Building a Scrambles G80 Bitsa

[Joker_Bones]

AMC classic spares list a foam block as the original fitment and a horse shoe rubber as an alternative.

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This is the foam block...

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I use an ally flat bolted to the lug at the top of the down tube and a smaller foam pad for my ally tank. The flat being a flat is flexible and offers a bit of cushioning.

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[clive]

With that big additional lug in the frame I would be inclined to use the horseshoe the other way up to help the tank clear the top rail.

[RichardS]

With that big additional lug in the frame I would be inclined to use the horseshoe the other way up to help the tank clear the top rail.

That's exactly what I was thinking. I've got over worrying about what _should_ be there.

I was wondering whether the lug thing in questin was to be used similar to the front tank mounts, but that seems not to be the case either.

[RichardS]

Richard, I see no reason why an alloy tank should have a different fixing compared to the late model steel tanks (CS and STD). As for the tube-in-tube penetration, I believe it's a fabrication not performed by the comp shop. Maybe someone fitted a BSA tank off a Victor model, which had this kind of central bolt mounting? The bolt hole is huge though and fitted too far to the rear to serve as a single tank mounting. Maybe it was intended as support for a head steady of a V-twin engine? Who knows. I see the penetration in connection with the fact that the front winged tank mounting had been cut off. It's a pity the individual damaged the frame this way. Now that it's there, your best option is to leave it be, even though it adds unnecessary weight to the frame. Repairing the damage would require replacing the spine tube .....
The crossing tube which now appears to be a turned part has an internal structure. I am pondering what it is - a spring? Furthermore, I believe the upper part of the penetration has a larger diameter when compared to the lower part. Another evidence the modification was a hotch-potch.

- Knut

I am pretty certain the apparently additional lug shown was original, it has the same brazing and general look and patina (rust) as all the rest of the frame.
It is not (was never) a crossing tube, it is two shaped grandiose 'washers' brazed on above and below the top tube.

I replicated and added back the upper piece, believeing it to be the tank mounting - it had clearly been ground off, but the lower part was in tact and as far as I can tell had always been there.

There's no way it could be for a BSA tank, it's right at the rear of the tank, BSA's did have somewhat similar mounting, but halfway along the tank, 6" in front of this.

Guess we'll never answer this, it's just another quirk of history.

Given the evidence, I wonder if I might not just remove that top lug-ette and braze a patch over the hole - it does seem to be rather in the way. Something to ponder over the weekend.

[dave16mct]

Could've been for an Ariel trials tank which was similar to BSA central bolt mounting but towards the rear. I've never seen that lug on an AMC bike.
Dave.

[RichardS]

Time came to make a decision as teh whole project was getting hung up on details - So I decided to take the plunge and amputated the spurious possible tank mounting boss-lugs and brazed patches over the holes before sending the frame for painting. A bit like a visit to the dentist it's better once over and done with.

In parallel to that I had a dry-run assembly to check that everything looked reasonable before painting and to check the gearbox and chaincase alignment.
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Apologies for the lousy photo 'composition', too much cr@p to tidy up. And as a passing remark, obviously the cylinder and head is just a space model and not the actual parts I'll use. More of the butchered melange I have collected over the years,the sadly shorn barrel came from a guy who was keen on grass track and removed 'excessive' fins for running on dope.

I acquired an alloy chain case many years ago, I can't find the part number listed in any of the Parts Lists so no idea which model it is from, but given that it's what I have I 'll just make it fit. The alternator housing is ample to accommodate the engine shaft shock absorber and I'm flirting with the idea of adding a small modern Japanese generator - time will tell.
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A simple spacer on the crank case is easy enough counterbored to register on the crankcase and similarly for the chaincase, but the hole PCD on the chaincase was inconvenient - slighlty different PCD and different angular position. So I welded the holes and will put a new trio near to midway between the originals. I would have been 'possible' to make new holes to line up with the crank case holes and use through-bolts, but they would have been awkwardly close to the original chaincase holes - near but not the same PCD and close to countersink distance. Better to give myself a bit of flexibility.
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In case nobody has ever said, the crankcase threads are 0BA which is pretty obscure these days, but 0BA is 6.0mm dia and 1.0mm pitch a modern M6 is a good enough clone. Yes, the thread form is different, but given the choice between a mullered mild steel slotted screw and a nice new countersunk HT cap-head I know which way I'll go.

I adjusted the thickness a bit to give clearance both ways when the engine sprocket is properly aligned with the clutch. It seems not all AMC clutches are the same either. I have a couple of hubs which move the primary chain by about 3/32", way too much for manufacturing tolerances, but barely enough to be 'obviously different'. I settled on an acceptable compilation of parts and it all looks like it should be OK.
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Have a good weekend all

[spookefoote1956]

Phwar

[Joker_Bones]

That looks like the inner chain case off of a twin, well it looks like the one on my G12 having the two locating dowels and the recess at the tail end, which would account for the slight mismatch with the holes and angle I guess.

Having swapped out my 350 alternator engine for my 500 comp style motor, I've just done the same job mating up the ally chain case...

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As the case was from a single there was no issue with the three front fixings, but as there's no threaded boss on the 55 crankcase for the centre fixing, I put an extra hole in the case and added a small bolt.

[Joker_Bones]

I reduced the boss on the back off the case...

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So that the case sat as far inboard as possible, just clearing the swing arm...

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That way I could get a DID heavy duty chain clear of the inner face of the chain case...

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This is with an AMC gearbox and clutch. When I did this job before, but using a Burman clutch centre and basket, the clutch sprocket was a little more inboard and the heavy duty chain rubbed the chaincase. A standard chain just! cleared it.

[Joker_Bones]

I'm flirting with the idea of adding a small modern Japanese generator -

I'm very curious about this...
I wondered as I was fitting the engine sprocket and shock absorber assembly if there was any way of dispensing with the shock absorber (there's cush rubbers in an AMC clutch) and fitting a rotor and stator.
What do you have in mind?

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