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I probably wouldn't want to locally heat it too much due to distortion. Are you sure that's a 100% iron cylinder? The corrosion has an aluminum look to it. I know nothing about Bultacos.
 

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Discussion Starter #24
quote:Originally posted by Geeto67

bent fins add character.
Good advice. I have so many other things to worry about on this, I'm not going to mess with it. If I ever get to the point where this is complete, running, and doesn't look like total crap for a thousand unrelated reasons (all big ifs), maybe I'll worry about it then.
 

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http://aircooledrdclub.org.uk/forum/forum_posts.asp?TID=32185&KW=barrels

You need to check out this guy's restoration of a cylinder that most of us would cast off as junk, including straightening of the fins. The OP's tag line is "OCD engineer"...indeed. If he can make that cylinder look and perform properly, you should be able to fix one fin. Just sayin'.
 

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Discussion Starter #26
quote:Originally posted by raven
Are you sure that's a 100% iron cylinder? The corrosion has an aluminum look to it. I know nothing about Bultacos.
quote:Originally posted by santa_ana
Barrel itself should be iron. The rest alloy. I wouldn't touch the fin, it's mega crappy cast.
Yes, you are both correct. It is alloy with an iron sleeve.
 

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Discussion Starter #27 (Edited)
Even though I've seemingly made no actual progress on this build, it's not dead, just simmering on a way-back burner most of the time. I HAVE managed to make the first rough cut of the swingarm plates out of 3/8" flat stock. Ooohwee — now that's some kinda' progress! ...Now I just keep moving them from one side of the workbench to the other.


I also stripped the paint off the swingarm. For some reason, somebody did a crappy job of painting the YZ125C swingarm white, and much of the surface was rusty and corroded where both the repaint and the OE black were worn off. Before doing any work on it, I wanted to strip it. Since I was doing this in my basement garage, which is directly under our kitchen and bedroom, I bought some odor-free "Citristrip."


Since most products that promise greater safety and fewer harmful fumes also tend to be generally less effective, I wondered if a product designed for stripping wood furniture would be able to tackle OE frame paint. It went on easily enough and was thick enough to cling well. Here's a hint: use a natural bristle brush, not one of those cheap foam brushes — not only could it not work into crevasses well, but as I was working with it, the stripper dissolved it into goo!


The instructions said to let the stripper work for "up to 24 hours," but I was really just testing whether it was going to work at all, so I only gave it about 40 minutes of soak time. Then, I pulled on the rubber gloves and scrubbed it haphazardly with a green scrubber sheet for all of about 90 seconds, then hosed it off in the driveway. Amazingly, despite a pretty slap-dash application and not much "cooking," nearly all of both coats came right off.


It needs another application, especially underneath the bottom where I hadn't bothered to coat it, but I was very impressed with the job Citristrip did.

Thinking through how to make this swingarm work is still what is slowing this build down. (Well, that and the fact that I am spending no appreciable time or money on it!) There's really no point in doing anything else until I have nailed down precisely how I want to mate the swingarm with the frame and engine. As many different designs as I've uploaded to this thread, there have been just as many variations since that I've investigated and abandoned. Unfortunately, I also discovered that the Bultaco Matador rear engine mount I bought off Ebay won't work with my Pursang frame —*even though the engine cases are the same, the position of the swingarm pivot relative to the engine is just slightly different. Grrrr. Perhaps I have more mounting plates to make.

What I'm realizing is that that this swingarm is really not an optimal choice for this build, even though it was the starting point for the whole idea. The pivot diameter is too narrow, the shock mount is too wide, and the whole thing looks pretty cobbled. I know many shadetree amateur weldors who wouldn't be satisfied with boogered-up welds like these.

 

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Discussion Starter #29 (Edited)
I discovered that the Matador rear engine mounting bracket that I bought on Ebay isn't going to work, so I had to design new rear engine mount brackets. I first drilled holes in two pieces of 3/16" thick high-density polyethylene sheet*—*one sheet with two 1/2" (close enough to 12 mm to work) holes on 100 mm centers, and another with a 7/8" hole.


The hole saw says 15/16" because this was just a test on scrap sheet.

I attached one piece to the motor and installed the frame tube through the other one. I made sure the front motor mount bolts weren't binding, then clamped the two pieces together.


I removed them still clamped together and drilled two small holes for machine screws, locking down the relationship between the motor mount holes and the frame tube.





Which then allowed me to generate an accurate design. I am going to try to replicate this out of HDPE as a final check before I send the design file off to be made out of 1/4" mild steel on a waterjet cutter.


I'm still undecided about the speed holes.
 

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Discussion Starter #30
A friend just read this thread and told me that I was making this too detailed and tedious to read.
Perhaps, but I want to document everything it really takes for an average joe without special tools or experience to build a bike from parts, including all the head scratching and blind alleys, and stuff that gets scrapped.
 

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Discussion Starter #35 (Edited)
And here we have the design for the assembled rear carrier assembly, which will tie the frame, rear engine mounts and swingarm pivot all together.


The second, smaller tube at the bottom is probably overkill, strength-wise, but it will reduce the chance of distortion due to suspension loads (although it's probably theoretically stronger than the swingarm itself!). More practically, it's an easy way to positively align the the swingarm plates and motor mount while welding.

The lower tube will have a third function, (probably) acting as the mounting tube for a crossover shaft if I choose to adapt my early right-shift crankcases to a modern left-side shift configuration, similarly to the BSA/Norton P92 prototype's arrangement in the photo below, (which I spent time studying closely when I was at the British National Motorcycle Museum this spring).


Bultaco did make left-side-shift motors after '75, but the design is a bit wonky, and some riders complain it's imprecise. I like this sort of external crossover better. Not only do I already have an early 5-speed crankcase, but it would be nice to not be limited in the future regarding what motor I can slip into this bike.
 

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Discussion Starter #36 (Edited)
I need to make sure that my new motor mounts keep the engine properly aligned with the frame; this is complicated because the engine is clearly but ever so slightly offset to the left in the frame. I also need to determine the drive chain offset from C/L, so I can machine the rear wheel sprocket carrier to match.


My solution is a hardware-store laser cross-hair level. I hung the frame and engine from the ceiling and aligned the laser down the centerline of the frame and along the very bottom edge of the crossmember tube I added. From there I've been able to determine the measurements I needed to within about a millimeter.

Just to record the numbers here for my own future reference:

Engine offset in frame: 4 mm to left
Rear crankcase mounting boss width: 31 mm
Counter shaft sprocket centerline to crankcase centerline: 68 mm
Counter shaft sprocket centerline to vehicle centerline: 68+4=72 mm

That last one is problematic, because the sprocket offset on the GS450T rear wheel is about 85 mm. That means I'll have to have about a half inch machined off the face of the sprocket carrier. I was hoping to avoid that.
 

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Discussion Starter #37
I realized that my twin-tube crossmember design (shown in the CAD drawing two posts back) has one fatal flaw — the lower tube/shift shaft housing would run right through the lower drive chain path. Oops. I'll have to re-think this so that the second tube is to the rear, between the main tube and the swingarm pivot.

Fortunately, it's a whole lot easier (not to mention cheaper!) to find that out now. That's why I'm spending so much time and effort in the sketching/planning stage, trying to catch all these issues.
 

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Discussion Starter #39 (Edited)
With some helpful advice from guys over at Garage Journal, I actually managed to drill through 3/8" plate steel (my swingarm mounting tabs) using an ordinary 7/8" bimetal hole saw from the local Ace Hardware. I used an aerosol can of Tri-Flow for cutting lube. My drill press's lowest speed of 620 RPM was probably a bit too high, but it worked. It only took around 30-40 seconds to go through each plate, and after drilling two plates, the teeth on the bit don't even appear noticeably worn.


Unfortunately, the hole saw didn't run true at all (the pattern of paint wear on the saw attests to that). I knew that before I started because it visibly wobbled on the arbor, but I went ahead and tried it to see what sort of hole size I'd end up with.

Roundness is remarkably good and both holes are similar, but the nominally 7/8" (.875) diameter ended up being 0.89" at the bottom of each hole and about 0.96 at the top, or a range of about 2–10% over spec. That's pretty sloppy tolerance for welding fit-up (this will be welded onto my 7/8" steel crosstube).

I'm going to try again with a slightly undersized 13/16" Lenox hole saw (better quality, with a screw-in arbor). Hopefully that will give me a usable part.
 

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I have great luck with bimetal hole saws. Even with my less than perfect drill press things come out OK. Not to ask a dumb question but,did you remove the pilot drill from the saw for the picture, or did you saw with out one?
 
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