Crankshafts

Exchange Crankshaft Part No. 0003-Siba 0004-Bosch

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.)

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.