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The article gives you instructions on how to make a vintage motorcycle small end bushing; also known as the little end or wrist pin bearing, top end etc. This applies to ANY small end, not just the DKW NZ.

It is not particularly easy and accuracy is everyting, so take your time! Replacements for the DKW NZ's are not usually available and the cheap ones you buy are too small becuase they are for the Russian IZH 350. The IZH version is also the split type.

1) Remove the piston
2) Remove the wrist / gudgeon pin
3) Remove the bushing from the piston small end.
4) Measure and take an average.
5) Make a new one.
6) Press it back in.
7) Ream if you really have too.

The original 1939 - 1942 DKW NZ350 small end bushing dimensions are W 24.93mm X OD 19.35mm X ID 15.09mm

How to remove the piston and wrist / gudgeon pin.

How to press out a wrist / gudgeon pin

You will need to make a special tool from an old plug socket and a peice of 8-10mm threaded rod to do this. Remove the 'C' clips if there are any and line up your extractor tool.

Once you have your extractor on very tight, apply heat up inside the piston. You mau hear it pink and as the wrist pin moves. Be warned these can be in so hard it's almost a joke. I found that if you tighten your extractor, apply alot of heat, let it cool for about 30-60 seconds the pin moves better than any other method. I think that it's because the two materials contract differently. Trust me when I tell you I stripped a few rods getting it out!

How to remove the wrist / gudgeon pin bushing.

How to remove a wrist / gudgeon pin bush.

Again you will need to make a special tool by turning down a peice of aluminium rod to around 5 thou under the bushing size so that it will slide through the piston rod end but also press out the bushing. Use a long bolt or threaded rod to crank it through. If it gets stuck, head the piston rod end a little.

wrist pin bush removed on a DKW NZ 350

Inspect the piston rod for obvious bends and cracks. Now that there is no piston fitted it's a good oportunity to feel for any play in the crank shaft bearing. You will feel that the NZ piston rod has quite a lot of latteral play but zero horizontal. Pull it up and down as the piston would and you MUST feel nothing. If you do, you need a new big end too.

Measuring the small end and wrist pin bushing.

how to measure the wrist pin bushing on a DKW NZ 350

Next you need to measure the internal diameter (ID) of the piston small end which will be the OD of the busing. You can see that I used a digital micrometer for the photo but I actually did mine with a Mitutoyo manual vernier with fine feed as well and there was a difference. Take five measurements and then an average.
My measurements where:
1) Digital 19.30mm
2) Digital 19.29mm
3) Digital 19.30mm
1) Manual Vernier 19.35mm
2) Manual Vernier 19.34mm
3) Manual Vernier 19.35mm

As the bushing will be a press fit, you need to make the busing OD slightly over sized and just how much is a matter of debate. I decided to make two bushes, of and one and two thou over sized to see how they press. You need to consider that the idea is that the bush doesn't rotate in the hole whilst considering that it will expand at a diferent rate to the piston rod. Hmmmmm.

The bush that was made to 1 thou (0.0254mm) over pressed better with no burr, so I would go for that unless you know differently?
So I will be making my DKW NZ350 bush OD 19.347 (avg) + 19.372mm.

Now do the same for the ID which in my case averaged to 15.09mm. When making the busing on a lathe, bear in mind that as you press the bush home into the piston small end, the internal ID may collapse in a fraction. On the DKW NZ 350 the wall thickness is near 2.0mm so it does not collapse much at all. Perhaps 0.01mm on the diameter.

I will be fitting a NOS IZH 350 piston at first over size and wrist pin so my ID will be 14.99mm not 15.09 as per the original.

Making the small end / wrist pin bushing on a lathe.

Making a two-stroke phospor bronze bush on a lathe

I use an EMCO COMPACT 8 Lathe; the original Austrian yellow ones with a power feed. It's a nice sized machine of around 70Kg.
Phospor Bronzes are not all the same, the material comes in different grades. I used a PB104 (UK).

  • PB102: General purpose phosphor bronze for engineering components EU/USA CW451K / C51000
  • PB104: High strength phosphor bronze with excellent wear resistance CW453K / C52400
  • PB1: Concast leaded bronze for medium/heavy load bearings CC481K / C91700
  • For the vintage motorcycyle rider, I would imagine that any would dobut some will not last as long; but you maye be talking the difference between 5,000 miles per grade; who knows?

    Turn down some phospor bronze rod stock to about 1mm oversize, so in this case and OD of around 20.5mm. Drill out the middle to undersize of around 1mm, so in this case 13mm. Swap out the drill for a boring bar and bore to size. Make sure you stop and measure until you get it spot on. My gudgeon OD was 14.99, so I made the ID of the bush 15.02 to allow for a reduction in ID when pressing the bushing into the small end.

    How tight should the wrist pin / gudgeon pin be?


    This is a question that is dabated all over but I couldn't find a straight answer so I did my research and asked as many "old school" engineers as I could find.

    The consensus was that the wrist pin should push in by hand and when tilted on it's side it should not fall out until you gently push it. This gives you a very good summary of how mine feels.

    Note in the picture above where I am parting off the finished bushing, the grove in the middle. I would always cut one of these so if the bush were to rotate in the small end then oil will still find it's way to the wrist pin. Also note the taper at one end for pressing and also note NOT TO REMOVE THE BUSH FROM THE LATE UNTIL YOU PART OFF. PB104-phosphor-bronze-bushing

    Pressing wrist pin bush into the small end

    pressing in small end bush

    You'll be glad that you put a small taper on one end of the bush now as you try and press this in straight. It is vital that it goes in as straight / parallel to the crank case as possible else the gudgeon pin will not be parallel and the piston will rub the cylinder wall.

    Line up the bush and wobble it around until it feels snug. You'd be suprised just how accurate you can be by feel alone. Once lined up you will need a modified version of your bush extraction tool to press it back in. This can be made from aluminium rod and 20 minutes on a lathe. Once your bush is lined up, and your tool inserted you can use your threaded bar as a measure of level. When you insert a long threaded rod through the new bush that is resting in the small end, the piston rod and the threaded bar will for a capital 'T' and you can take measurements and use your eyes from that. If you take your time, you can get it pretty accurate.

    In the photo, you will see some burr on the new bush, this was from the 2 thou oversized one, so I reduced it to 1 though for easier pressing. Note: you cannot put it back in the lathe, you need to make another!.

    Cutting the oil holes in a new bronze bush

    Place a dowl or plastic tube in the bushing to prevent scoring the inside and put a sock in the crank to stop burr falling in!. cutting-oil-holes-in-bushing-1 oil-holes-in a DKW NZ350 bushing cutting-oil-holes reaming-bronze-bush

    Reaming
    In the last picture above you can see that I used an adjustable reamer to deburr and check the size. This is high risk, so don't do it unless you have too. You will NOT be able to ream parallel, so don't even try.

    The finished and insalled NZ350 bushing.

    new bronze bushing installed in a DKW NZ350

    Finally testing the wrist pin fit - perfect!

    wrist-pin-installed on a DKW NZ350

    I've written this because I've never done it before and couldn't find much info on how to do it, just 100's of people asking how. I hope you find it useful and feel free to add any comments or tips.

    Comments
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    Comments:

    Julian   28/02/2019 at 20:49:56
    No problem. Im glad it was of use!
    Reply -------------->

    alan   28/02/2019 at 20:29:03
    Thanks a lot. This was exactly what I was looking for.
    Reply -------------->
     - Julian   28/02/2019 at 20:49:10
    No problem. Im glad it was of use!

    Derek   05/02/2019 at 04:31:22
    Than you I just read this well written and concise.
    Reply -------------->

    Bob Dunford   17/09/2018 at 11:56:08
    I like this. Hope you will do more of these how to tips.
    Reply -------------->

    Jake   08/08/2018 at 20:56:26
    Please help me find a new split cage needle roller bearing for 250cc Zundapp.
    Reply -------------->

    Ed   01/04/2016 at 15:20:20
    Dude did this last?
    Reply -------------->
     - julian   13/04/2016 at 19:18:51
    Yep, all okay so far.....engine sounds good but ive not done many miles yet.



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