Honda 650 single

unclerob

Been Around the Block
Hello all, I've been posting over in the UK section about a project I've been slowly working on for the last couple of years, the idea was to build a sort of 60's single cylinder race bike replica using parts from the Honda RFVC singles. Have been slowly refining the design and have just started on the 8th one which I'm building for a friend....so thought I'd start a thread here on it...
A few pics of completed ones to start with...
 

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unclerob

Been Around the Block
Yes, am going to....starting with the frame! This is the 8th one I've made and each has had a bit of modification to improve it, now fairly happy with the design. Its made from 11/4 CDS tube, 18g wall, the first couple I made pretty much freehand but then made a jig so it goes together pretty easily now.... the biggest problem I was having was distortion at the headstock where 5 tubes join to it....even getting the joints as tight as I could and being careful with the welding sequence it still moved. After they are done I was having them checked and straightened at a frame alignment place but never liked the thought of how much stress was being put into that area. Then hit on the idea of making the bend in the diagonals a bit different so that there was a small gap between them and the top rails and leaving the joint unwelded while it was straightened....so that the bend altered slightly as the headstock was twisted....then welding in gusset plates afterward to lock it all together...
 

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unclerob

Been Around the Block
Thanks! As for registration, unluckily I killed my lathe boring out some bar to make the headstock so had to use the ones from the original frames.....! Probably best to say no more other than this one which is for a friend and another I'm doing after are all getting the boring stuff like indicators, mirrors and such like and one or the other will be having an IVA....
 

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unclerob

Been Around the Block
And while I'm here a bit more about the frame design....the original one from the SLR and FX 650 just has the swingarm supported in the middle with the outer ends of the pivot bolt supported by the footrest plates. I've changed that so the main support is from 2 pairs of plates spaced apart 10mm either side which are then welded to the diagonals running directly up to the headstock. These plates also take the lower rear engine mount which is a long stud with spacers either side of the crankcase....these can be made to an exact length so as to get the chain line spot on.
At first I'd made the 2 front engine mounts..to the head and the downtube as part of the frame....have now changed that to seperate bolt on mounts....I make them deliberately a bit too short and then when the motor goes in I bolt the 2 rear ones up tight and shim the front ones to about a 2mm gap before doing the bolts up. Puts a little bit of tension into the structure and hopefully adds a bit of extra strength....thats my theory anyway! What it definitely does do is make everything easier to make....hard to weld it all in place and expect engine mounting bolts to line up exactly....
 

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cardinal

Been Around the Block
As a former bicycle frame builder I love seeing the frame fabrication pics and the jigs! Pardon my ignorance, but why are there two exhaust pipes if this is a single? (not familiar with that engine at all)
 

unclerob

Been Around the Block
Its a 4 valve single and each port has its own pipe....some versions had 2 carbs as well! Also the valves are arranged radially instead of being paralell to each other....makes for a very complicated follower set up....and probably not really worth all the trouble Mr. Honda went to!
 

The Limey

Evil English Villain
Weren't they twin ports to make a radial head design a bit easier? Either way, cracking job there matey. If I weren't living off a pension I'd commission one of those beauties.
 

cxman

Over 1,000 Posts
DTT SUPPORTER
only one of the carbs has idle jet and circuit

and it has progressive linkage to open the second carb later
 

der_nanno

Faster!
cxman said:
only one of the carbs has idle jet and circuit

and it has progressive linkage to open the second carb later

Much like Yamaha XT600 engines. Speaking of the RFVC-design... it's really just as horrible as it sounds. If maintained correctly they are ... ok, but if several Summers working at a Greek motorcycle rental has taught me one thing: as soon as they aren't, well, they're ticking time bombs with worn rockers and cams. And as seemingly everybody abuses them, both are very hard to come by for a reasonable price. But that's slightly off topic and those frames ARE KILLER. Please keep showing more of the fabrication work!
 

The Limey

Evil English Villain
Funny you mention the XT. For my next project I've been thinking about slipping a big XT single mill into an FZR400 chassis, make a kind of supersports single. Seen the XT engine in a Cagiva Moto frame and it caused spontaneous failure of my Y front elastic, such was the extent of the swelling in my groinal region.
 

unclerob

Been Around the Block
Not sure I'd call the RFVC design horrible....more just unnecessarily complicated! I once ran a business servicing and renting out the 250RS singles for London couriers....they really were time bombs! Cams on those ran directly in the head so the whole top end was buggered in one go....
But you're right they do need regular oil changes to last, the donor bike I used for the previous bike (the red one, 62) I got off ebay for £200 odd with an Mot (god knows how) and 78,000 miles! Luckily for me though I'd also come across a very low mileage front on write off that had a cracked case mount...so a nice lot of good bits for the rebuild.
Just finished this frame yesterday and painted it....and only forgot to weld on one bracket....usually its more so some assembly pics soon....
Re the big single in a light frame idea that was how this project really started with me....I had a couple of Honda singles and was planning one in a Cagiva Mito frame....went as far as buying one with a dead engine but decided I wanted something more classic as it were....spoked wheels twinshock etc so ditched the Mito idea and made my own frame ...incidentally its not only the Honda singles I pick on to rip apart...have recently done the same sort of thing to a Hinckley Trident, a CBR400 and currently looking for a nice innocent Bonneville to chop to pieces!
 

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unclerob

Been Around the Block
Was going to post some assembly pics but then realised I left the camera at the workshop....so heres some from before of making the tanks instead....
I'm ok with welding sheet metal but not so great with making it into nice curvy shapes, but I made the first one from steel and it took forever, hours of hammering, my neighbours started throwing things at me and it didn't come out that well either!So Plan B was to make a fibreglass mould of it, then make a tank shaped box in ali and fit it into the fibreglass cover using expanding foam....
 

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unclerob

Been Around the Block
Beginning assembly now, the swingarm is the stock Honda one with the monoshock bracket removed and brackets for twin shocks added....using NJB shocks which have 2" of length adjustment. During welding the bottom legs of the frame distort in slightly so I use a length of studding and nuts to push them apart....saves struggling and damaging paint
Next is the oil tank, it bolts to the frame at the rear with 2 rubber bushed bolts, then another bracket underneath with 2 rubber buffers on it is bolted to the frame downtube....as its tightened up it forces the tank against a 3rd rubber mount.
Idea being to avoid having brackets welded directly to the tank....
 

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