Benelli250 2C/ Guzzi 250 TS ĎAssoí piston numbers
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Thread: Benelli250 2C/ Guzzi 250 TS ĎAssoí piston numbers

  1. #1
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    Benelli250 2C/ Guzzi 250 TS ĎAssoí piston numbers

    Hi everyone, Iím in the process of rebuilding a Benelli and a Guzzi version of these 250 2-stroke twins. Iíve got 8 pistons, all made by Asso who supplied the originals to the factory. My pistons have all come from alloy barrelled versions with chrome bores. They look identical and all the key measurements are the same allowing for very small variations to suit a particular chrome bore. However 4 of the pistons have the number 2090 cast inside and 4 have the number 2002 cast inside. The only difference I can find is that the 2090 pistons all weigh about 138g -141g and the 2002 pistons all weigh between 147g and 150g. Several of the engines I dismantled had one of each number although one of them could be a replacement. I believe, but donít know for certain, that the pistons in the earlier iron barrelled versions may be different but as Iíve said all mine came from standard 56mm alloy barrelled motors. Can anyone shed some light on these numbers? I would be grateful for some definitive information. Many thanks, Fr. Brian.

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    Senior Member R Powell's Avatar
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    Richard Powell
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    Hi I am not sure about the numbers but the pistons have a letter stamped on them as does the chrome barrel and these have to match. As a barrel is made and nickasiled they them Machine it to size if it is out of tolerance they remachine it and fit a slightly larger piston to suit! The letters are stamped on the piston crown and the top of the barrel!
    The Benelliman
    1981 Benelli Sei 900
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    1981 Benelli 250 2c
    1976 Benelli 250 Quattro
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  3. #3
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    Asso pistons

    Hi, thanks for your reply. I know about the chrome bore sizing chart and my barrels are all marked with the appropriate letter code. The chart is in the official manual which I have. The pistons were marked by using red and green points or no point at all. The trouble is that any coloured points have long since been burnt off. However the chart in the manual gives the correct measurements for each grade of piston so with a little bit of care I can determine which piston matches which cylinder bore. As I said in my original post itís the two different piston casting numbers that Iím puzzled about. The only difference I can find is the different weights of the pistons. As luck would have it the two very best pistons that I have are differently numbered and about 9 grams different in weight. So Iím still looking for some definite information about the numbers. Thanks.

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  5. #4
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    Asso pistons.

    Further to my posts above: I rang a friend in Devon today who has restored a couple of these bikes and still has a few bits left over. He checked his spare pistons and told me that the two he has numbered '2002' have hard chrome faced top rings which would mean they are intended for the early iron barrelled type Benelli (1972-1974 approx) Chrome rings were never used in the later alloy hard chromed bore type( late 1974 onwards Benellis and Guzzis) I believe that the Guzzi version was never produced with iron barrels because I have a issue of 'Motociclismo' from November 1974 with a test of the just launched Guzzi 250 TS featuring alloy cylinders, unpainted, and black painted cylinder heads. The other pistons that I have are numbered '2090' and about 8 or 9 grams, on average, lighter than '2002' numbered pistons. I'm coming to the conclusion that pistons numbered 2002 are for the iron barrelled versions hence the earlier number and chrome rings. The later numbered 2090 pistons are for the alloy barrelled versions and are slightly lighter and have plain cast iron rings to suit the chrome bore. To some extent this is my best guess but you can never be sure about Italian production consistency! I found some of the earlier 2002 pistons in my alloy barrelled engines complete with chrome rings which, as you know, is an absolute no-no. Some eejits had obviously fitted earlier pistons and rings without knowing what they were doing! The important thing I think is to use pistons of the same number and check bore clearance and of course never use chrome faced rings in a hard chrome cylinder. I hope this might help someone.

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