Category Archives: Engineering

Smart and Brown stripdown continues…

I’ve stripped the saddle off and have had the chance to see the good and the not so good.
The mechanism which shifts the main power travel feed to either the cross slide or the main saddle seems to be in really good condition and is a joy to behold.

Feed direction indicator on front of apron
Feed direction indicator on front of apron
In fact I think most things seem to be fine although I plan to diss-assemble, clean and lubricate everything even if there’s no actual damage.
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Both the top slide operating screws and nuts are very worn though as well as a few other bits and pieces.
Fortunately, I’ve made contact with somebody who has most of the bits I need, which is great.
The saddle is worn though and I imagine the top slides too, so there needs to be some precise measurements of wear and then some decisions made about refinishing work.
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This is a fairly big deal, but it can be sorted out.

I think when I’ve got the bits to repair it sorted out, then I’ll put a cover over her and leave her until the end of the season and just do some research on bed repair in the meantime. I don’t want to hurry this as it’s likely that I’ll have this lathe for a very long time. That’s my plan at least. Now I need to get back to the bikes. Really my priority has to be my Super squirrel engine, otherwise I’ll have nothing to ride this year. The Silk Scott racer’s frame is high on the agenda too.
Last week I picked up some 5mm MS plate to make some engine plates out of for the Norton model 18 and I have a plan for the Triumph engine…

Lathe inspection begins

Well, It has not been sitting in the corner of someones de-humidified workshop for 60 years with a dustcover on.

The cross slide and compound slide both had a bucketful of backlash, so I took them off last night to check out the leadscrews and nuts. I’ve stripped the cross slide completely and the leadscrew was pretty badly worn, with some damage which actually chased out the brass nut.

Cross slide lead screw
Cross slide lead screw
Amazingly I just found one on ebay(!) and snapped it up. I haven’t stripped the compound slide yet but I’ll probably do it tonight.
The next thing to do is to take the saddle off and look at the underneath.
Cross slides stripped
Cross slides stripped
Part of me really wants to strip the thing completely and have the bed re-ground, and scrape the saddle.. but that’s a reasonable amount of work.

Also I’ve found that the brass helical back gear drive from the main spindle is badly worn, but it might be ok. Wonder what the spares situation would be for that?IMAG0611

The more I look at this lathe, the more I like it. The details are superb.

Model M: forward/reverse
Model M: forward/reverse

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The eagle has landed!

A few extra bits of bracing on Sunday morning and a trip to Screwfix to buy a chain block and Sunday afternoon I lifted the lathe off the trailer! I did have to let the tyres down to get enough clearance to pull the trailer out but actually it all went perfectly. I borrowed some 1/2″ OD stainless rods that we’d had in an auction lot through work and dropped it onto those.
Once down I screwed an anchorage into the floor of the workshop and used the chain block again to drag it in, moving the rollers as necessary. It took a while but it’s now in position. Very satisfying.

Now I just need to get it to work...
Now I just need to get it to work…

I spent an hour or so last night cleaning, lubricating and inspecting it and can see there’s a bit of work necessary. There’s definitely wear on the tenons and the bed but I’m sure that I’ll be able to get it good enough for general work.

The lathe installation

It’s going to take me a few weeks to get to the point of it actually working but the next step is to actually get it into the workshop. It’s not that easy a thing to do, since the lathe isn’t exactly light but since it was such a lovely day, I thought I’d spend it cobbling together a gantry out of the half rotten pile of decking we removed from our garden at the end of last year. I’m loathe to post a picture lest the whole thing collapse tomorrow when I try to lift the lathe… but I’ll be optimistic!
I did think about hiring one, but I could well do with saving what I can for fixing bikes and trying to do some racing this year. I’ll post pictures tomorrow if it’s a success and if it isn’t I’ll pretend nothing happened.

The weekend’s spoils

The main point of this weekend was to go up to Worcester to pick up the lathe that I bought from a fuzzy picture on ebay. It’s a Smart and Brown Model M Mk2 toolmakers lathe from the 1950s and is soon to be manhandled (400kg?) into my little workshop. I’m going to need to get a phase converter as it’s three phase but I am really pleased. I think you can get much more for your money if you buy three phase.

It looks better blurred...
It looks better blurred…
This cost me less than ¬£200 (although a phase converter will probably cost as much). It’s a proper little tool-room lathe and it’s got collets and a three jaw chuck. I’ll just have to start picking bits of tooling up here and there and hope the thing works when it’s all in.

Also, I went up to see my dad to pick up my Scott racer’s cylinder block which he’s had to inspect. I thought I’d bring a few more Triumph bits down too but what I didn’t figure on bringing down was another bike; A Silk Scott.

Ten years ago or more, he bought this Silk Scott from Roy Lambert (not the late John Underhill as I had originally thought. Apparently John had owned it and sold it on previously).

The Silk Scott had been George Silk’s first incarnation of a Scott based motorcycle and had applied 1970’s two stroke tuning theory to the ports and the pipe of an otherwise pretty standard Scott engine. The cranks had been improved and the crank chamber sealing was done with a conventional rubber seal instead of the spring loaded metal to metal gland seal that the original Scott design used. He also created a better oil pump using, I believe, a modified best and lloyd pump design from the vintage period. He had a frame made for it out of Reynolds 531 by Bob Stevenson at Spondon which was basically a copy of the frames Spondon made for the small Yamaha racing bikes.

Yamaha AS-1
Yamaha AS-1

Here’s a picture of the Silk Scott prototype.

Silk Scott prototype

The road bike’s rolling chassis was finished with Spondon 38mm forks, a single sided twin leading shoe front drum and a mechanical disk on the rear. Aluminium rims and a light alloy tank certainly kept the weight down here at least.

Basically it’s a complete racing chassis, built to house an engine which had changed very little from 1928.

The soon-to-be Moss Silk Scott racer!
The soon-to-be Moss Silk Scott racer!

George undoubtedly released more power from the engine, but at a cost. Scotts are not a ‘Schnuerle loop scavenge‘ engine, they are a crossflow engine with the exhaust port and transfer ports opposite each other in the cylinder and using a deflector on top of the piston to send the transferred inlet gas into the top of the combustion chamber, thus scavenging the cylinder.

Deflector piston
Deflector piston

The Scott is notable as a two stroke for having a great amount of torque at low revs, probably because the design does not depend so much on gas velocity to achieve a decent scavenge. Loop scavenge engines, with the transfer ports adjacent to the exhaust port can be susceptible to losing charge directly through the exhaust if the revs aren’t high enough. There are other factors at play, but the torque of the deflector piston Scott really surprises people used to later loop scavenge designs.
If however, as is often done when tuning a loop scavenge engine for greater power, you raise the exhaust port and the transfer and extend the inlet duration, you tend to lose the bottom end. Maybe that’s ok when you’re able to get a engine producing a significant amount of power a bit higher up the rev range, but Scotts aren’t so keen to rev with that massive hump on top of the piston and also have completely unbalanced cranks, relying only on flywheel damping.
Plus the fact that the standard (long stroke) Scott only ever had a three speed box, and the Silk Scott only a four speed box doesn’t enable you to cover a narrower powerband and you start to see where modifications to the original design require an overall consideration of how these things link with each other.

Our plan is to build the Silk up with one of my dads racing engines, which only slight differences to the original port timings but has much better gas flow, and work to make a decent set of exhaust pipes to exploit the low rev range available.

We originally planned to make the Silk Scott a test bed for the engines we built for people so we modified the frame with the intention of doing a detachable front cradle to aid engine swapping.

It’s going to be tight to get it done this year, but I’m going to get on with it and see how it goes.

What have I done?

I like many people have a love/hate relationship with ebay. It requires a certain level of discipline, which late at night or when struck by the ‘I can’t miss this opportunity’ feeling, seems to be lacking in me occasionally. Mostly I find that these purchases were good ideas, but immediately afterwards I’m generally found shaking my head at my own impetuous behaviour.
So what did I do? I bought a lathe.
God knows what condition it’s in, I couldn’t afford to pay for anything that looked like it had sat at the back of someones workshop unused for 60 years so I’m fearing the worst. I know what it once was though and that was a very high quality 1950’s 4″ tool-room lathe, with screw-cutting capabilities. I’m just hoping that I can deal with whatever issues it has.

It’s a pretty difficult job to work on vintage bikes without having machine tool capabilities and a lathe is pretty fundamental. Especially if you are racing/ breaking bits. Fork spindles, spacers, gearbox bushes, hubs and drums, brake shoes, footrests, the list goes on.

I’m arranging to pick it up next weekend, so I’ll post pictures after that.