A Stuart 7A resurrected

[deverett]

Some time ago, I acquired a Stuart 7A as a long abandoned project from our favourite auction site. A lot of work had been already done but there was this nagging thought that a problem with the machining had caused the abandonment considering the condition of it. Still, it was worth the gamble. 'Collection Only' reduced the interest and so on a trip over to England, I arranged to meet with the seller at a convenient motorway service station to pick up the bits.

Here's what I got in the box



There was also this fearsome paraffin blowlamp which was an original Stuart item I was told. Long before the days of gas burners.


And underneath that low was a pleasant surprise


I thought it would make an interesting winter project.

To ease myself into the job, the first part tackled was the boiler hand pump. This was stripped and examined. All appeared OK, so it was reassembled and painted.


Next the engine was dismantled and de-rusting of the parts was undertaken. I'd heard that soaking in Coca Cola was one way to go about things; after all see how shiny coins come out when soaked! Results were so-so. A wire brush on the Dremel and some emery cloth worked much better.

Starting the reassembly with the crankshaft, I couldn't work out how the cap nuts had been secured (or remember how I removed them).


There was no clearance for any spanner that I had. The caps visited the milling machine and I enlarged the recesses so that a box spanner would fit the nuts.


So far everything was going well.

Dave
The Emerald Isle

 

[deverett]

I spoke too soon.

The main problem that I have come across so far is that the bottom holes in the cylinder had been put in the wrong location and were not spaced at 90 degrees. The bottom bolt hole of the cylinder as seen in the picture goes directly into the steam passage.



The task now was to drill new holes in the correct positions. I didn't want to put in any more holes than necessary, so the way I went about it is as follows:

The engine was mounted on a rotary table and the crankshaft was lined up with the X axis using a DTI. I used the bottom cylinder cover as a jig.



The bottom cylinder cover was placed on the standard and lined up as accurately as possible and clamped. The holes were then spotted through with a transfer punch.

A centring plug had already been turned up and then put in the drill chuck so that the bore could be centralised under the mill spindle. Dials were zeroed, then the table was moved over so that the cylinder bolt PCD was under the spindle.



Using a sticky pin to pick up the first punch mark, the cylinder bottom cover was secured so that a drill would go through an existing hole and a hole was drilled in the standard. Rotate the table so that the next hole was in line and so on for the 4 holes.



The standard with the new holes.



The two new holes on the right lined up correctly, but you may notice a slight discrepancy at left, particularly with the top one. I can live with this, to save drilling any extra holes.

Dave
The Emerald Isle

 

[deverett]

Next job was to get the cylinder correctly lined up and mounted to the standard.

After the 4 holes in the standard had been drilled, the crankshaft was lined up again with the X axis and the cylinder was put in place. Using a DTI across the port face, the cylinder was lined up as accurately as possible with the port face perpendicular to the crankshaft.



The steam chest was bolted to the port face and the valve gear temporarily assembled to check that the motion was free. When I was satisfied, the engine was removed from the rotary table and put in the bench vice to spot through the new holes into the cylinder bottom.



Now the cylinder could be mounted in the milling machine vice and the marks picked up with the sticky pin, each one drilled and tapped in turn. The picture shows the second hole being lined up.



The 4 new holes drilled and tapped, the old holes will be filled with JB Weld in time. (Note one of the original holes going through the steam passage).



Dave
The Emerald Isle

 

[Walsheng]

This is going to be fun to follow. Don't get to see a lot of reworks and thanks for all the pictures.

John

 

[digiex-chris]

can you describe picking up the punch mark with the sticky pin? Is it just a visual thing? Watching for movement as it enters and exists the punch depression?

 

[deverett]

The transfer punch leaves a very small dimple on the cylinder flange. When using the sticky pin, the way I do it is to ensure the point is running true with the spindle turning about 500 rpm. I have an old toothbrush for cleaning out the Tee slots and use the back of this to bear against the point to get it true (No chance of sticking ME!) When true, the spindle is stopped and I bring the point down to a very small distance above the flange - say 20 thou, not IN the dimple. Then it is just a matter of eyeballing the point above the dimple along the X and Y axes and moving the table as required until the point appears directly above the dimple. Good enough for the girls I go out with!

A similar method can be used in the lathe to centre a punch mark on a job in the 4 jaw chuck. The tool for this is usually called a pump centre. It is spring loaded and the point bears in the dimple while the other end sits on a centre in the tailstock. A DTI bears against the pump centre near the point. Turning the chuck by hand, you can see how far off true the job is from the DTI deflection. Adjust the job until there is no DTI deflection and the dimple is now running true.

Hope this explains everything for you.

Dave
The Emerald Isle

 

[digiex-chris]

Thanks! Makes sense.

 

[deverett]

To encourage me to press on with the work, I had a trial assembly of the bottom part. Looking good to my eyes - at least better than when I first took it on.


Next on to the valve gear.

There is no means of oiling the eccentric shown on the drawing, so I decided to make and fit an oil cup. Using a slot drill, I put a flat on the sloping side of the eccentric rod where a centre drill could start. Fortunately, I had a long series centre drill and slot drill. However, I needed to extend the tap. I have a few reusable extenders which have come in useful over the years.


These are simply pieces of round bar with a hole in each end to cover the shank sizes of the taps that I have. A grub screw locks against the square of the tap and this is trimmed in length so that it does not protrude and foul the job.


This pic shows the hole for the oil cup and the cup itself.


And the eccentric assembled.

Dave
The Emerald Isle

 

[deverett]

I now need to start assembling the valve gear and get this looking like an engine. However, another previous mistake has manifested itself: The threaded hole above the exhaust in the port face for securing the steam chest had been made oversize. The stud just dropped straight through. To rectify this, I enlarged the hole to 4BA and inserted a screwed plug which was filed flush with the port face. A drill was poked through the exhaust hole to clear the excess of the plug.



The plug was drilled and tapped 7BA. To find the correct place for the hole, the steam chest was fitted using the other 5 studs and the location spotted through the steam chest with the clearance size drill, just sufficient to make a dimple in the new plug.



The steam chest cover was undersize to fit flush with one side of the steam chest.



So I made a new one. A convenient piece of 3/16” steel was cut to the correct size against the steam chest.



Held in the bench vice by the clamp, the hole positions were transferred by the usual transfer punch leaving 6 dimples in the cover.



The cover was mounted in the milling machine vice and the six holes were picked up by the sticky pin and drilled in turn. Note the parallels placed so that the through holes would not hit them!



Thinking after doing it, if I had to do this job again I would prefer to spot/drill the holes first and trim the plate to size afterwards. It was just that the piece of steel used was an odd shape to start with and had to be squared up before I could hold it easily.

The outside of the original cover had a decorative (?) recess in the centre. This was replicated on the new cover. A punch mark was put in the centre of the embryo cover and it was then glued to a superglue chuck held in the 4 jaw independent chuck and centred using a pump centre.

A DTI probe was put against the dark part of the centre and the chuck turned by hand and adjusted until there was zero runout.



The superglue chuck is just a piece of aluminium bar with some light concentric rings. When the job has been completed, the glue joint can be destroyed by gentle heating. When the chuck is next used, the face can be skimmed to give a true surface.

There was no cylinder cladding (cleading according to one old British railway company) so I made up mine from a piece of litho plate which was easy to cut and finally trim. A piece of cereal packet was initially used to make a template, but was not very successful so I just used the bodger’s way of cut and try in the end. 10 BA was the size of screws used to hold it in place. To reduce the chance of breaking a small tap, I drilled for a 65% engagement. Plenty strong enough for this purpose.



The last job was to trim the studs so that they all had the same amount of threads showing above the nuts. Next, a couple of coats of paint – primer and then the traditional Stuart colours. Nearly the end now.



Dave
The Emerald Isle

 

[ShopShoe]

Nice Job. The colors really do look right.

Running Video?

Thank you for posting.

--ShopShoe

 

[deverett]

Not quite finished yet. When the engine is finished, I will tackle the boiler and the hope is to put the whole lot on a board as a 'steam plant'.

Dave
The Emerald Isle

 

[deverett]

Although cylinder drain cocks are fitted, if used as designed the condensate would blow over the flywheel. This is how most ST models are made, but I decided to make drain lines so the condensate would be directed to the ground.

Using some copper from a piece of earthing wire, I made up templates for the 1/16" bore copper tube. When satisfied with the shape, I bent the tube to follow the template. I don't have any 10 BA round head screws long enough, so will be using 8BA for straps to hold the pipes against the standard. (The copper tube came from a scrapped water cooler fountain).



I wasn’t sure how to secure the drain lines to the cocks, hesitating about soldering them in place. There was a suggestion about making pipe unions and screwing them, but in the end I took the lazy way out and soft soldered them in place. I will be running on compressed air only, so the soft solder shouldn’t be a problem.



This is how it has turned out so far, but not yet run. Still need some pipework to go to the boiler.

This will be the last post for a while until I finish work on the boiler. Other non-engine jobs are taking precedence at the moment.

Dave
The Emerald Isle