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Thread: The Sensible Twin

  1. #1
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    The Sensible Twin

    Subtitle should probably be "The bucket hacksaw"...

    My version of what Speedpro has done - but unsupercharged...

    Currently on final assembly. As Speedpro's version, the RHS of an FZR250 four - but this one is resleeved and bored to 53mm to use GSXR400 pistons.
    Which coincidentally use the same dia gudgeon pins as the FZR. They're roughly 4mm higher compression height so a barrel spacer is required. As is a camchain 2 links longer. Straight out of the DID catalogue, from memory a GSXR400 one again.
    I'm using a Kelford profile on the inlet cam, std exhaust.
    Wallace bored the block to suit the liners I machined up, then finish bored them after I'd installed them.

    Pic is cases ready to go together. Oil galleries to the unused mains tapped and grubscrews inserted to block them off. Crank cut, oil holes radiused and then it was linished. 3LN long rods used. I'd have preferred to use the early short rods but out of the roughly 1 1/2 broken motors I got from Husa, there was only one short rod...The mind boggles at the mixture that may have been used. Pistons are currently 30g heavier than the originals so I may yet have a cutting session.

    More to come as it gets done.
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  2. #2
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    25th March 2004 - 17:22
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    So what sort of frame will that go into? FZR seems the easy choice, if a little fat & heavy.
    I've been told. Dreaming`s free.
    Think I'll go, back to sleep.
    Everybody listen, voices in my head
    Everybody listen, do yours say, what mine says?

  3. #3
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    Quote Originally Posted by F5 Dave View Post
    So what sort of frame will that go into? FZR seems the easy choice, if a little fat & heavy.
    I really haven't decided yet. The easy choice, not having an FZR frame handy, is the CBR250RR bare frame i have laying here. Given the motor has one wide shoulder, anything narrow will have a carb going through a frame rail.
    The alternative of an assymetric trellis, right down to a single sided arm with the shock on the left where there are no cylinders, has appeal.
    As i remarked to husa, where did i put the pic of the prototype White Power single sided fork...Assymetry gone mad, LOL.

    Bear in mind too that i'm not looking at it as a Kart track bike. It looks like we're on biggerer tracks here for the forseeable future.

  4. #4
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    Cool. Can we see how you blank off the cut end of the cylinder & head, and are you using the standard rotor?

  5. #5
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    Linear bearing thingy, yeah I remember reading about those, maybe in ACMN.
    I've been told. Dreaming`s free.
    Think I'll go, back to sleep.
    Everybody listen, voices in my head
    Everybody listen, do yours say, what mine says?

  6. #6
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    Quote Originally Posted by speedpro View Post
    Cool. Can we see how you blank off the cut end of the cylinder & head, and are you using the standard rotor?
    Yes, you will, I took some lessons from you, lol....Then did it differently.

    At present a cut std rotor will be used. Ign will be total loss as i have no accessories to power. I first saw a cut alternator rotor on Sidecar Bob's 600 engines for Pinky. If it's good enough for the IOM, etc...basically, the swaged edge is turned off and the magnets removed, then the outer edge is cut back to the edge of the raised triggering lumps. So it's just used as a carrier for the trigger pieces. In the past i have made up alloy rotors with a steel trigger shoe screwed to them - adjustably mounted on the OE boss. This sucker has multiple triggers which i don't feel like duplicating. Any adjustment will have to be by offset keys.

  7. #7
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    Quote Originally Posted by F5 Dave View Post
    Linear bearing thingy, yeah I remember reading about those, maybe in ACMN.
    Very cryptic Dave - but I assume you're referring to the single sided fork, yes AMCN rings a bell.

  8. #8
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    Next installment. Cases together, barrel on. Used rings and pistons but the rings still had the ID marks discernable so not too bad.
    Didn't lighten the pistons - lets see how bad it shakes first. The sides of the domes had to be trimmed slightly to fit the chambers.
    A std base gasket, a 4mm alloy spacer and a .020in copper head gasket made by anonymous which came with the bits...then .010in off the barrel as a cleanup cut gave a measured .032in squish. That'll do.
    The valve cutaways only needed the ex side made closer together. A shaky hand and the die grinder did that.
    With the cams dialled in there's plenty of valve to piston clearance. I could have cut the head to get more compression but it measures up at 11.8 at present, again, that'll do.
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  9. #9
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    Quote Originally Posted by Grumph View Post
    I really haven't decided yet. The easy choice, not having an FZR frame handy, is the CBR250RR bare frame i have laying here. Given the motor has one wide shoulder, anything narrow will have a carb going through a frame rail.
    The alternative of an assymetric trellis, right down to a single sided arm with the shock on the left where there are no cylinders, has appeal.
    As i remarked to husa, where did i put the pic of the prototype White Power single sided fork...Assymetry gone mad, LOL.

    Bear in mind too that i'm not looking at it as a Kart track bike. It looks like we're on biggerer tracks here for the forseeable future.
    VFR400NC30 rolling chassis....? Swingarm on the left.....
    6 grand for 2 grands worth of fun. - F5 Dave

  10. #10
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    Quote Originally Posted by HenryDorsetCase View Post
    VFR400NC30 rolling chassis....? Swingarm on the left.....
    Too narrow - the V4 is quite a skinny thing at head level, albeit heavy.

    Other options may be opening up.

  11. #11
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    A bit more progress. Head on, cams in on marks that I had to re-establish. They are of course swapped end for end and slots cut for cam location at the outer end. Machining there done by Wallace again, at least he's now got a cutter for that job...
    Inlet is a Kelford grind, cut into the base circle, no welded buildup. About .020in more lift and around 20 degrees more duration at running clearances.
    Springs are still a long way from coilbind, still about .25in clearance retainers to seals. Biggest shim needed was a 265.
    This would be a good hopup for a std FZR250. If it seems exhaust restricted, I'll get the exhaust cam done too. There's room in the valve cutaways.
    They're in on 103/104 lobe centers which I know works on bigger FZR's.


    Sorry Speedpro, I'm not teasing you, I'll get pics of the other end.
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  12. #12
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    Fill the inlet port bottoms if they are like other period Yamahas.
    I've been told. Dreaming`s free.
    Think I'll go, back to sleep.
    Everybody listen, voices in my head
    Everybody listen, do yours say, what mine says?

  13. #13
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    Quote Originally Posted by F5 Dave View Post
    Fill the inlet port bottoms if they are like other period Yamahas.
    Not needed for once IMO. Surprisingly good. Just some short side radius work needed.

    I know what you mean though, the FZR600 Kev had was beautifully done - and we think by someone in Wellington.

  14. #14
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    Will be interesting to see what type of rpm it can actually achieve safety (20tho twin = awesome)

  15. #15
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    Quote Originally Posted by TALLIS View Post
    Will be interesting to see what type of rpm it can actually achieve safety (20tho twin = awesome)
    Define safely, lol...Pistons are 30g heavier which is pretty well 20%....Yes the rods are nuggety, particularly compared to say ZX400, but...

    I'm thinking around 15 will do for a start. I see the stock power curve actually peaks around there so it's just passing air above that.

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