Building a Scrambles G80 Bitsa

[RichardS]

This could end up being a long thread as there are kind of two projects in one. The main object is to resurrect the somewhat butchered late competition frame I have remarked on before into a G80 bitsa scrambler. The other is to convert my '49 trials to it's original 350cc and use the 500cc top end for the scrambler above.

As there are few issues cropping up regarding the rigid trials iron, I'll start a new thread for that.

I have a resonable pile of AMC parts, but from different models and years so given the damage done to the frame there's no point getting too excited about "originality". I will be sympathetic and use AMC parts, but rivet counters will probably blanche as I catalogue my mix and match approach on this occasion.

As I have remarked earlier someone had butchered the frame by variously grinding off lugs and drilling spurious holes in places, I assume in a lame attempt to save weight. One critical detail is that the tank mounting lugs had gone, so before I can send the frame off for finishing I needed to sort that out or I'll wreck the paint doing it later.

The original lugs are heavy tube brazed into the head casting, but with the casting sockets missing I decided to fabricate a short stub to braze in their place and a cross piece to become the actual tank support.

The originals are stub tubes, flattened on the under-side only to leave the top flat face near enough in line with the tube top OD. This is not how tubes usually flatten when squeezed in a vice or belted with a suitably large hammer. So I determined I would need a die and to clamp the tube to the table on the press so that all the forming would occur one side.

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I'm reasonably happy with the result, 'though if I did it again I'd put more bend in the middle on the cross piece and shorten the stub on the frame.
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As remarked earlier, I have decided to fit some Norton Roadholders and Norton 8" hub wheel and that has precipited changing the head bearings to taper rollers. I had mooted just making some adaptors and keeping the original ball races, but they're not very lovely and prone to floating about in the frame. I made a plug to fit through the ball race and register in the frame to use as a reference to ensure my new bore is concentric with and normal to the steering axis. I then mounted the frame on the mill and do the deed.
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A bike frame is not easy to mount and is distinctly springy which is bad for machining. So I made a couple of rough and ready brackets with Vees as clamps from scrap 1" bar and bolted those to angle plates bolted straight to the mill knee in place of the usual table. Took a few days to think through, a couple of hours to set up and 20 minutes to do the job, but that's how it goes. I noticed only just in time that the counter-bore for the taper roller outer ring is in danger of breaking through the edge of the head casting so I built it up around the outside before machining and ground it back to a fair looking curve.
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[RichardS]

I was dismayed to find the rear wheel bearings are trashed due to rust damage, presumably from condensation. It's very frustrating as they are otherwise perfect, but series of axial rust grooves in the race and flats on the rollers is not going to perform well however much grease I slap in there now. They are a non-standard bearing even within the Imperial range (groan, but no surprise) so I elected to make some adaptors to fit standard metric taper rollers and matching seals. Unexpectedly, the original seal liners are 24.0mm dia so standard metric seals are readily available and this contributed to the overall choice, I made the 1-3/4"-42mm bearing adaptor to accept the lip seal as well, all in one part.

A metric ISO-32004 (20x42x15) fits adequately inside 1-3/4OD, over 5/8ID and all I needed to do was add a little length to the 5/8"-20mm liner to make up the difference in length to fit with the original bearing spacer. Yes, as an alternative I could have made a new spacer with a bearing pressed onto each end, just chose not to.
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I made a small modification to the bearing adjuster/retainer so that the new seal will fit directly into that.
Change from £20 for 2 bearings and 2 seals and I spent a couple of hours at the treadle amusing myself making swarf - and it keeps me out of trouble.

I need to rebuild the wheel as the rim and spokes are badly rusted so while I have it all apart I can polish the hub.
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I 'm not keen on the ribbed hub which is effectively impossible to clean let alone brighten and much prefer the clean diabolo shaped versions. So while it's all stripped bare I did that deed too and the shorn hub machined beautifully and barely needs polishing. I though I'd keep the two outer most ribs as marginal additional stiffness for the spoke supporting edge of the hub.
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The drive pegs are rather sad too with flats worn on both the drive and over-run sides, but after polishing up a bit and rotating 90deg I'm hoping they'll be adequate. They are distinctly small for the job they do, the equivalent parts on my Panther are at least an inch in dia and on a significantly larger radius. I may yet feel compelled to improve the design, but for now I'll try to ignore it.

Thanks to respondents here who confirmed that the Norton and Matchless versions of the AMC gearbox are not at all conveniently interchangeable I decided not to pursue that approach. In very rough terms, the triangular end is horizontal on the bottom when fitted to a AJS/Matchless, but horizontal on the top when fitted to a Norton. I could have messed about with the mounting plates, but the result would have been distinctly inelegant and ungainly so I acquired a Matchless case off fleaBay and transplanted the newly serviced guts from the Norton box I had intended to use.

The waterjet cut alloy engine plates should be ready next week so I can check the assembly before sending the frame for painting. I'll update some more then.

[dave16mct]

Great thread. Thanks for posting all the pictures etc. Really interesting, keep it going! Glad you didn't keep the Norton gearbox case, it'll be much neater with the Matchless case.
Dave.

[mdt-son]

Hi Richard,
> I decided to fabricate a short stub to braze in their place and a cross piece to become the actual tank support.

I am impressed with your achievements, but would like to express a word of warning. If the short stub was braze welded to the malleable lug in a butt welded joint, as it looks to be, I don't think it will hold, especially under dynamic loading of combined shear and a bending moment. What you could do to make the joint last, is drill a hole (5/8" -- 3/4") in the malleable lug centric to where the casting stub used to be. Be sure not to penetrate the down-tubes. Prepare a special connector by turning. The connector consists of a cylindrical rod to the same length as the drilled hole, and a hollow cylindrical head, which has the diameter equal to the ID of your short stub. Length of the head should be approx. 3.5 x ID of the stub. Prepare the short stub by brazing the connector into place. Then join the stub w-connector to the bore by traditional brazing. Finally, fix the short stub to the malleable lug's surface by braze welding. This will produce a shear and bending resistant joint with some flexibilty to withstand dynamic loads.

- Knut

[Joker_Bones]

I do enjoy these build threads keep it coming!
We have similar projects but kind of mirror images... I built a 350 bitsa year before last, teledraulics and AMC front wheel, Norton back wheel and have just rebuilt a 500 comp style motor for it.

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I made a schoolboy error when I swapped a Norton box into a Matchless case... I used the gearbox sprocket that came with the Norton box, not noticing it was thinner than the Matchless sprocket, 1/4" instead of 3/8". Once the primary cases were fitted it was out of sight when I fed the rear chain on. It was months and miles later before it dawned on me what I had done.

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[RichardS]

Hi Richard,
> I decided to fabricate a short stub to braze in their place and a cross piece to become the actual tank support.

If the short stub was braze welded to the malleable lug in a butt welded joint, as it looks to be, I don't think it will hold, especially under dynamic loading of combined shear and a bending moment.

- Knut

I broadly share your concerns and although I _think_ it will _probably_ be OK, that's a long way from fully confident.
- but there wasn't anything left in the casting to put a hole into
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- it's been ground down to barely 1/16" (1.6mm) thickness around the tubes, less in places and is gone altogether in the area where the stub tube is brazed on. After initially attaching the stub in place I did 'forge' (bend) a tongue forward to meet with the remains of the bottom of the original casting so the stub is attached 'all the way round', but it's uncomfortably close to hanging in fresh air

The stub is pretty substantial 1-1/2" dia x 3.5mm wall ( sorry for the mixed units) and I did contour the face to match the remains of the frame castings so it is a bit more than 'just stuck on', but you have pin-pointed an area I was already uncomfortable about - bugger!
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I think the only viable option is going to be to add a couple of small gussets, one each side, below the stub, maybe one larger one up the middle above the stub instead would be less obtrusive and could be much larger. I'm leaning for the latter as I'm pondering on it.

[mdt-son]

Richard, I didn't know the malleable lug is hollow between the legs. Fitting a 45 degree gusset plate up the middle is very good. Let it extend to the outer edge of the flattened tube, and allow a certain height here, ca. 10 mm.

- Knut

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[RichardS]

Here's the lastest update, I think the gusset should keep us both reasonably content. The brazing is a bit below par, I initially tried using a newly acquired TiG set as it seemed a nice option, but it made a monumental mess. Clearly I have a way to go learning to handle that. I recovered with a good old gas torch, but it's a big lump of iron so acts as an efffective heat sink and the braze spreads as the hot zone is too wide and at the same time it's easy to overheat the relatively thin gusset. It'll look OK once it's been to the painters

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The new wheel rims turned up early ( I was quoted 3 weeks, 10 days ago!) so I bunged the rim on the restored rear hub. Not trued and centred yet, but a blind man would be happy to see it.
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Obviously I'll need a new spindle too in due course or at least the nuts and spacers as what's there is badly rusted. It's only the tip of the spindle that's visible when it's installed so a polish up on the ends and it would be OK I guess.

[RichardS]

I am having a dry-run assembly of the bike before I send the frame off for painting and I am confused about the rear tank mounting. I have an alloy tank and I bought the Rear Mounting Rubber from the AMOC which turns out to be a horse-shoe shaped part that evidently goes around the frame tube. Is that correct for an alloy tank?

The frame has a kind-of lug with a 1/2" hole which I assumed was for a rubber mushroom bung of some sort. If not that, what is is for?

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Did I buy the wrong rubber part?

[mdt-son]

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