16 HP Adler motorcycle pistons & 18 HP Adler pistons

The Adler engine is a 250cc air cooled twin cylinder two stroke with nominal bore and stroke of 54mm,
and compression ratio of 5.75:1. It was made in Germany from 1951 to 1958.
The MB250 motor made from 1954 to 1958 was very similar to the earlier M250 motor made from 1951 to 1954.


The Workshop Handbook says:
Cylinder --------------------- 54 H 7 +0.015 -0.00mm
Piston ----------------------- 53.96 +0.01 -0.00mm
Play at lower piston edge ---- 0.030 to 0.060mm
Gudgeon pin ---------------- 15.00 +0 -0.0025mm
(all dimensions in mm)

The 16HP pistons are unusual in that they have a full circumference skirt with rectangular transfer port windows offset to the front of the piston.
The overall length from piston lip to skirt is 67mm.
The standard piston crown is lightly domed by about 2mm and is stamped "53.96".
The wrist pin has a diameter of 15.00mm and is retained by wire circlips.
The wrist pin is offset to the rear of the piston by approx. 1.5mm.
  • For more info on piston pin offset, Click Here, and Here

    • The width inside the piston for the con-rod wrist pin bearing is 21mm.
    The distance from centre line of the wrist pin to piston lip is 30mm.
    The 16 HP MB250 piston weighs 170 grams, and was typically branded inside the skirt: NURAL 1761.
    The 16 HP Favorit piston weighs 150 grams, and was branded: MAHLE 54121.

    Adler pistons Adler pistons Adler pistons ADLER MB250
    Click on an image to enlarge it.

    In each picture, the Favorit piston is on the left, MB250 on the right
    They have 3 rings.
    The top ring pin is in the centre of the bridged inlet port and the other 2 pins are equally spaced around the piston
    at 130 degrees, with an angle between them across the front of the piston of 100 degrees.
    Each ring on the 16 HP MB250 piston is 2.4mm in height.
    Each ring on the 16 HP Favorit piston is 3.1mm in height.
    The body of the MB250 piston is thicker and the wrist pin bearings have no strengthening webs.
    The body of the Favorit piston is thinner and the wrist pin bearings have curved strengthening webs.


    The MB250S (sport) and the MBS250 (Sprinter) had a 2 ring piston. Effectively the top ring is eliminated which means the inlet port bridge is not required.
    The inlet port on the 18HP engine therefore is a single opening. The exhaust port on the 18HP engine is 36mm from TDC whereas the 16HP engine was 38mm.
    The 18 HP pistons had no transfer port windows - just traditional full sized cutouts. The 18 HP piston weighs 145 grams, and was branded: MAHLE 548xx.
    Notice the inlet port in the barrels (one oval hole).

    18HP piston and barrels 18HP piston

    Motorrad Stemler of Germany have replacement piston kits priced at around 85 Euro each. Their part number is 00-00-0-001-456.
    They are available in oversize steps of 0.5mm from 54.0 to 56.0mm so the first oversize is 54.5mm.
    They appear to be 4 stroke Mahle pistons into which someone ("HGM") has machined transfer port windows and fitted ring pins.
    They are in fact low-silicon alloy Italian pistons and because of greater rate of expansion, require the barrels to be bored bigger.
    The original Adler clearance was 0.03 to 0.06mm - these Italian pistons require 0.08 to 0.09mm clearance.
  • Click here for the Stemler note that accompanies the pistons
    Because of this, these pistons may be a good fit for worn barrels.
    Be prepared for a rattley motor until hot.
    See www.motorrad-stemler.de

    The Adler Museum of Germany also have replacement piston kits priced at around 92 Euro. Their part number is MB250-54.

    The only other pistons that offer the critical characteristic of correct skirt length are early (1960-63) Yamaha YD3 pistons. Keep an eye on eBay for these.

    Also available from Stemler as part number 00-00-0-003-304 at 276 Euro a pair, are better quality Adler pistons. They are possibly genuine old stock.

    5 pistons 5 pistons
    Here are some of the alternatives - Click on an image to enlarge it

    The nearest common and modern piston is from a 1960-63 Yamaha YD3.
    They are high quality aluminium silicon alloy and therefore have a low co-efficient of expansion.
    Several bikes in Australia are using them successfully.
    This pistons dimensions are the same as the Adler.
    Piston kits are available from Yamaha sources or eBay.
    Standard piston part number is 148-11631-00-96, 1st O/S(+0.25) = 148-11631-10-96 etc
    Standard rings (2 of) = 148-11610-00-00, 1st O/S(+0.25) = 148-11610-10-00,
    2nd O/S(+0.5) = 148-11610-20-00, 3rd = 148-11610-30-00, 4th = 148-11610-40-00.
    Gudgeon pin is part No 148-11633-00-00.
    They are reasonably priced and available in four x 10 thou (or 0.25mm) oversize steps.

    US YD3 piston repro YD3 pistons YD3 rings YAMAHA YD3
    Click on an image to enlarge it.

    An Italian source that the German Club members use is PistoniGol
    They do a 2 ring piston in 4 oversizes for E77.50 each.
    Their Adler piston is 70mm high and would need shortening and scalloping to clear the crankshaft.
    Since it has 2 rings high up on the piston, it would be suitable for use in the 18HP engines also.
    GOL pistons Italian GOL piston

    A German Company called Wahl "makes" a Mahle piston with an L section top ring and with a (very) small top ring land.
    It is quite expensive at E220 ea. Michael Liebig has a pair.
    wahl pistons Wahl Mahle pistons          Wahl invoice Wahl Invoice

    Another German Company called OBS "makes" pistons. These are likely cheap Italian pistons. Michael Liebig has a pair and he says they rattle until warm.
    These are quite likely the same pistons that Stemler supply.
    OBS advert OBS details OBS pistons OBS pistons
            

    The next nearest common and modern piston is from a Suzuki GT250.
    Several bikes in Australia are using them successfully.
    This piston is 5mm short in the inlet skirt - so the inlet port timing will be longer than original.
    This means that the motor revs very well but has a flat spot on the torque curve at about 3500 RPM
    It has only 2 keystone rings (letter T on the ring to the top).
    The ring pins are separated by about 80 degrees and this puts the ring tips nicely between ports.
    The gudgeon pin is 14mm not 15mm in diameter so the little end bushes must be replaced.
    Piston kits are available from BB Importers in Perth.(sales@bbimporters.com.au)
    They are reasonably priced and available in 10 thou or 0.25mm oversize steps.

    GT250 piston SUZUKI GT250
    Click on an image to enlarge it.
  • For more info Click Here
    • Another promising alternative is to use the pistons from a Kawasaki KH125 models K7 through K10 from 1992 to 1998.
    The problems are: the Kawasaki piston diameter is 55mm not 54, the inlet skirt is short by 4mm, and the ring pins at 60 degrees apart are in the wrong place, but it is a fairly easy job to relocate them. Good news is that the little end pin is 15.000mm. These pistons can be obtained from WeMoto UK by ordering over the Internet priced at around 24 Pounds Sterling or 32 Pounds each with postage. An interesting possibility is to use the KH125 con rod kit also which would result in needle roller bearings at top and bottom. The centre to centre length of the rod and the crank pin diameter are not known definitely. Con rod kits are available from WeMoto also at www.wemoto.com priced at around 43 Pounds Sterling plus postage.

    KH125 rod KH125 piston KAWASAKI KH125
    Click on an image to enlarge it.