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I managed to track down this engine from an attic in Slovenia where it had sat for decades. It was common after the war for engines of any vehicle to be removed and installed into farming equipment.

The 350cc two-stroke was known for its simplicity of manufacture and repair, reliability and excellent power-to-weight ratio.

These three factors combined offered some significant advantages over the equivalent four-strokes such as the NSU 351 OS WH. Anyone that has checked tappets will appreciate that it's not ideal doing it when it's -20C with wet hands!. This engine has a case manufactured stamp of 5/(39) (May 1939) with a release number stamped Nov 1944 (1442XXX / 72)on the same case. Most of the parts inside are dated May to Nov 1939.

This was nothing unusual because hundreds of engines where made as spares but then fitted to bikes later on; especially when aluminium started running out and things became critical in the war. In addition the NZ 250 and 350 had interchangeable frames and engines, so the frame number of a NZ 250 for example is never tied specifically to a 250 or 350.

As the engine has been sat for so many years, it's just to risky to install it in the bike and try and run it. It makes sense that whilst it's on the bench to strip it, inspect and rebuild.

Removing the cylinder / barrel

As there are broken cylinder fins, I decided to remove the cylinder, repair the fins and check the piston condition.

The cylinder is cast iron so you cannot weld it with mild steel and a MIG (well you can but it won't be strong). You really need a TIG and nickel rods but if you can't have access to a TIG, use stainless steel wire in a MIG and remember to pre-heat the cylinder in an oven before you weld it!, then alow to cool very slowly in a bucket of fine sand. If you don't the welds will crack when the engine gets hot.

For more information about DKW cylinders see:
NZ 250, NZ 350, IZH Cylinders and differences

When pulling the cylinder off, be careful not to rest the skirt of the piston on the con rod (as I did) and chipped the skirt!. Now I have no option but to change the piston, which as it transpired would have been required anyway because of play in the small end bushing / bearing. Now you have the cylinder off measure the bore with a vernier in a few areas and as deep as you can.

Casting cylinder head covers (Uberstromdeckel)

Okay so you wil no doubt be thinking "is this guy nuts!", but I couldn't find any new cylinder head inspection covers as they always seem to get lost, so the only choice I had was to make my own. This is for geeks after all!!

Probably like 99.9999% of the world I've never cast anything in aluminium before so it was an experience and also a success (well after a few goes and methods). Lost wax seems to give the most accurate copy, but lost foam is much easier and quicker.

I will create a page shortly on casting your own engine parts from aluminium. Fire, molten metal, smoke, heat and success, it's got it all!.

Piston ring inspection

You can see here that they look flush with the piston; so are either worn or stuck down (or both). The way to check is to squeeze each one shut visually inspect the gap. If you can get them to almost touch then remove the top one with your fingers and check the gap when fitted in the cylinder.

For more information about DKW piston rings see:
How to check piston rings

Piston inspection

Check the piston for any damage, cracks, burns or distortions. Once you have done that, clean it up on a gentle buffing wheel and check again. If all looks okay, measure the diameter with a vernier and make a note of the measurement. Measure a few times and take note of the average.

For more information about DKW pistons see:
NZ350 and IZH 49 pistons

Check the piston for play on the small end bushing in two directions. 1) Vertical: hold the piston with both hands and pull it up as high as it will go, feel for any play or clicking when rocking at the top of the stroke. Replace if you can feel even the slightest vertical play. This will make a pinking type noise when the engine is running. 2)Rotational: hold the piston in one hand along the gudgeon pin (wrist pin) like a coke can and twist like you were trying to pour out the coke. A slight play is okay (perhaps 1mm of skirt movement at the bottom when rocking), any more and it needs to come off to replace the bushing.

Small end bushing/bearing inspection

The picture shows the small end of a DKW NZ350 piston connecting rod . You can see the press-fitted phospor bronze bushing. Note the drilled oil holes and the wear inside. On more modern bikes this bush has been replaced with a small roller bearing.

As it happens the replacement piston has a gudgeon pin that is tad smaller than the old one, so this bushing is now of no use. The DKW NZ gudgeon is about 15.09mm OD and the replacement (IZH '49 NOS) is 14.99mm OD . The only option is to press the old one out and make a new bushing from scratch!

For more information about how to make a DKW small end bushing see:
How to remove a piston and make / replace a wrist pin small end bushing.

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Hugh   06/04/2017 at 15:55:58
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 - James   06/04/2017 at 15:57:12
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