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Discussion Starter #1 (Edited)
[Not So Low] Budget Parts-Bin Bultaco

Since someone asked, "Building a bike from scratch, worth it?" elsewhere on this forum (http://www.caferacer.net/forum/general/16705-building-bike-scratch-worth.html), I thought I would start a build log on my own budget scratch build. Okay, so at this point it's not anything that could even be called a "build." It's more like "holding spare parts up to each other," seeing how they might someday be part of a functional motorcycle.



In this picture:
Frame – 1977 Bultaco Pursang 370 Mk10 Model 193
Swingarm – 1976 Yamaha YZ125
Fuel Tank – Early '60s Rex KL35
17" Rear Wheel & Axle – 1981 Suzuki GS450T
19" Front Wheel & Axle – 1980 Suzuki GS850

I also have a few more assorted smaller bits around the garage.

This all started almost six years ago with the swingarm. I saw it on eBay and I thought, "hmm, that would make for an easy monoshock conversion," So I bought it for $23 shipped, without any sort of clear use for it other than for "something...someday." I next got the matching Suzuki star-spoke wheels from two different guys locally, and they reminded me of an old concept drawing I'd briefly toyed with about 16 years ago for an old Pursang 360 parts donor...



My old Buls are long-gone, but I've been pining for another one. Once I stumbled on this late-model frame, enough of the ingredients were present for me to start envisioning a monoshock Bultaco streetbike.

Don't expect lots of frequent updates, but I'll post here as I slowly continue developing the project. Since this is strictly a toy, my cost-control strategy is to wait and slowly acquire suitable parts that show up on Craigslist or Ebay for remarkably cheap prices...but I'm already getting excited enough about this to become an impatient hunter. I currently have a line on a pretty 4-speed 250 Matador motor and a 175 Alpina 5-speed, but both are more than 200 miles away.

...patience, my boy. Patience...
 

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Discussion Starter #5
quote:Originally posted by joe1028

six years for that! hell i don't think i will live to see it finished...ha ha
i like the look-a modern metrella
joe
Well, I hope *I* live to see it finished! Actually, I really didn't think about building something much at all for most of that six years. I only got the frame a little over a week ago. Now that that gives me a more specific direction to head in, I'm going to devote some more effort and resources to it. I picked up a front end yesterday and I'm waiting for a monoshock I bought off Ebay to arrive.

And thanks for the encouragement so far, guys. I'm not sure quite what I'll end up with, but my intention is that, even though this will definitely be a budget build, it won't be silly looking or a bodge job.
 

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Discussion Starter #7

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Discussion Starter #8 (Edited)
My first goal for this project is to have a rolling chassis I can push around the garage by the end of the year. To that end, I've picked up a GS650L front end. But lots of things are dependent on lots of other things —*rake and trail are dependent on fork length, which also affects ride height, which is also determined by the choice of rear shock and how that's mounted, etc...

So, I'm starting with the most basic task, attaching the swingarm to the chassis. The YZ125C swingarm may be nicely braced and ready to accept a high-mounted monoshock, but it has one major drawback that makes it a less-than-ideal choice: it uses a really wimpy 12mm swingarm pivot bolt. I initially played with the idea of going with a larger diameter bolt, especially when I discovered that the 15mm Suzuki front axle I have is just the right length for the Bul frame (which is about 27mm wider at the mounts than the swingarm). But the ID of the swingarm's mounting tube is only slightly less than 24mm, so there's just not enough room to go thicker on the bolt. If I used a 15mm bolt, the inner and outer bushing surfaces would have to be no thicker than 12 or 14 gauge sheetmetal to fit in there. Wafer-thin bushings would be no better than the skinny bolt. So, I'm stuck with a 12mm bolt and the stock Yamaha nylon bushings. Realistically, that shouldn't be a problem. I'll most likely end up with a motor that doesn't put out too much more than the YZ's original 18 HP, and street use won't stress it like banging hard landings off whoops and doubles would.

The Bul originally used the swingarm pivot for the rear motor mount. Considering the skinny pivot bolt I'll be using, and the fact that the Yamaha swingarm would have to have a gap cut out of the center to accomodate it, I've decided to let the swingarm design dictate alternative engine mounting, rather than the other way around. After all, I have yet to even determine exactly what engine is going to power this thing. (Like I said up top, every decision affects another decision down the line, so the order you do things makes a big difference.)

I am, however, now presented with a separate issue —*the swingarm mounting holes in the Bultaco frame are 14.3mm in diameter.


So, how do I "drill them smaller" to center the pivot bolt? Several people have said, "make up spacers," but don't think collars 1.15mm thick and as wide as the mounting plates are a practical option. I could weld up the hole and then drill it out to the proper size, but how do I ensure the two sides stay in alignment? Like most guys, I don't have any high-accuracy stuff like a jig or machine tools.

Right now I'm thinking that I could weld a pair of proper diameter washers to the frame on either side of the mounting holes. I don't have to center the pivot bolt in the existing holes, I just have to keep it aligned horizontally in the frame. So I could simply install the washers on the pivot bolt and let the bolt rest on the bottom of the holes in the frame while I welded them. (Note that the gap is at the top in the diagram below.) Once I torqued the pivot bolt down, I'm assuming that there'd be a lot of compression friction keeping things in place, so that the welds wouldn't bear too much of the stress directly.


(To Scale)

This all seems workable, but I'd like to get feedback on two things:

#1: Is my washer idea any good, or is there a better option? How thick do they need to be? Could I get by with only welding them on one side?
#2: Where do I source a strong 12mm bolt about 230mm long?
 

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Discussion Starter #9 (Edited)
Well, duh. Just writing this up made think of something I had not considered: using the Suzuki axle as a pivot bolt and simply having it turned down to 12mm —*the threaded portion is already necked down as it is.


I could simply use an expanding reamer to enlarge the right side frame hole to fit the axle's original diameter, have the spacer made up with a 15mm bore, and have the axle turned down far enough along its length to accomodate the swingarm. I'd have to take more care to make sure that I welded the washers onto the other side of the frame in the proper alignment, but I am thinking a few 90-degree magnetic holders and some careful measurements with a calipers before I weld should get me there.
 

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Rather than a pair of washers on the right, why not another spacer like the left. 12mm ID through out, say 20mm OD, necked down to 14.5mm (or bigger if you're up for reaming the frame anyways) OD to pass through the frame. Reinforce the frame with a new washer on the outside with the ID set to match the OD where it passes through, trim the necked down portion of the spacer so the nut seats against the new washer before running out of thread or hanging up on the spacer? That way you don't have that small void that is currently present between the washers.
 

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Discussion Starter #11
quote:Originally posted by Kurlon

Rather than a pair of washers on the right, why not another spacer like the left. 12mm ID through out, say 20mm OD, necked down to 14.5mm (or bigger if you're up for reaming the frame anyways) OD to pass through the frame. Reinforce the frame with a new washer on the outside with the ID set to match the OD where it passes through, trim the necked down portion of the spacer so the nut seats against the new washer before running out of thread or hanging up on the spacer? That way you don't have that small void that is currently present between the washers.
I get it. Makes perfect sense, great suggestion. That's exactly the sort of help I was hoping to get on this forum.

Hopefully people will find it useful and interesting to see all the iterations, changes and blind alleys I go through in thinking through this build.
 

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Discussion Starter #12 (Edited)
I have not done much work on the bike other than lots of measuring and test-fitting, but here's an update.

I revised my swingarm arrangement thanks to Kurlon's suggestion. A new swingarm bush arrived and the good news is that the threads on the Suzuki axle fit inside the bush no problem, so this will work. So, here's a third "no weld" version (I probably will still weld the red washer to the frame, but that will be only for secondary support).



If I can figure out exactly where the swingarm needs to be located to center the back wheel this evening, I'll drop the pivot bolt off with a retired machinist I know tomorrow morning and have him turn it and make up the spacers. In related news, I really, really want a lathe.

I also fitted up the forks and a rear shock to the bike temporarily and mocked up this illustration. The shock is an XR100 unit, which won't be the final component, but it provides an idea of what length I need (it's 10.5") and gives me some something to work with for right now. The forks are a bit long, but I am planning to use clip-ons mounted above the top triple clamp, so hopefully I can make them work without further modification.



If you want to see the raw photo, without the tweaks, it's here: http://www.tanshanomi.com/project-x/PB030031.JPG
 

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Discussion Starter #13 (Edited)
The forks are from a Suzuki GS650L cruiser. I was originally looking for a set of standard GS450 or 550E forks, because I thought the leading-axle design used on the "L" moddels would look too "cruiser-ish" for the sporty look I'm after. But when I found these locally for $65, including the steering stem and trees, I couldn't pass them up. Then I recalled plenty of standards and sports bikes had leading-axle forks back in the day: Ducati GT, Royal Enfield Interceptor, /5/6/7 BMWs, the KZ1000ST... and then it occurred to me that leading-axle forks were the PERFECT choice for this bike. Why? Well, because...

 

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Discussion Starter #14 (Edited)
Okay, so I've not made a lot of recognizable progress, but I have been picking up more parts, as well as doing lots of measuring and head-scratching.

After thinking things through more, I've chucked the whole idea I originally had for mounting the swingarm. I just didn't like the idea of relying on a 9-1/2" x 12mm bolt to support both the swingarm pivot and the rear engine mount, with nothing else tying the rear of the frame cradles together. But thanks to my random selection of the YZ swingarm, I'm limited to that pivot bolt diameter unless I either start hacking up the swingarm or toss it and start over with another style of swingarm. I'm not real keen on either of those options at this point. Not to mention that I'd be out the $72.95 I've spent on new swingarm bolt, bushings, shims and seals!

So instead, I've decided to divorce the pivot bolt function from the frame cross-bolt and rear engine mount, as shown (this is a plan view from above):



In order for the the swingarm bolt to clear the lower frame cradle during insertion and removal, It will have to move back a little more than 2-1/2". That will increase the wheelbase that amount, which might help the ride and stability a bit (who knows how this thing will handle!) as well as give me a little more room for a longer rear damper unit.

I bought a length of 0.156-wall 7/8" DOM tubing and cut it to fit between the frame plates. Yea, that's a bit heavy, but with the swingarm cantilevered out that far, it'll potentially exert some serious torsional forces on that tube. Additionally, the tube ID conveniently matches the 14mm existing mounting holes in the frame side plates. I also bought a piece of 3/8" flat stock that I'm going to make the axle mounting plates from (the blue pieces in the diagram). Even though 3/8" plate steel feels massive, I could put in some gussets (shown as dotted blue lines). I'm interested in people's feedback on whether that's necessary.

There's no way my little 110 MIG welder will get decent penetration on .375 plates (or even .156 tube) but I should be able to tack weld it together and then take it to a "real" weldor to make it all solid. I haven't decided if the cross tube and drive-side plate should be welded to the frame. The whole assembly could conceivably just bolt in place, since the rear engine mounting plate(s) would prevent the assembly from rotating (once I determine what engine I'll be using!)
 

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Discussion Starter #15
I haven't updated this in several months, mainly because I haven't progressed enough to have anything new to show. I decided that I definitely want to go with a Bultaco engine, for several reasons: 1) I like Bultacos. I've owned three of them, and they're great motors. 2) I know Bultacos (See Point #1). 3) Any other motor would require additional frame modification, and thus time and expense. 4) I would like the finished product to be "a Bultaco" rather than a mongrel combination of engine and frame. That might also help me get a street title for this bike.

So I've slowed down while looking around for just the right Bul motor. Even though any Bul 5-speed engine will fit my frame, they're not all identical. I am being a bit picky about the particular model I go with. In keeping with the budget aspect, the asking prices for complete Bultaco motors vary widely. I would rather bide my time and find something that's not at the top of the price range.

In the meantime, I bought a set dummy set of Bultaco engine cases to use while making up the swingarm pivot. I also bought a stock rear engine mounting bracket, which will require me to either cut the engine mounting tube in half, or bore a hole in the engine bracket the OD of the tube. Not sure which way I am going to go at this point, but I've definitely reined in (or perhaps restricted) my options a lot by settling on an engine design.
 

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Discussion Starter #16
I thought the XR100 shock was a bit dinky-looking, and I knew it wouldn't have the spring-rate or damping to handle the bike's mass. So I scored a rear shock from a Kaw ZX-7 on the cheap. I knew it would be total overkill, but I figured I'd rather have a near-hardtail than something bottoming out all the time.

Holy Schmoly, the thing is the size of a small pony. It's a waste of $17 as far as this project is concerned, but as a paperweight it'll sure hold down a lot of paper on my desk.
 

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Discussion Starter #17
Just as an update, I haven't abandoned this project or anything, I'm still slowly collecting parts and fitting them together. As I said when I started this thread, it is going to be a very slow process, since I am trying to do this in my spare time at minimal cost. The overwhelming issue (other than a lack of free time) is that everything affects everything else, and I am REALLY trying to think through all the steps in order to not waste resources on fabbing something in such a way that it doesn't work down the line.

Also, I am trying to fabricate/modify stuff without spending too much on tools. For example, I devised all sorts of cunning ways to accurately and easily enlarge the rear wheel slots in the swingarm by the required 2 MM, but in the end I just carefully used a Dremel tool. It was time consuming, and could have potentially screwed up my swingarm, but it cost me nothing to do.

I'll try to post some pictures of my progress soon, if anybody still cares.
 

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Discussion Starter #18
Just as an update, I haven't abandoned this project or anything, I'm still slowly collecting parts and fitting them together. As I said when I started this thread, it is going to be a very slow process, since I am trying to do this in my spare time at minimal cost. The overwhelming issue (other than a lack of free time) is that everything affects everything else, and I am REALLY trying to think through all the steps in order to not waste resources on fabbing something in such a way that it doesn't work down the line.

Also, I am trying to fabricate/modify stuff without spending too much on tools. For example, I devised all sorts of cunning ways to accurately and easily enlarge the rear wheel slots in the swingarm by the required 2 MM, but in the end I just carefully used a Dremel tool. It was time consuming, and could have potentially screwed up my swingarm, but it cost me nothing to do.

I'll try to post some pictures of my progress soon, if anybody still cares.
 

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Discussion Starter #19
I've been too lazy to take photos of my progress, but I now have enough parts that I need to stop collecting and start making them all work together. But this is definitely a "back burner" project, and I've only been able to devote a couple of hours a week to it. I'm starting to get a clear picture of the enormity of the task. Every step in trying to make a motorcycle out of junk takes more effort and expense than you can predict.

For example, take a look at my cylinder.

I found a jug from a 200cc Bultaco Model 45 recently on Ebay. Despite the seller's assurance that "ALL FINS ARE GOOD," I was disappointed to discover once it arrived that the bottom fin was bent on the
left-hand side. I got it for under $150 (incl. shipping), so it still isn't necessarily a bad deal ...but it's not the killer deal I thought it was, and it has added one more thing to my list of to-dos.

Can this be straightened, and if so, what's the proper technique? So far I've been told to heat it with an oxy torch, to only use a somewhat lower heat such as propane, to heat it to about 500 degrees by putting it in an oven, to NEVER try to heat cast iron...simply bend it very, very slowly with light taps on a wooden wedge between the fins.

Or am I just tempting disaster if I try to fix it? If it's too risky, I'd rather have a bent fin than a broken off fin, I guess, but I'm kind of bummed about the appearance. It's an otherwise good barrel.

 
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