Building a Scrambles G80 Bitsa

[Joker_Bones]

Richard...
What are you planning for a gearbox adjuster/stop? I notice you don't have the holes in your engine plates.
This comes to mind if it's of any relevance to your build...
I made up an adjuster/stop in the style of the adjuster on the twins.

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It occurred to me that it only acts as stop in one direction and I wondered what happens to the gearbox under drive. Is it pulled forward by the primary chain, pulled backward by the rear chain or are the forces equal and opposite.
Whatever the theory... In use, on two different occasions, I found that the gearbox had pulled forward leaving the primary chain flapping and the rear chain tight.
The first time I assumed I hadn't sufficiently tightened the top gearbox mounting and re-tightened it tight! It wasn't long before the gearbox had moved forward again.
I wonder if it is down to the ally engine plates, being relatively soft and relaxing no matter how much the the top mounting is initially tightened.

[Groily]

It occurred to me that it only acts as stop in one direction In use . . .on two different occasions, I found that the gearbox had pulled forward leaving the primary chain flapping and the rear chain tight.

It was for that exact reason that I made a heavy-duty second adjuster on the timing side for my 650, operating between the two fasteners shown in your pic on that side. Similar to the set-up on Burman boxes in fact. So there's a pull-me-push-me and a push-me-pull-me on opposite sides. Gearbox can't move a millimetre, and is also set square. It had previously had the very annoying tendency you refer to.
Your work is very pretty J-B, as ever - mine in this case was 'farmyard'!

[RichardS]

Richard...
What are you planning for a gearbox adjuster/stop? I notice you don't have the holes in your engine plates.
This comes to mind if it's of any relevance to your build...
I made up an adjuster/stop in the style of the adjuster on the twins.
<snip>

I had originally intended to use an adjuster with a 'post' bolted to the engine plate and an eye around the gearbox top bolt as on G3C, then when I got the Matchless gearbox case thought I would put it between the engine plates as yours 'as it ought to be'. But now I am filling that area with the oil tank I intend to go back to the locating eye scheme. I would prefer to put it on the drive side, but I can see it will not be possible to access it under the oil tank and hidden behind the chain-case so I'll have to live with it on the timing side.

I too had wondered which direction was the net chain tension and it will obviously depend on which gear you are in at any given moment. In general I would have expected the greater tension in the rear chain, pulling the gearbox backwards towards the rear wheel - so your adjuster should be fine. So much for theory

[Joker_Bones]

Groily...
That is kind of you to say so, it did make me chuckle though...
I made up and fitted a second adjuster/stop forward of the gearbox.

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When I had finished I didn't think it was "very pretty", so I made a cover.

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[Joker_Bones]

Richard…
Not because it “should be” just conversation …
I read somewhere that at some time the CS singles acquired a second adjuster/stop for the AMC gearbox. The late ‘50s parts lists agree with this listing two of all the adjuster parts for the CS models, even the gearbox shell is a different part number for the CS.
I don't know the form of these adjusters, I've never been up close and personal with a genuine CS, all my bikes are bitsas.
On my other comp style single, which in true bitsa style has a burman gearbox in a later duplex frame, I made up two eye bolt type adjusters and managed to squeeze one in on the drive side.
Had to shave a bit off the chain guard on the inner primary case and as you might imagine it requires a degree of patience to adjust.

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[56G80S]

I can only echo the comment about the skills exhibited, so far ahead of anything I manage.

On my lower output single I found that the AMC gearbox, despite the stop arrangement, had an initial tendency to move towards the rear and tighten the primary. After a change of any kind I would have tools with me, check after a few miles and re-adjust by the side of the road; it would then settle.

Johnny B

[mdt-son]

56G80S, the gearbox shell moving rearwards despite presence of the adjuster/stop bolt is not possible, unless the adjuster bolt nut unscrews itself from vibration etc. In the above pictures, I note some of the respondents did not fit the counter nut. The adjuster should be clamped in the crosshead to prevent what you describe.

Picture shows a genuine 1964/5 G80CS rear engine plate and adjuster. Note lock nut.

G80CS engine plate and adjuster.png

- Knut

[mdt-son]

Mine neither.
Tightening up the pinch bolt on an N15 last year, ohhh joy...
IMG_20220924_210057.jpg
Way too much clearance between the hole in the slider and the OD of the spindle.
New slider from Andover Norton and I sleeved the spindle to a close fit.

Youch! That's the worst I've seen, no way can you "repair" that.
I've had three over the years and all came to me pre-cracked just adjacent to the pinch bolt. A couple I had welded and it was adequate, though not wonderful. The other I changed the forks so it became irrelevant.

Dresda company offers a service in which the Norton pinch eye and bolt is removed by machining the slider, and a bolted cap is provided, AMC/Triumph style. The solution looks very decent and preserves the slider, as all modifications are performed cold / at room temperature. Cap holding bolts are invisible. As for the cap, I can't say whether it has a steel ingot or not. Cap looks rigid enough, and clamping forces are not that high anyway.

- Knut

[RichardS]

Sorry for my rather long absence here, but I have not been entirely idle between the daily grind. I was rather concerned at the apparent (well actual) lack of compressioni I noted in casually turning the engine shaft by hand and it transpired that the push rod tube seals in conjunction with the tubes weere preventing the head from closing down onto the barrel - at all! 'twas right drafty in the gap.

I checked the 'shims' against the head seals (only 020") and even with them removed completely the head still didn't seal so I ended up using smaller than provided o-rings at the base of the tubes and finally managed to get a head-barrel seal. Standard 5.3mm section o-rings seem to do the job fine.

That in turn meant that the pushrods were now too long to allow the valves to close, even with the adjusters fully retracted so it was open it all up again, and shorten the push rods as suggested by JB above. I'm annoyed I wasn't more insightful in the first instance - I could have done that from the start, but live and learn....

I made a brace for the chain case that approximately replicates the stud in the crank case of later engines. Mine is just bolted to the engine plate and after a first scrapped attempt - I must have mis-measured as it was exactly 1/16" too short! Grrrr, - seems pretty robust for the task.
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I wanted to make some progress with actual 'assembly', but can't put the chain-case and clutch on finally until I have resolved the oil tank, so that's next. I had been agonising over the oil tank, but finally caved-in and started cutting sheet to the general design of the cardboard version I'd made after suggestions last November - between and over the engine plates, under the carb. I've done a fair bit of welding over the years and some alloy welding with gas, but that's not very elegant and by modern standards very antique. So when a guy in the next villge was selling an old, pretty monstrous, but working TiG set for an acceptable sum it seemed fate was trying to decide for me. Deal done, I dragged the machine home (all 215kg of it!). Fortunately there are now a couple of options for inert gas that don't require a BOC account - their rental costs have sky-rocketed in recent years - so off to the alternative gas-man for a bottle of argon and start playing.

That, of course, was only the beginning and the short story is that like any other skill, it's a steep learning curve to start with, despite and in some ways because of my previous experience. Initial results were not very pleasing and an agony to achieve. I got as far as this, but realised for visible welds on the top of the tank it was simply not going to be acceptable.
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I'm really not proud of that and it quickly became evident that TiG welding, alloy in particular, without a foot-pedal control was pretty fraught and unlikely to produce satisfactory results. No pedal came with the welder, so that was added to the list of new wants. As I have ofen observed in the past, it's a matter of buying tools to make tools to mend tools... to do a job that you should probably not be trying in the first place.

Oh, yeah, before the welder I needed to make so long overdue folding bars so I could make a half-decent job of the tank shell. Another day playing in the workshop, but the result is satisfying compared to trying to make a neat and accurate bend holding the work in the vice and belting it with a hammer.
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Making tools to mend tools.......
I bought a pedal off fleaBay for less than the price of its military spec. Amphenol connector that's required to interface to the machine, but of course, the internal electronics were not compatible - they're all pretty standard, just different standards!
It's a simple circuit with a minimal power supply and a voltage divider for the output and to my utter amazement I had a suitable voltage regulator in the shed from a shovel-full of random unused components I once recovered from the skip at work, 30 odd years ago - TaDaah I knew they would be useful one day !!
It then took a couple of days to reverse-engineer the correct connections having found that someone had re-wired bits of the control panel at some point 'to make it work' no doubt, but not as the designers intended.

I had deliberately started practicing with the bottom of the tank which will never be seen, so the inevitable errors and corrections won't be obvious unless the tank is removed. The pedal transformed the controlability of the process as well as the work produced. And in conjunction with a couple of hours trawling YooYoob turotials I can now produce acceptable welds which at one point seemed a pipe-dream.
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Not perfect, but as BR used to say 'getting there'

[RichardS]

It's not quite finished yet as I have been concerned about how to mount it to the bike. Alloy is not good against vibration so any welded-on tags and lugs would be very susceptible to cracking in service. Equally, 'lose' mountings wear with vibration and alloy is soft and not wear resistant. I have settled on welding two thick-walled cross tubes right through the tank and I will put studs right through those and both engine plates with a slightly over-length rubber hose over the studs snug in the tube. When all bolted up the tank will end up rubber mounted on the compressed/swollen hose and captive on the studs. Time will tell if it's a success.
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Pilot hole for the tank mounting, I'll open it out to the size of the rubber-hose-come-bush later

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Including the welder, the gas and the materials it's probably running to about £500 for the tank - pretty steep, but then a custom made one-off tank from anyone willing to make it would not be a lot less - and I've got the welder and experience left to fight another day. 'course, if you count my time, even at minimum wage it's probably nearer a grand.

On the bright side, the next one will be less than half that price.


I fitted the rear mudguard - an ANO after-market alloy part - what the hell has happened to prices?!? The last alloy mudguard I bought was about a tenner - OK, probably 25 years ago, but now £80 gasp! But at least having done that I could then put the bike on its wheels for the first time in some decades.
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The rear frame loop seems rather close to the wheel, but as I needed to get new rear suspension units I got them longer than the random old scrappers I had lying around. The new ones are 13-1/2" centres and seem just about adequate, I'll see how that goes when riding it. When I got the frame, the rear loop was bent upwards, very obviously not original as there were tool marks and damage to the tube at the bend, but I now slightly regret straightening it out, the extra clearance would be nice.

After the tank, the remaining significant task is to make some foot rests and then I can put the chain case & exhaust on which will look like real progress. I have a box of mixed foot pegs, some AMC, but no competition type. In any case I like modern folding foot rests for this so I guess I'll fabricate some brackets to bolt between the foot rest and centre stand lugs - a bit further back than original.

Oh, one annoying SNAFU in closing, although the cardboard tank could be fitted and removed easily enough albeit with a bit of fiddling. I guess alloy is rather less flexible and I have mis-judged the size slightly so had to remove one of the carburettor studs to get enough clearance to actually get it into place. Bugger!
Seems a cardboard cut-out is not all it's cracked-up to be, It's annoying, but I'll survive......

I'll report back in a bit....
Cheers & Happy New Year all