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Making a large cc thumper


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I've been thinking, if the Buell Blast is essentially half of a Buell XB9S V-Twin, how hard would it be to take a CVO Fat Bob 110 cubic inch engine and make it a single? 55 cube thumper (900cc) would surely have some amazing torque!

I'm thinking it should have something like 50hp/50tq all under 6,000 rpm. ­čśĆ

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im not sure that you get a 55 into a buell blast. but... if you could it would be amazing basically what a 750 gsxr is, stuff a 750 into a 650 frame. lighter with more power.

I'd be fine if it doesn't fit into a blast frame. Throw it into a Sportster frame or even a custom one. I'm more interested in if it is basically as easy and taking the rear jug off to make a 55" single out of a 110" v-twin.

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I guess it *could* be that easy. Fab spacers for the "knife" con-rod since it might be lighter than the "fork", a single intake manifold, cap off the unused hole and tappet block port. You'll need a single fire coil since the dual fire would need the circuit complete between the two plug leads. I suppose it would look best if the front cylinder were used.

I'm thinking in terms of EVOs here...........may or may not be the same for a Twinkie.

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I guess it *could* be that easy. Fab spacers for the "knife" con-rod since it might be lighter than the "fork", a single intake manifold, cap off the unused hole and tappet block port. You'll need a single fire coil since the dual fire would need the circuit complete between the two plug leads. I suppose it would look best if the front cylinder were used.

I'm thinking in terms of EVOs here...........may or may not be the same for a Twinkie.

I'm thinking I could use a Blast coil or similar. Also, it'd be really awesome to use a FI engine as the base. Should be able to use a piece of software to shutoff the rear cylinder and make it work. A guy used Buell FI stuff to inject his blast that way. He used ECUSpy or something like that.

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the clutch and transmission would have an easy time with half the cubes. would there be ballancing alterations needed, is there a ballancing shaft in that motor?

That's what I'm not sure of. I don't think there would be balancing issues because it is a single pin crank (I'm pretty sure anyway) with a 45 degree v-twin. Which basically means that half the cylinders should be half the vibration. The two cylinders don't cancel each other's vibes out like on a parallel twin or I4.

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If you're talking about an Evo big bore (or standard Evos too), There is no balancer. One would think that there should be less vibration w/ one less piston and rod on the rotating assembly.

W/ Twinkies, the "A" engines didn't have balancers; these are the ones in the rubber-mount applications such as the Dynas and touring bikes. The "B" engines do have balancers; those are the ones you'll find in the Softails.

My assertions about the Twinkies may or may not be true anymore, but they were through the last decade.

It might be less hassle/expense/superfluous architecture to use an Evo big bore; you'll just have one hole, tappet block port and half the cam you're not using. The Twinkies have a lot more monkey motion you'd theoretically be paying for and not using.

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I guess one could make the argument for using the Twinkie and just leaving the rear cam and secondary cam chain out, but I don't know if there other complications w/ that. It may be that simple though.

I sorta wonder how smooth a "B" motor might be w/ a lighter rotating assy, but I'm not familiar w/ how much they shake as designed either. I've only got experience w/ "A" motor Twinkies.

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Yeah, if I'm going to actually do it, an EVO is out because the biggest I could get it would be about 750cc. So a Twin Cam/twinkie is where it's at. Good info on the A and B motors though. I'm not very well versed in Harley engines yet, so any help is appreciated.

I won't what I could jam a Twin Cam single into? Also, I suppose it would no longer be a "Twin Cam" eh? I'd be making it a "Single Cam Single." Maybe I could call it an "SCS" or something.

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Hmmmm, I haven't laid a tape measure next to any, and I certainly don't want to present myself to be "big papa Harley wizard el supremo", but I suspect the rubber mount rigs (Dyna and/or Touring powertrains) should be a bit more compact than those for the Softails.......but I'm just spitballin' there. Anyway, the Dyna chassis is fairly compact in comparison other cruisers, and the Dynas are about the cheapest "big twin" H-Ds you can buy. It might expedite the project to just buy a whole Dyna and go from there. I would like to assume you could buy a crank and 1 piston for the big bore engine, but that could be just a fantasy. From that point you just have to block stuff off and sort out the EFI. I bet if you tried to buy an entire CVO 110 engine, it would be as much as an entire Dyna, and then you'd be tossing stuff anyway.

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Stuffing a modified H-D engine/tranny rig into something else could just kill you in details, and may just end up w/ a white elephant in the end. I'm not positive there would be a lot to gain from a frame swap unless you went to some kind of sport bike frame..........and again - details. H-D motors are pretty tall. You may have height issues no matter what you do depending on how much the engine is stroked; I'm admittedly ignorant of what they do to get 110 cubes out of the twinkie. Even custom frames will often come w/ a caveat that they will only tolerate engines of a certain and/or stock stroke.

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Well I appreciate your insight. I was thinking the same thing about buying a Dyna, maybe one with high mileage etc, for cheap (ish) and then using that as a starting point. Once I figure out how to get it running well on one cylinder, then I could do the big bore swap. I'm not sure if I can get "half" a kit or not.

Anyway, after that if I chose to I could swap the drivetrain into another bike or modify the Dyna chassis to my liking. I'm just worried about weight. They weight 630lbs dry or so. Course I'm sure that taking a jug, piston and rod off would save weight...but how much? 15lbs or so maybe? I'd like to see the end result be a 1,000cc single cylinder "standard" style bike (similar to the sportsters) and weighing in around 500lbs dry.

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It's true you'll be battling weight. Rigid-mount Sportys were weighing a tick over 500lbs (the standard ones anyway), and when they went to rubber mounting the bikes gained 40lbs of weight, and in some cases more. I suspect that most of it lies in the beefing up of the frame which no longer uses the engine as a stressed member, and the necessary bracketry to allow the exhaust to move w/ the engine and still not flap around. The rubber mounting really complicates things.

There can be a lot of weight saved by substituting the steel fenders for ABS or glass, and perhaps some weight could be saved w/ aluminum wheels. I think the suspension ought to be adequate. If things are done properly you can get these bikes to handle pretty well...........it's just that there is a lot crap on the market as well as misguided maintenance taking place earning H-Ds questionable reps.

I once had a squirrel on a sportbike pull a hairbrained pass on me in a canyon; going around me entering a right hander on a blind curve. He didn't bargain that he was passing a former roadracer out for a ride on his (custom home-built) softail. I decided to ride "with" him through the rest of the twisties, enjoying his panicked double-takes in his mirrors. When we got to a long straight, he promptly went WFO to gain some distance. It wasn't worth a ticket for me. Granted, not all riders are created equal and he certainly could've handed my a** to me, but my point is that a 560lb softail with floorboards can make a respectable showing in the twisties.........squids notwithstanding.

You might find the Dyna-based bike to be adequate. I have my doubts that you'll find a smaller/lighter frame w/ enough room for the H-D motor/tranny rig, but I hope you do!

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It's true you'll be battling weight. Rigid-mount Sportys were weighing a tick over 500lbs (the standard ones anyway), and when they went to rubber mounting the bikes gained 40lbs of weight, and in some cases more. I suspect that most of it lies in the beefing up of the frame which no longer uses the engine as a stressed member, and the necessary bracketry to allow the exhaust to move w/ the engine and still not flap around. The rubber mounting really complicates things.

There can be a lot of weight saved by substituting the steel fenders for ABS or glass, and perhaps some weight could be saved w/ aluminum wheels. I think the suspension ought to be adequate. If things are done properly you can get these bikes to handle pretty well...........it's just that there is a lot crap on the market as well as misguided maintenance taking place earning H-Ds questionable reps.

I once had a squirrel on a sportbike pull a hairbrained pass on me in a canyon; going around me entering a right hander on a blind curve. He didn't bargain that he was passing a former roadracer out for a ride on his (custom home-built) softail. I decided to ride "with" him through the rest of the twisties, enjoying his panicked double-takes in his mirrors. When we got to a long straight, he promptly went WFO to gain some distance. It wasn't worth a ticket for me. Granted, not all riders are created equal and he certainly could've handed my a** to me, but my point is that a 560lb softail with floorboards can make a respectable showing in the twisties.........squids notwithstanding.

You might find the Dyna-based bike to be adequate. I have my doubts that you'll find a smaller/lighter frame w/ enough room for the H-D motor/tranny rig, but I hope you do!

Excellent story, and good info. I think that with some work I could get a single cylinder Dyna down to 500lbs dry. I'd need to shave 130lbs, but eliminating one cylinder, fiberglass fenders, alloy wheels and maybe some frame customization I think it could be done.

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FWIW, concerning the weight reduction..........the claimed weight of the lightest softails was 622lbs, as published by H-D, back when I built mine. I used a Daytec "Street Scene" chassis and mostly H-D parts. Even the wheels are a Heritage spoked front and a Deuce rear. The rear fender is a Milwaukee Iron strutless (steel), and I used plastic "custom" fender from H-D for the front, the name illudes me right now. The forks are OEM Heritage forks w/ the big Road King-style nacelle headlight.

I said all of that to say that mine only ended up weighing 560lbs dry, as I mentioned earlier. I dunno what H-D is hanging on the softails to make them that much heavier. Granted, my bike is pretty bare bones, but does have things like 2 mirrors, a horn and blinkers. I made my own bracketry out of aluminum in many cases, but I still can't believe it made the difference of 60 lbs.........and I wasn't really even trying that hard to keep it light; I knew I was building a sled and figured it would be what it would be. Ending up lighter than what the sales propaganda claims may not be as hard as it may seem.

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