Benelli 900 SEI engine rebuild
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Thread: Benelli 900 SEI engine rebuild

  1. #1
    Senior Member R Powell's Avatar
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    Richard Powell
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    Benelli 900 SEI engine rebuild

    Am just taking the engine out of my 900 SEI that I have owned for the last 33 years to give it a much needed rebuild. It has been using lots of oil and smokes quite a lot! As well as the compression on the no 1 cyclinder being 60 psi and the others 120 psi.
    i have purchased a new barrel and 6 Pistons from Germany and intend to have these and many other parts fitted in the engine.
    Benelli 900 SEI engine rebuild-image.jpeg Benelli 900 SEI engine rebuild-image.jpeg Benelli 900 SEI engine rebuild-image.jpeg

    Will take engine out of the frame with the help of 2 friends during the week! And then I intend to take it to Selwyn Motorcycles for rebuilding. This will include various improvements that Selwyn has made to many of his Benellis. I rode Liz's Benelli six on a trip to Germany and told Selwyn that it was the smoothest bike I had ever ridden!
    I hope I get mine to run like that, I will keep you posted on the progress.
    Attached Thumbnails Attached Thumbnails Benelli 900 SEI engine rebuild-image.jpeg   Benelli 900 SEI engine rebuild-image.jpeg   Benelli 900 SEI engine rebuild-image.jpeg   Benelli 900 SEI engine rebuild-image.jpeg   Benelli 900 SEI engine rebuild-image.jpeg  

    Benelli 900 SEI engine rebuild-image.jpeg  
    Last edited by R Powell; 03-20-2016 at 12:46 PM.
    GeorgeE111 and DenH like this.
    The Benelliman
    1981 Benelli Sei 900
    1989 Benelli Sei 900
    1976 Benelli 125 Enduro
    1981 Benelli 250 2c
    1976 Benelli 250 Quattro
    1992 Ducati. 907 ie

  2. #2
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    J Davenport
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    Sounds expensive Richard. Do you know what mods Selwyn does to his bikes to make them run smoother? Good luck with the rebuild.

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    Senior Member R Powell's Avatar
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    Richard Powell
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    The Barrell and pistons are the best part of £1000, some of the improvements Selwyn makes are carburettor rejetting, electronic ignition, making all the conrods the same weight, replacing the drive chain from the crank to the gearbox and smoothing the inlet manifolds. Mine has always sounded like a bag of bolts when it is cold and has always had some hesitancy in the rev range . The bike I rode in Germany had none of those issues and ran well from cold. Am hoping know that the bike will serve me well until I hang up my riding boots!
    The Benelliman
    1981 Benelli Sei 900
    1989 Benelli Sei 900
    1976 Benelli 125 Enduro
    1981 Benelli 250 2c
    1976 Benelli 250 Quattro
    1992 Ducati. 907 ie

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    Quote Originally Posted by R Powell View Post
    replacing the drive chain from the crank to the gearbox
    Hi Richard,
    Will be interesting to read your comments after the engine rebuild is completed and then its performance.I am also interested to know if 900 sei has prime chain tension blade factory fitted as the 750 sei's did not have one and I ask my mechanic to get the tension blade fitted while rebuilding.
    Good luck with all the work Richard.
    Cheers
    Jerzy

  6. #5
    Senior Member R Powell's Avatar
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    Richard Powell
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    Hi just an update on my progress, I am picking up my rebuilt engine from Selwyn motorcycles this week. On close inspection Selwyn found various gears and shafts in the gearbox worn and wear in the clutch. This has all been rectified and the balanced crank has been refitted with the new barrel and Pistons. The cyclinder head has had the valves ground in and all new valve stem oil seals fitted. As I use the bike a lot I thought it was prudent to have any part worn parts in the engine sorted. This bike should now last me till I die!
    will post some photos on refitting the engine and let you know how it runs!
    The Benelliman
    1981 Benelli Sei 900
    1989 Benelli Sei 900
    1976 Benelli 125 Enduro
    1981 Benelli 250 2c
    1976 Benelli 250 Quattro
    1992 Ducati. 907 ie

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    900 sei oversize pistons

    Hello from Greece, new here this is my first post.I own 4 sei's among others.Two 750s and two 900s.i have started the restoration of one of the 900s, the other is a parts bike.
    Is anyone familiar please with the modification of the honda pistons to fit the sei cylinders ? the 61mm diameter will be bored ,the problem is with the height of the piston (taller) which is over the DTC.Except a thicker gasket what else is suggested ?
    Will soon post pics, thanks.
    Last edited by buckley; 07-07-2020 at 06:56 AM.

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    Member Cosmo's Avatar
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    I fitted Yoshimura CB 550 pistons in my Sei decades ago. It wasn't too onerous as I remember, and the same can be done to a 504. I think just the gudgeon pins needed tweaking and a one off copper head gasket. My Sei 820 isn't stroked and runs Honda valves and guides and a Ravasio cam too. If you're building up a larger capacity overbored 900, bigger alternative pistons may have to be machined I suspect, so you may have some research and measuring to do to find some suitable donor parts. I'm sure it's been done by someone on here who isn't a restoration purist.

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    Quote Originally Posted by Cosmo View Post
    I fitted Yoshimura CB 550 pistons in my Sei decades ago. It wasn't too onerous as I remember, and the same can be done to a 504. I think just the gudgeon pins needed tweaking and a one off copper head gasket. My Sei 820 isn't stroked and runs Honda valves and guides and a Ravasio cam too. If you're building up a larger capacity overbored 900, bigger alternative pistons may have to be machined I suspect, so you may have some research and measuring to do to find some suitable donor parts. I'm sure it's been done by someone on here who isn't a restoration purist.
    Thanks my friend, i assume you speak for the 750.In its case its a more straight forward situation with the 550 pistons .750 sei bore X stroke 56X50,6 and honda 550 58,5X 50,6.
    In my case i speak for the sei 900 ( bore and stroke 60X53,4) .The only "suitable" honda piston comes from the 750 sohc (bore and stroke 61X63).There were two types from the cb 750K1-6 (the kit i bought) and from the cb 750 f1 f2 last of the single camshaft .These were even taller.Pins are ok i will overbore to 61 but i must shorten the height of the piston ...i was told that its been done.
    For use to any friend ,its a case study and the only way to recondition an 900 engine. Will post results...i welcome any help.
    PS excuse my poor english ...

  10. #9
    Member Cosmo's Avatar
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    I do recall speaking to someone quite a few years ago who successfully fitted Kawasaki gpz305 pistons to a 900 Sei. I looked up the bore X stroke which is 61 mm × 52.4 - it's also a two valve sohc engine derived from a z250, so he probably used oversized pistons, but you're only going to get similar cc out of those, not a 1000.

  11. #10
    Senior Member R Powell's Avatar
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    Richard Powell
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    The 900 Sei oversize pistons are impossible to get, Kawasaki gpz305 pistons fit with some light machining but are again impossible to find. I bought a standard barrel and 6 pistons from Benelli de in Germany a few years ago so they might be able to help. I also heard someone there may have found alternative pistons. The 900 Sei and Benelli 604 shared the same piston but were made in smaller numbers than the 750 and 500 which can use Honda parts.
    if you do source some pistons let us all know there are lots of owners who may need them in future,
    The Benelliman
    1981 Benelli Sei 900
    1989 Benelli Sei 900
    1976 Benelli 125 Enduro
    1981 Benelli 250 2c
    1976 Benelli 250 Quattro
    1992 Ducati. 907 ie

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