now approximately in the middle of the stroke height, one drips oil into the angle between piston pin and cylinder wall all around and turns the piston rather exactly into the upper dead center. The pressed up oil is wiped clean and now the piston is pushed in the direction of the piston pin with force against the cylinder side. Oil swells out, but the piston must remain stuck, the oil must not sink back any more or only very minimally. After that, press the piston again with the thumb in the opposite direction, now oil must come up again and must not sink back or only very insignificantly after the thumb pressure has ceased. As soon as the piston springs back from one side of the cylinder, it means that the connecting rod is bent to the opposite side. So it must be carefully straightened with two Englishmen or better two turning irons. Whether the news was enough, or. Whether you have adjusted too much can be checked cheaply and quickly with this thumb pressure method. One should not let this work of the control after heavy clamps and all the more with each assembly after a dismantling of the engine be annoyed, it saves one mostly much more annoying later clamps. With new engines these conditions are of course all given, I describe this only so in detail, because it comes into question for used and also by any people subsequently ground out and reassembled engines.
If the engine is new or has been rebuilt according to the above, there will be no difficulty in running it in, so you can proceed in the following schematic way:
The new engine, which may not have made a sound at all, is warmed up for half an hour by simply driving it. You won't be able to rev up to top speed in the gears, but you can always go up to 20 km/h in the first, 40 in the second, 55 in the third and 80 in the fourth. Solo transmission presupposed. (These figures are higher than those given in the operating instructions, but they have proven themselves to me.) It's no problem at all to stay in the gears for a very long time on long uphills, as long as you don't need full throttle for that. (A line with the grease pencil on the throttle grip and the handlebar tube is much more practical than it looks, at least you can always see how much throttle you have. By the way, you do not need to fear full throttle even with a brand new engine, you can leave it once 10-15 seconds when accelerating or overtaking a dawdling front man quietly.)
After a thorough warm-up, one accelerates from 2/3 of the maximum speed on as even a road as possible and without headwinds as possible to full throttle once on the. The engine must answer this with a clear acceleration, paying close attention to the speedometer. The RT 125 must accelerate from 55 km/h up to 70 km/h in 10 seconds, the RT 200 must accelerate up from 60 km/h to 75 km/h in 7 seconds, the two RT 250 must accelerate from 65 km/h to 90 km/h in 13 to 14 seconds at the most. If these acceleration times are not achieved at the first attempt, the only thing that helps is to run the engine for another half hour in the described manner with 3/4 throttle. Afterwards, however, one will achieve these acceleration times without further ado.
In this way, the engine has indicated that there is no more unprogrammed resistance in the power unit, it "goes free". At this point we can choose a free route, and even if the operating instructions warn against the highway, I am personally in favor of the highway, because there you don't have to pay so much attention to other people and you can concentrate on your engine:
You pull up to full throttle from about 2/3 of the maximum speed, count off your 10-15 seconds within which you have to get close to the maximum speed, and beyond this number of seconds, for better or worse, you leave the engine at full throttle once – that is, if necessary, after no more than an hour of total operating time. Of course, we pay close attention to the speedometer, which must not drop. Likewise, turn your head a little so that the wind noise in your ears stops and you can hear the dark sound of the engine. One has the famous "cautious" "turns" have two fingers ready on the clutch lever.
If after about 30 seconds of full throttle the dark sound of the engine changes and becomes brighter, then it is high time to take back the throttle and let the engine roll on for a few minutes with half throttle. If the sound of the engine does not change and the engine does not tire, leave it at full throttle for about one minute, and only after this minute return to half throttle and let the engine catch its breath. If everything went smoothly, you give full throttle again after a few minutes, and after 5-6 such attempts, you can try to leave full throttle for several minutes. The majority of all engines will behave according to this program, they will be fully throttle-proof after only one hour of operation, so what is written here refers to a few examples, as they occur in every series production.
If the engine gets tired or changes its sound after one minute of full throttle, then it is said that it wants to jam soon. Of course, one of the causes described at the beginning, such as a bad piston pin fit or a crooked connecting rod, can be to blame, so these possibilities should be eliminated by checking beforehand. Another possible cause for unprogrammed sticking is a too low spark plug. Furthermore, the carburetor must be kept perfectly clean and the fuel tap and fuel supply must be carefully cleaned, so that partial fuel starvation is not the cause of jamming. Likewise, make sure that the ignition setting is correct, although it should be noted that this is inevitably the case with new machines, but that it is suspect once the machine has been in the hands of a workshop.
(If a workshop is angry with me because of this repeatedly expressed mistrust, I am gladly ready to admit that I do not mean this workshop, of course, but only the competition. With this competition I have however even experience!)
The exact pre-ignition values can be found in the technical tables, the procedure for the ignition control is described in detail in the electrical chapter. If a piston tries to stick four or five times in a row after running at full throttle for about 2-4 minutes each time, then it doesn't matter, you go home and let the engine run for 2-3,000 km in everyday use. Here you can flay it without any problems, you can let it jubilate in the gears for half a minute, occasionally a whole minute with full throttle, you can also leave it at full throttle for 1-2 minutes, just not full throttle all the time. After 2-3,000 km of such operational driving, one makes renewed attempts at full throttle on the highway. You will then find that the engine will now tolerate full throttle for many minutes until it tries to jam, most likely it will no longer jam at all, and if it still jams, then it may still be that in the meantime the carburetor or the fuel supply have become dirty again, so clean it.
With solo engines you probably won't have any trouble at all, but with sidecars the engines may still tend to seize occasionally, but only after at least 10 minutes of uninterrupted full throttle running with occasional over-revving. In such cases, it is advisable to use the next higher main jet, which is given in parentheses in the tables. This is a pure hitch setting, which does not increase fuel consumption at all and has no influence on performance and spark plug pattern. See also chapter Carburetor.
Basically, one should be warned against the procedure of the so-called running-in nozzle: This is an over-the-thumb recipe, which, however, is still followed by many workshops to this day. The next larger main jet is simply used compared to the normal setting. In my experience, the DKWs with the normal factory setting are easily jam-free, only to the heavy team operation, so only to pronounced hunting in the team is justified a number higher nozzle compared to the normal setting.
By the way, we only need to think of two possibilities that can still cause a two-stroke engine to jam, even if it has 30,000 km on the clock and has long since been ready to be worn out: 1. If you have an engine on a long uphill slope possibly in the gears highly slammed and very hot driven, if you have then on the other side downhill a beautiful full throttle straight and there actually with full throttle and possibly overspeed, thus more than otherwise usual top speed behind goes, then the famous careful finger on the clutch is recommended under all circumstances – under these circumstances also the best engine can once jam.
2. But by no means does clamping always involve full throttle. Just when you think you are cruising along comfortably at half throttle and about 2/3 of the maximum speed, it can happen that you have a tailwind, the presence of which you do not notice at all. But this little tailwind can knock over any cooling system. The engine can suddenly seize up with a loud screeching noise. These are things you have to know, they can happen with any four-stroke engine, no motorcycle engine is immune to them.