Building a Scrambles G80 Bitsa

[RichardS]

It's been a while with business committments really getting in the way of progress, but I have finally dragged this project forward.

After cleaning the spurious maybe-a-tank-mounting off the top tube I sent the frame for powder coating and it came back looking pretty good. Compared to some powder coating I had done some years ago which wsa utter rubbish, this is actually as good as real stoving as far as I can tell.
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I assembled the fork yokes and needed to decide what to do about adjustment. The original yokes had a nice fine thread and matching locknut, but the Commando yokes I'm using with the Roadholders have the tube down from the top (not up from the bottom) and a short, coarse thread on the bottom. That's not really suitable for adjustment purposes and although the thread is OK for just clamping, it's too short to allow double nuts. So, I made a spacer to go between the taper roller inners, deliberately slightly too short and then shimmed it to the right length to get a very slight bearing pre-load. I hate shims as a rule, but sometimes they're a necessary evil.
I was a bit concerned at the nut potentially coming loose, so I added a radial grubscrew to lock it with a slug of lead down the hole before the grub screw so I don't mangle the thread.
I wanted to have the top and bottom dust caps in stainless, but I found it too difficult to work - no great surprise, but a bit annoying. I ended up spinning them from alloy which flows easily. I added some felt rings inside to keep the dust and bugs out and the grease in.
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I seem to have spent an inordinate amount of time making various odd thread nuts, but it keeps me off the streets at night. The rear end is nearly sorted with SS nuts for the wheel spindle and brake. And the alloy chain guard I got at an auto-jumble eons ago is rather fetching too - I believe it is genuinely AJS, 'though may be an after-market product not OEM. It has the right mountings in precisely the right places and the right profile, was an alloy guard an option?
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I have also now added the proper barrel and piston to stop the cob-webs and bird's nests while progress is slow. On dismantling the donor for the cylinder and piston, I was disturbed to find that there was an evident "A hits B" issue with the exhaust valve.
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This is worrying and odd for two reasons. The parts are all standard, the cams were standard, not wild high lift ones and the piston also standard AE from the late '70's when I bought and fitted it. It is weird again because after initially rebuilding the engine way back, I did have a broken head sleeve nut (I still have the fragments) and subsequently replaced it. I am sure I would have noticed the piston damage then had it been there. So how did it happen later? Sticking valve? Maybe, but there is no evidence of that, the valve is perfectly in order, undamaged as far as I can tell, not bent and no obvious sign of collision.
After cleaning the scuffed area of the piston I coated it with marker and assembled the head on the engine and turned it slowly by hand. No scratches evident on the piston so I conclude it is at least clear now.

[RichardS]

I need to repair one of the fork sliders which has the standard Roadholder pinch-bolt crack. Not a great design detail that in my experience, I 'm not sure I've ever seen one that's not cracked. I have machined off the pinch-bolt lugs and I will add a couple of studs and an end-cap similar to Teledraulics. Once that's done and I've polished ths sliders I can get it onto its wheels which should be a milestone.

Obviously the rear suspension unit in the earlier photo is trashed. Being oil filled I can't have it powder coated, so I may have to get some new ones or swallow aerosol paint, but that's not really up to the job. And I need to make some caps for the swinging arm pivot - they were missing when I got the frame.

One frustrating stumbling block to over come is that I have a nice, resotred BTH mag for it, but after spending half a morning finishing off the water-jet cut alloy mounting plate and meticulously duplicating the original bolt holes, I found the damed thing has different mounting hole pattern compared to the normal Lucas part. So much for standardisation!

Than just to add insult to injury, a casual wave in the general area shows it's a different centre height too. Jeez, I have a BTH-KD1 on the trials bike and no drama, looks like another £30 with the waterjet man, no offence to him, but I'd just as soon not be spending it again.

[Joker_Bones]

So how did it happen later? Sticking valve? Maybe, but there is no evidence of that, the valve is perfectly in order

Is that actual collision damage to the piston? If I was just looking at the photo my interpretation would be some material had been filed away from the piston to avoid such contact with say a larger diameter ex valve?

[Joker_Bones]

I need to repair one of the fork sliders which has the standard Roadholder pinch-bolt crack. Not a great design detail that in my experience

Mine neither.
Tightening up the pinch bolt on an N15 last year, ohhh joy...

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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.

[Groily]

So much for standardisation!

Than just to add insult to injury, a casual wave in the general area shows it's a different centre height too. Jeez, I have a BTH-KD1 on the trials bike and no drama, looks like another £30 with the waterjet man, no offence to him, but I'd just as soon not be spending it again.

I guess there's only so much standardisation possible in these things, given manufacturers' foibles and preferences Richard!

In terms of spindle height, the motorcycle world is divided into 35mm and 45mm heights from the base. Lucas and BTH, and others, made some of each height. Eg Lucas N1 or SR1s for AMC singles at 35mm, all magdynamos and many other base-mount mags at 45mm. That difference followed manufacturers' preferences.
Threaded hole / dowel patterns on bases also followed manufacturers' preferences. If you ever see a Lucas magdynamo from a range of Ariel singles you'll see there is a 5th threaded boss on the underside, in the middle . . . because Ariel needed one. N1s made for stationary engines are, many of them, different from N1s for motorcycles. Some manufacturers preferred to use a strap and dowels to locate instruments - BSA in particular. Permutations galore, unfortunately.

On parallel twins, most are flange-mounted and do interchange between marques - a BTH KC2, with its fatter drive end flange needing longer studs, and its horizonatally-emerging HT pick-ups which can foul against cylinders, is a straight swap for a Lucas K2F if it will go on. Triumph used both in the '50s. KC1 BTH and K1F Lucas will interchange too, and you'll see either on Vincent Comets etc. You'll see a mix on Velo singles too.

So there are things that will and things that won't interchange, but not because of the magneto makers failing to standardise for selfish reasons. Key dimensions, tapers, contact breaker and camring sizes, brush dimensions etc etc were settled about 100 years ago, but that didn't get round needing to make different sub-classes of instrument, with the two main spindle heights and other variations, to suit the manufacturers.

A 35mm spindle can often be raised with a 10mm plate quite easily, but shaving 45mm ones down is fraught with risk.

[RichardS]

So how did it happen later? Sticking valve? Maybe, but there is no evidence of that, the valve is perfectly in order

Is that actual collision damage to the piston? If I was just looking at the photo my interpretation would be some material had been filed away from the piston to avoid such contact with say a larger diameter ex valve?

Hi JB,
yes the damage is exactly that, not file marks. I can say that for two reasons, 1)I fitted the piston from new albeit 40 years ago and definttely did not file the piston, 2) the damage is bright bare metal whereas the rest of the piston crown is mildly sooty as you'd expect.

[RichardS]

Groily,
thanks for the interesting reply. I was having a mild rant, but it's interesting to hear the internal parts such as cam rings are common yet the overall mountings are not. I knew there are various mounting / clamping options, but glibly assumed the holes would be in the same places. The fact that there are two specific centre heights is handy to know.

As I was once told: "the great thing about standards is that we can all have our own"

[Groily]

Groily,
thanks for the interesting reply. I was having a mild rant, but it's interesting to hear the internal parts such as cam rings are common yet the overall mountings are not. I knew there are various mounting / clamping options, but glibly assumed the holes would be in the same places. The fact that there are two specific centre heights is handy to know.

It was a good mild rant and highlights things people have to bear in mind!
I think I said 'dimensions' were standardised, rather than the actual bits though - 'cos they aren't the same. You can graft bits between makes, but it's not that easy for reasons too boring to go into here. And can be made a whole lot more confusing by the differences necessitated by opposite directions of rotation.
It was the First World War that drove early demands for standardisation - the military needed some flexibility to keep things running, and so various manufacturers were asked (obliged really) to make it so that their offerings could be used interchangeably. All through the vintage era you'll find near dead ringers for 4 cylinder mags for cars etc, and similarly for bikes, from different makers.
It was also in WW1that the Brits and French felt obliged to break the patents held by Bosch, on whom pre-war everyone had been excessively dependent, especially for aero engines. I think (but am not going off on a reading expedition right now!) that that is how there came to be 'American Bosch' in the USA, making ringers for the German instruments. It's also how some of the British eg particularly ML (and other) magnetos bore an astonishing resemblance to the Bosch ones.
After the Great War, it was 'reparations' which included (I think this is correct) the right for manufacturers who had broken the patents to continue, so Bosch couldn't regain what had been a valuable monopoly. Didn't stop them making some of the very best magnetos though, right through to the sorts of things that were seen on racing Rennsport BMWs.
Every time I see the Bosch logo on a washing machine or anything else they make these days, I smile and ask myself how many folk know that it's a magneto armature. I hope they stick with it forever!

[Joker_Bones]

yes the damage is exactly that

If that amount overlap occurred at the full lift of the valve with the valve still in contact with the rocker/push rod/cam it's difficult to imagine that something more catastrophic didn't occur.

So perhaps a sticking valve does explain it with the overlap occurring with the valve not in contact with the rocker allowing it to be pushed back. I don't know the exact relative positions of the piston and ex valve as they go though that part of the cycle so I am struggling to figure how that could happen.

[Joker_Bones]

Every time I see the Bosch logo on a washing machine or anything else they make these days, I smile and ask myself how many folk know that it's a magneto armature.

I didn't know that. But now I do... I'll be dropping it into conversation any chance I get