Picture shows Sachs engine as fitted to KR200 viewed from output drive side (exhaust silencer not fitted). Also shown is SIBA black box control system as fitted to all KR200 models up to chassis number 70643, (reported date of introduction 6th April 1959, when it was superseded by the BOSCH control system. Black box systems are interchangeable and a BOSCH engine will work with a SIBA control and vice-versa.

Picture shows Sachs engine as fitted to various scooters viewed from output drive side (exhaust silencer not fitted and the chain wheel is hidden by the extended casing). Note that the SIBA voltage regulator is carried on the engine as is the ignition coil.

To investigate the different types, I took out my stock of barrels in the garden and spent some hours measuring them. The barrel for early Messerschmitts has no K at the inlet and can be recognized by the V-shaped lower edge in the inlet, and also both corners in the top of the inlet have the same shape. The inlet inside the barrel, it is 38mm wide and the slots for the transfer ports in the bottom of the barrel is 26,5mm wide.

The barrel for later Messerschmitts has a K moulded at the left side of the inlet, and the shape of the inlet is a bit different. It is 40mm wide and the bottom is V-shaped and also has a small groove in the bottom. There is an irregular shape in the top of the upper right corner. The width of the slots for the transfer ports is in most cases only 25mm, but I also have some K-barrels where the slot is 34mm! Otherwise I can?t see any difference from other K-barrels.

Finally we have the “go fast”-barrels. They have no K at the inlet and can be recognized either by a moulded screw head at the left side of the base or a spotweld outside the left transfer port. The inlet is 43mm wide and is not V-shaped on the bottom edge. It is almost rectangular except for the irregular shape in the upper right corner. Another difference from all other barrels is the position of the exhaust port. The normal distance between the top and the port is 42mm but those barrels have only 40mm. If you put a finger in the transfer ports (from the base) you can also find that they are slightly bigger than in other barrels. I’m almost sure that these barrels both come from the Swedish Fram-King-Fulda and also the graveyard tractors. The width of the slots for the transfer ports is 34mm and this explains why I often have been confused why the fit of new pistons is so bad! I often have to cut of 5-6mm from the piston to make it fit the transfer ports. It is a good idea to first put the barrel into one of the crankcase halves, and then adjust the alignment of the transfer port. Then do the same with the other half. Finally put the crankshaft and piston in one crankcase half and with the barrel on you can see how much you have to adjust the slots in the piston. As I am keen on originality I wouldn’t call this tuning, it is only to optimise!

Anders Johnsson, Sweden

Supplementary note from David Garner:

I have most of the Sachs archive documents relating to the Sachs 200 engine. These are titled “SMD Mitteilung”. (SMD = Sachs Motor Dienst). That is Sachs Motor Service in English. I will translate them all in the coming months and publish on the site. There was an exchange service to allow existing German owners to change to the lower power piston and barrel. New registration documents were issued certifying the lower output that granted the less than 10HP insurance rate.

This part was originally developed by Messerschmitt Club Deutschland some years ago and includes many improved features not available on the original. These are aimed particularly at improving the reliability and durability of the various moving parts. Only the counterbalance weights are re-used from the original crankshaft, all other parts are new. You must remove the compression plates from your original crankshaft. These are presses metal plates covering the crankshaft counterbalance weights and their purpose is to reduce the air-space in the crankcase to increase the crankcase compression. To remove them, gently prise up the edges that secure the pressings to the counterbalance weights using a blade or small screwdriver. Avoid bending the pressing because it will be difficult to straighten later and may foul the crankcase. There are four types available as described in the following technical details:-

0003 Siba Exchange Crankshaft with Aluminium Connecting rod and Bronze small end bush. ?215 with exchange. (?50 extra with no exchange.)

This is identical in appearance to the original Sachs crankshaft. The Aluminium connecting rod has a much stronger roller bearing assembly fitted to the big end. A new bronze bush is fitted and jig bored to the correct size ready for use. Note that the key for the Dynastart is replaced by a standard type, (original has a rounded profile and is unique to the Sachs engine). The technical details are as follows:-

New precision ground taper shaft for mounting the flywheel with correctly machined keyway.

• New precision ground drive side shaft including harder material for the camshaft.
• Larger big end bearing assembly with lower lubrication requirement
• Modified original conrod (with part No. 1604)
• Precision jig-bored small end bush
• New cam – as original smooth ground finish for long contact breaker life.
• Camshaft end drilled and tapped M4 to facilitate retention of electronic ignition timing flag.

0003a Siba Exchange Crankshaft with Forged Steel Connecting Rod and Roller bearing small end. ?255 with exchange. (?50 extra with no exchange.)

The forged steel connecting rod has a much stronger roller bearing assembly fitted to the big end. A roller bearing small end bush is fitted. This is retained by a plastic tie wrap for transit but is fully floating after assembly. Note that the key for the Dynastart is replaced by a standard type, (original has a rounded profile and is unique to the Sachs engine). The technical details are as follows:-

• New precision ground taper shaft for mounting the flywheel with correctly machined keyway.
• New precision ground drive side shaft including harder material for the camshaft.
• Larger big end bearing assembly with lower lubrication requirement.
• Roller bearing small end (with part No. 0003a)
• New cam as – original smooth ground finish for long contact breaker life.
• Camshaft end drilled and tapped M4 to facilitate retention of electronic ignition timing flag.

0003a uses modern Motorcycle Crankshaft Bearing techniques to allow running at 50:1 two stroke oil mixture. Synthetic oils are safer at these low concentrations and correct mixing with the fuel is vital. With this mixture smoking is almost eliminated except at full throttle.

0004 Bosch Exchange Crankshaft with Aluminium Connecting rod and Bronze small end bush. ?220 with exchange. (?50 extra with no exchange.)

0004a Bosch Exchange Crankshaft with Forged Steel Connecting Rod and Roller bearing small end. ?235.50 with exchange. (?50 extra with no exchange.)

Technical specifiaction as for above Siba types but with 7.5:1 taper for Bosch Dynastart.

Caution, never fit main bearing inners without at least 1mm shim behind. It is almost impossible to remove the bearing as no gap is available to fit the extractor jaws. It is extremely difficult to fit a new small end bush correctly reamed to the required tolerance. Hence, new MOC supplied crankshafts have a jig bored bush already fitted to the correct factory tolerances.

There have been no reported instances of failure since the introduction of these new crankshafts. It should be noted that earlier MOC supplied crankshafts did not include the new cam offered as standard on current stock.

It can be very expensive to neglect the original crankshaft. All are now more than thirty five years old and many have pitted or rusty big end bearings. These can fail with catastrophic results, often destroying the engine. You have been warned!

Comaprison of Steel and Aluminium connecting rod.

The original Sachs connecting rod is fabricated from Aluminium alloy. It is lightweight and has smooth rounded contours to aid gas flow at high engine revolutions. One disadvantage is that, in case of catastrophic failure of the big-end bearing, the bearing housing tends to peel apart and the remaining connecting rod smashes through the gearbox. This often damages the crankcase castings beyond repair. Some Messerschmitters believe the original type is capable of higher revolutions than the alternative steel type described below. The reciprocating weight of the connecting rod and piston are counterbalanced by the crankshaft counter weights. It is not possible to completely counter the forces of the piston and connecting rod at all engine revolutions and the chosen amount of counter-balance is always a compromise.

The new forged steel connecting rod is considerably stronger than the original aluminium type. There have been no reported failures since introduction of this type so we do not know how it behaves when the bearing fails. However, the modern design does tolerate a much lower petrol to oil ratio and the stronger design would reduce the risk of catastrophic failure described above for the aluminium connecting rod. The roller bearing small end is a significant advantage in reducing friction, again contributing to the ability to run weaker oil to petrol ratios. There is a small weight penalty and the steel rod is not as smooth in contour as the aluminium type.

To summarise, each system has different advantages and it must be for the owner too decide, using the above information, which one suits a particular driving style.

Bosch and Siba type crankshafts – Explanation of differences.

Siba crankshafts have two types.

1. Up to 1956, identified by big end bearing without lubrication slots.

2. Post 1956, identified by four lubrication slots visible around the connecting rod main bearing.

Take care not to confuse these with the Motorcycle versions which have a 20 mm diameter main bearing shaft diameter, (I believe some of the 1955 KR200s had this type though I have never seen one). All have 25mm main bearing shaft diameter. The distance between the crankshaft bearing mounting faces is nominally 45mm. The actual dimension, as built at the factory, is etched into the side of the connecting rod. This is intended for use in calculating the number of shims when assembling the engine.

The crankshaft has a 1:10 taper for the dynastart mounting with an unusual semi-circular keyway.

Bosch crankshafts are identified as follows:-

Fitted to KR200s from late 1958 with Bosch dynastarter. The distance between the crankshaft bearing mounting faces is nominally 50mm. The actual dimension, as built at the factory, is etched into the side of the connecting rod. This is intended for use in calculating the number of shims when assembling the engine.

The crankshaft has a 1:7.5 taper for the dynastart mounting with a standard keyway.

While the engine is out, it is a good idea to take the cylinder off to inspect the piston rings, transfer ports and exhaust port. Removal of the exhaust nut normally needs some kind of special tool and the original ‘c’ spanner is the ideal way to get a good grip on the nut. Other methods include a chain wrench as used by pipe fitters and the last resort, a hammer and screwdriver or chisel. The nut is always damaged by the last method and it should be used only in an emergency.

If you are fitting a new silencer, or have dismantled the silencer from the front pipe, the correct procedure is as follows:-

1. While the cylinder is off, carefully clean the exhaust threads with a wire brush. Be sure to remove all debris from the threads. Do the same with the exhaust nut using a sharp spike such as a toothpick to clean out all debris. Lightly coat the exhaust thread with copper-based anti-seize lubricant and do the same to the nut. This will ensure that the nut can be fully tightened and will be easily removed at the next service inspection.
2. Fit the front pipe to the cylinder with a new exhaust gasket in place. Tighten it to the point where the front pipe can still be moved while the pipe is aligned to the silencer.
3. Smear exhaust joint compound both on the pipe and the internal bore of the silencer. Note that this compound hardens within a few minutes so the silencer assembly must completed quickly.
4. Fit the mounting studs to the silencer brackets and tighten the nuts.
5. Fully tighten the exhaust nut.
6. Fit a ‘u’ bolt clamp to the exhaust to silencer joint and tighten fully.
7. Finally, fit the exhaust nut-locking device as shown in the photograph. This is a 42mm ‘u’ bolt clamp. The picture shows a factory original exhaust nut. MOC supplies a much stronger nut with machined slots for the ‘c’ spanner. Bend one leg of the clamp to make contact with the exhaust nut as shown. Fully tighten the clamp.

I have never had a nut come loose using this method. The ‘u’ bolt clamps, copper based anti-seize lubricant, and exhaust joint compound are available from Halfords and many car accessory shops in the U.K. or your local auto parts shop in the U.S.A.