dkw nz350 cylinder/piston size

Basic info on DKW NZ 350's

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andyy1
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dkw nz350 cylinder/piston size

Post by andyy1 » Fri Jan 20, 2017 7:05 pm

hi all, what size piston should i use with a cylinder that has a bore size of 72.86. would a 72.5 be suitable ?
Frank
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Re: dkw nz350 cylinder/piston size

Post by Frank » Sat Jan 21, 2017 12:17 am

Hi,

A 72,5 mm piston is too small for your bore.

On original war time German pistons the clearance was 0,17mm, but with other more modern pistons the clearance are smaller.

Normally the cylinders are bored to a standard measure (72mm, 72,5mm, 73mm, 73,5mm and in some cases up to 74mm) and the clearance comes from that the piston is smaller than the cylinder bore.
How much smaller the piston should be (the clearance) than the cylinder, is given by the piston manufacturer (depending of the material etc. used in the piston) and the piston is already machined to that tolerance.

If you take a standard cylinder (72mm) then the original war time piston probably had a diameter of 71,83mm.

Probably your cylinder have had a 72,5mm bore and is now worn to 72,86mm (you will not find a suitable piston to that dimension), so the correct way would be to have the barrel bored to 73mm and find a new piston (73mm) for it.

The best way is to buy a new good quality piston (forget those cheap pistons for 30-40€) and take the new piston and the cylinder to a machine shop, where the guy measures your new piston (which normally have the clearance stamped on the doom of the piston) and the grinds your cylinder to the the correct diameter.

//Frank
andyy1
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Re: dkw nz350 cylinder/piston size

Post by andyy1 » Sat Jan 21, 2017 7:25 pm

hi frank. thanks for the reply. thats really helpfull. i will do as you suggest. i know people use izh pistons in there engines. would the gudgen pin be the same diameter for the small end ? regards, andy
Frank
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Location: Finland

Re: dkw nz350 cylinder/piston size

Post by Frank » Sat Jan 21, 2017 9:16 pm

Hi,

The gudgeon pin is same on the old IZH models. Pistons from the old IZH are possible to use in DKW.

I have no experience from IZH pistons myself, but according to old bikers here in Finland who have a lot of experience from IZH, tells that if you find old original Russian NOS pistons from 50`s or 60`s they are quite ok in quality and useful, but avoid those new pistons sold as IZH, on ebay and in Poland for some 30-40€, they are just crap.
Not worth to destroy your cylinder and engine with that s**t.

I have one of these new pistons (from a well known seller in Poland) on the shelf and I would only use it as an ashtray :evil:

If you plan to put your bike in a museum, or on display, then you can use those cheap pistons, but if you plan to use your bike, then go for a quality piston from a German manufacturer!

Here is a good advice from Thomas: viewtopic.php?f=1&t=2010

Have a nice weekend!

//Frank
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