Portable Engine Model Engineer Article by Tony Webster

Well the "corrected" drawing that has been published in 4429 STILL HAS ERRORS, lets hope someone will be third time lucky when the second revision is published.

Also hope no one has made the valve chest yet as the tail rod bush detailed in 4429 may be a bit of a loose fit, I find drilling a 3/16" hole does not give much thread engagement on the specified 3/16x40 thread!! The 1/4" hex on the bush has also been drawn as 1/4" across corners not AF

Couple of 1/16" dimensions on the valve block would also help beginners.

Edited By JasonB on 01/05/2012 19:45:22

Hi there

the blind bush clearly states 1/8 inch.

Where is the 3/16 hole dimension? It appears to have been left off my issue.

The 1/4 inch Hex is given.

Nowhere does it state corrected on the the cylinder saddle drawing.

It says the incorrect drawing was printed and this is the correct one, not corrected one.

Where would you like the 1/16 dimensions, in the corners of the cavity or on the recessed edges of the valve?

Either way, who can say if they are 1/16 or not.

What errors are there on the cylinder saddle drawing?

It was corrected by Tony.

Edited By David Clark 1 on 01/05/2012 22:55:20

Edited By David Clark 1 on 01/05/2012 22:56:08

Off out to work but will reply later. to drawing errors

"Nowhere does it state corrected on the the cylinder saddle drawing.

.......................................................

It was corrected by Tony."

I think you just said it yourself

J

Edited By JasonB on 02/05/2012 07:55:52

Blind Bush.

1. This is shown with a 3/16x40 male thread. if you look back at issue 4427 the valve chest that this is meant to screw into is shown with a plain 3/16" hole at each end. That is why I said that I hope no one has made the chest as it should obviously now have had one end threaded 3/16x40 for the bush to screw into.

While correcting the valve chest Its worth pointing out that there is no form of gland shown on the rod side either so may be worth sorting that out at the same time.

2. the drawing may well say 1/4hex but that is not what the drawing shows. If you look closly at the hex part it is drawn with one of the flat faces facing the viewer which means there are two of the corners of the hex top & bottom in line with the 1/4" dia of the end of the bush. This would show that the bush is made from 1/4" round stock and the hex is machined into that so the max AF hex size you could get is 0.216"

If the 1/4 hex dimension shown on the drawing is correct as you say then the part will need to be made from 1/4" hex stock and the larger distance across the corners should be shown on the drawing.

Valve

1.Yes 1/16" should be on the recessed edges of the valve. "Either way, who can say if they are 1/16 or not." This is exactly why the dimensions should be there we should not have to make an educated guess or in the case of a beginner an uneducated one.

Valve rod

Would have been good to show the flat on teh 1/8" reduced dia part but as its in the text I'll let that one go.

Drawing of the corrected saddle to follow. Any thoughts on that sketch I posted on the guide bars? as the 3/8" dimension you measured would throw a lot of other things out.

Edited By JasonB on 02/05/2012 14:11:45

Cylinder saddle issue 4429

As said above I feel this drawing is still wrong, the reason being that the position of the 1/4" steam feed hole and two of the 4 mounting hole are dimensioned at 3/4" and 3/8" in from the edge respectively. They also have the same dimension in from the end of the inverted cylinder plan that was in issue 4426. The problem is that the base of the cylinder is only 1 15/16" long but the saddle is 2" long so no allowance has been made for the 1/32" difference at this end but it has at the other because the holes are shown at 9/32" which is the 1/4" + 1/32".

Click on the pic below to get a larger view of what I feel is correct in black and what the drawing in 4429 gives is shown in red.

Comments from Tony or David awaited

Jason

Just back to the valve again.

The valve chest is shown with a cavity 3/4" wide yet the drawing of the valve has the 13/16" dimension fitting into this 3/4" space

EDIT when the valve is assembled with the nut this will increase the whole assembly by another 1/32" giving an overall width of 27/32 which is an even more impossible fit into the 3/4" space

Edited By JasonB on 02/05/2012 16:36:34

Any comments yet on the errors I have raised that were in 4429, would be nice to get an answer before 4430 drops on my mat.

Oh and of course all the other queries over the last six weeks.

J

Hi There

The following will be published in ME 4431.
Some changes will be made if I get the information in time. Comments are welcome. This text is an attempt to get it into order.

The drawings for the portable engine have some errors and omissions. After discussion with Tony Webster, we have come up with some clarification and corrections.

The boiler barrel is eight inches long, with 3/8in. cut off at the firebox end for the lower one third of the circumference.

There are five bushes in the boiler, there is one each side of the barrel at the front so there is a spare for a hand pump. The hole sizes for the bushes can be obtained from the bush drawings.

The holes for the eight corner stays for fixing the horns should be 7/32in. dia. i.e. the 1/4in. dia. is reduced slightly in diameter to provide a 'head' to stop them falling through when silver soldering. The head is on the outside which makes it a two operation job. Reduce to 7/32in. dia. part off, chuck the other way round to centre, drill and tap 4BA. (It also cleans the bronze surface for silver soldering.)

The overall width of the firebox is 4 3/8 inch. The inside width of the horn plates is 4 3/8. Clearly there is no room for the head of the corner stays.

I think the stays go right through the inner and outer firebox side with the inner side holes being
7/32 and the outside being 1/4 with the outside of the stay flush with the outer firebox?
Also, can you supply a sketch of the corner stays to help clarify this?

In the end section to the right of the drawing on page 364, the 11/16in. dimension to the first row of stays should be from the top of the foundation ring as shown in the door plate drawing to the left. Also the 1/4in. should be for the foundation ring only and not include the firebox wrapper plate.

The front to back location of the water gauge bush and manifold will be detailed in a further instalment of the series. There is no harm in having a static connection to the water gauge, i.e. a pressure gauge where there is no movement of steam or water through the manifold. I agree that there must not be a connection to the blower or injector etc. from the water gauge. This would have a misleading effect on the water level in the water gauge.

The 1/2in. hexagon for the top water glass connection should not have the 3/16in x 40 tpi hole drilled and tapped yet. Drill 6BA clear and tap the hole in the boiler 6BA. Fix with a 6BA brass screw.
After the boiler has been silver soldered the screw is drilled out and the outer part tapped 3/16 x 40. This is for a banjo pipe connection to the pressure gauge.

Boiler: The height of the backhead is 6 inches.
The height of the firebox is 4 1/4 inches, neither is critical.
The wrapper plates are probably overlong and will need trimming to match the flanged plates.

The 7/16in. boiler tubes should be 20 swg; a standard thickness that should be available from your friendly model engineer's stockist. The tubes rise towards the front of the boiler.

Continued in next post.

Before fixing the skirt length (see below) check that the lower tubes will fit the lower holes in the firebox tubeplate, clear of the inside of the barrel.
The important dimension, when setting out the tube positions on the firebox tubeplate, is that the top row of tubes should be 11/32in. from the inside of the flange, (or 15/32in. from the outside of the flange if marking out on the other side of the tubeplate). Do not drill the 7/16in.dia holes until the flanges have been made.
When the drawing was redrawn for publication the firebox tubeplate thickness of 3/32in was not transferred.
The 'skirt' i.e. the firebox inner wrapper should be 5/16 inch below the outer wrapper. The foundation ring should be 1/4 inch square all round but you may be only able to obtain 6mm square which will be fine.

The outer door plate (backhead/front plate) and the firebox doorplate should be parallel to each other and vertical.
The firedoor and the water gauge bush should be on the same horizontal centre line and the stays should be equally above and below this centre line and maintain the 11/16in vertical centres. They are offset 1/8in. from the lower ones to place them nearer the centre of the area alongside the firehole.
Get the blowdown bush as low down as you can, i.e. 7/32 inch above the foundation ring.

The drawing says two off regulator bushes, as far as I can see one fits the top of the barrel and the other goes on the backhead.

Q. Should there be any thickening pad inside the boiler to mount the crankshaft bearings? A. The Crankshaft is mounted around the boiler by a strap so no thickening plate should be needed.

Q. Should there be blind bushes to mount the firedoor Hinge

Q. Confirmation of an available thickness for copper in substitution for the 3/32 inch and 1/8 inch thickness shown on drawing. A. Go slightly thicker not thinner.

Q. Will the cylinder stud holes be detailed on the valve chest drawing? A. The positions for the 10 off valve chest stud holes are detailed with the valve chest.

Q. For the cylinder drawing the text says machine the valve face back to lines 1 and 2 but drawing shows it at least 1/32 inch away from them. A. Machine to the line as the text says.

Q. The length of cylinder and protrusion of end faces are not shown on the drawing? A. They can be found in the text.

Q. There is no depth for the steam port slots. A. They must not beak into the cylinder bore.

Q. Width of valve chest given as 3/8 inch? A. This should be 1 3/8 inches.

Q. No length is given for the piston rod. A. I am awaiting the length.

The 1/4in piston rod dia. should be stepped down to 3/16 inch and threaded 3/16 x 40. To save creating a stress point at such a step, taper at 60 degrees inclusive and 'countersink' the piston with a combination centre drill before drilling tapping size for 3/16 x 40.

Q. Crosshead has no thickness given. A. 3/8 inch should be fine.

Q. No depth for piston ring groove. A. Make deep enough to take packing or alternatively use O ring charts to fit an O ring.

Q. No length of 1/4 x 40 thread on piston rod. A. Make to a length to screw into the crosshead as far as the slot, approximately 5/16 inch.

Q. No depth for cut out in crosshead. A. 9/16 inch will be OK to clear little end.

Slide bar drawing

Q. The plan view gives width over the outside of the bars and bracket as 15/16 inch. A. This is correct.

Q. The plan view gives one end of the bars at 13/16 inch centres and the other end at 11/16 inch centres? A. This is correct; you will have to relieve the slide bars at the left-hand end, as drawn, to clear the oval section.

Q. The end elevation gives the bracket as 1 inch wide and the bars project beyond the bracket. A. The bars should not project outside the 1 inch dimension. I think the 1 inch is also related to the 1 inch diameter that enters the cylinder.

Q. The oil cup does not show the hole carried through the slidebar. A. It goes right through the top slidebar.

The valve chest

The blind valve end is shown with a 3/16 x 40 male thread, if you look back at issue 4427 the valve chest that this is meant to screw into is shown with a plain 3/16 inch hole at each end. That is why I said that I hope no one has made the chest as it should obviously have had one end threaded 3/16x40 for the bush to screw into.

While correcting the valve chest it’s worth pointing out that there is no form of gland shown on the rod side either so may be worth sorting that out at the same time.

Cylinder saddle.

Q. Is this the actual view of a developed shape of the curved part.

Q. The 1/8in. dimension at the bottom left is wrong, it should probably be ¼ inch.

Q. The two 9/32 inch dimensions on the left seem wrong as the hole will break through the edge, 11/32 inch would be nearer what is right.

I feel this drawing is wrong, the reason being that the position of the 1/4 inch steam feed hole and two of the four mounting holes are dimensioned at 3/4 inch and 3/8 inch in from the edge respectively. They also have the same dimension in from the end of the inverted cylinder plan that was in issue 4426. The problem is that the base of the cylinder is only 1 15/16 inch long but the saddle is 2 inches long so no allowance has been made for the 1/32 inch difference at this end but it has at the other because the holes are shown at 9/32 inch which is the 1/4 inch + 1/32 inch.

continued below

Edited By David Clark 1 on 11/05/2012 19:08:05

The valve chest is shown with a cavity 3/4 inch wide yet the drawing of the valve has the 13/16inch dimension fitting into this 3/4 inch space.
When the valve is assembled with the nut this will increase the whole assembly by another 1/32 inch giving an overall width of 27/32 inch which is an even more impossible fit into the ¾ inch space.

The valve chest has the valve rod sliding directly in it with no gland. Surely this is incorrect?

Is it correct that the cylinder does not sit centrally on the curved flange front to back, or should the 3/4 inch dimension for the position of the 1/4 inch hole be 15/32 inch to allow for the 1/32 inch projection of the cover faces beyond the cylinder.

regards David

Edited By David Clark 1 on 11/05/2012 19:12:03

Hi Jason

On page 2, you have transposed the 3/8 and 5/16 dimensions.

5/16 + 5/16 for the slide bars + 5/16 for the slot = 15/16 which is correct.

If you make the right hand diameter 1 inch when it is cut away to take the slidebars it will probably be about 15/16 at the edges.

The centres of the slide bars should be 11/16 inch, not 13/16 as drawn. (The screw centres are still 11/16 and 13/16 respectivley.

regards David

Edited By David Clark 1 on 11/05/2012 19:38:24

Hi There

Most of these dimensions are easy enough to figure out but I have spent a lifetime in engineering. This is what sets us apart from the 'Armatures', experience and common sense.

regards David

Hi David and indirectly Tony.

I have looked through the above and list below my comments. I have tried to make it a bit easier to follow by starting each paragraph with J or T so we all know who is saying what. I have only listed things that are still awaiting answers or that I do not agree with. David if you could forward this to Tony It would be much appreciated.

T.The overall width of the firebox is 4 3/8 inch. The inside width of the horn plates is 4 3/8. Clearly there is no room for the head of the corner stays.

J.How do you get this figure? If boiler barrel is 4.0” and there is a 3/32” wrapper that makes a total width of 4 3/16” (4+3/32+3/32)

J only other thing I can see a possible clash with is the cylinder saddle will foul on the large bush on the firebox crown but will await the final drawings

JQ. Should there be blind bushes to mount the firedoor Hinge

TA........

JQ. There is no depth for the steam port slots.

T.A They must not break into the cylinder bore.

J Or the exhaust passage drilling which is nearer the surface

JQ. Crosshead has no thickness given.

TA. 3/8 inch should be fine.

J. NO this will not fit the bar spacing it should be 5/16 to match the flat on the end of the cylinder cover as described on page 502

Slide bar drawing

J Basically my hand drawn one is correct so David you may use this if you want in the mag but see last few items at teh bottom

The valve chest

J.The blind valve end is shown with a 3/16 x 40 male thread, if you look back at issue 4427 the valve chest that this is meant to screw into is shown with a plain 3/16 inch hole at each end. That is why I said that I hope no one has made the chest as it should obviously have had one end threaded 3/16x40 for the bush to screw into.

J.While correcting the valve chest it’s worth pointing out that there is no form of gland shown on the rod side either so may be worth sorting that out at the same time.

T.A........

Cylinder saddle.

J.Q. Is this the actual view of a developed shape of the curved part.

J.Q. The 1/8in. dimension at the bottom left is wrong, it should probably be ¼ inch.

J.Q. The two 9/32 inch dimensions on the left seem wrong as the hole will break through the edge, 11/32 inch would be nearer what is right.

J. I feel this drawing is wrong, the reason being that the position of the 1/4 inch steam feed hole and two of the four mounting holes are dimensioned at 3/4 inch and 3/8 inch in from the edge respectively. They also have the same dimension in from the end of the inverted cylinder plan that was in issue 4426. The problem is that the base of the cylinder is only 1 15/16 inch long but the saddle is 2 inches long so no allowance has been made for the 1/32 inch difference at this end but it has at the other because the holes are shown at 9/32 inch which is the 1/4 inch + 1/32 inch. See also my drawing in a previous post and wrong items in red

TA.....

J.The valve chest is shown with a cavity 3/4 inch wide yet the drawing of the valve has the 13/16inch dimension fitting into this 3/4 inch space.
When the valve is assembled with the nut this will increase the whole assembly by another 1/32 inch giving an overall width of 27/32 inch which is an even more impossible fit into the ¾ inch space.

J.The valve chest has the valve rod sliding directly in it with no gland. Surely this is incorrect?

T.A.....

T.On page 2, you have transposed the 3/8 and 5/16 dimensions.

T.5/16 + 5/16 for the slide bars + 5/16 for the slot = 15/16 which is correct.

J. No the 5/16 is mentioned in the text as the size to machine the flat on the cylinder cover, this sets the vertical gap at 5/16” you yourself said you went out to the shed and measured the width as 3/8

T.The centres of the slide bars should be 11/16 inch, not 13/16 as drawn. (The screw centres are still 11/16 and 13/16 respectivley.

J. This just contradicts what you said above if the bars are spaced on 11/16” cts then the overall width will be 1” not 15/16” (11/16 + twice half of 5/16 = 16/16 )

T.If you make the right hand diameter 1 inch when it is cut away to take the slidebars it will probably be about 15/16 at the edges.

J. Outer edge of slide bars should be 1” as two reasons above so to give the bars full support the spigot on the end of the cylinder cover should be 1.048” as shown on my version of the drawing

Have a good evening, Jason

Edited By JasonB on 11/05/2012 21:06:10

David,

Speaking as an amateur in terms of my own engineering experience, I don't think experience and common sense should be seen as a prerequisite to overcoming someone elses shoddy workmanship. Whether amateur or professional, you need a set of accurate and well annotated drawings to convert 2 dimensional pictures into 3 dimensional stuff that hopefully fits together, works and looks like what the originator intended.

I accept that errors will occasionally creep in for various reasons. However in this instance the number and range of errors and omissions highlighted by Jason in just a few magazine issues is frankly well beyond acceptable especially in a magazine that aspires to promote model engineering excellence.

I'm sure there are a great many model engineers who have no formal engineering education or experience who look to this magazine and others like it for inspiration and guidance. I personally have always held engineering and engineers in high esteem. If this fiasco is representative of British engineering then I shudder to think how something as complex as Concorde ever took to the skies, always assuming it wasn't actually designed as a submarine and something went wrong between the drawing office and the machine shops!

Regards, Martin.

Martin,

I think your comments about Concorde has upset everyone in the Patchway/Filton and surrounding Bristol area and I was involved at the time with the drawing office and this project. Around 1950 my father taught me the rudiments of how to use a Myford ML7.

Now task in hand
David is the Editor and cannot build every item that arrives on his desk. Yes I am sure there are articles that come in where the drawings have a multitude of errors. Try the Rob Roy ones for starters.
I built a 7.25g Loco from a series in ME and the author assured me that as he was useing CAD and every drawing was spot on. Have a look at the "Anna" forum. Some chaps gave up after spending a lot of dosh..
Is everyone who is inputting into this thread actually building this project it would be an interesting survey.

David

I think I have worked out why text is going under adverts it is so simple it is silly will PM you

Bob

Hi Bob

Sorry, it wasn't my intention to upset the good people of Filton especially since I have to work there occasionally! I was hoping the exclamation mark punctuating the end of my offending sentence would have indicated its tounge-in-cheek nature, perhaps I should have included a nice big smiley.

On the subject of Concorde, I well remember being in Bristol the day she came back home for the very last time. I was up on the downs by the suspension Bridge and it seemed Bristol in its entirity had turned out to welcome her back. An amazing and emotionally charged day, one I shall never forget. Its sad to see her ending her days parked up all alone in the far corner of Filton airfield.

Anyway, back to the subject of this thread, I appreciate that David can't build every model to prove the design and drawings but surely they should be proof checked prior to publication to weed out all or most of the errors that Jason has pointed out in his extensive postings? Whilst professional engineers may well be used to dealing with poor drawings, surely a construction series aimed at beginners should be backed up with accurately annotated plans? As an amateur with no formal engineering training I have enough problems dealing with my own shortcomings when I'm making something, surely I shouldn't have to sort someone elses cock-ups aswell before I can confidently start making swarf?.

Regards, Martin.

Hi There

I do check for errors but usually I am checking that the illustrator has not altered anything by mistake.

We are on a production line, 3 magazines every 4 weeks and no holiday or sick cover if any of the production team take time off.

Also, we usualy check each part of a series. Not the entire series.

We do one part of a series and forget it until the next part is due.

We do not have time to edit an entire series at once.

This is why the valve chest 3/16 thread got missed.

regards David

Hi There

Concorde looks like a brilliant design.

When it used to visit the Bournemouth airshow, it used to bank and turn just in front of my house.
It was a superb sight.

Unfortunately I don't think I worked on any Concorde parts unless you count the candlestick jacks that supported the wings? during maintainence.

regards David

The following will be published in ME 4431.
Some changes will be made if I get the information in time. Comments are welcome. The text is an attempt to get it into order.

The drawings for the portable engine have some errors and omissions. After discussion with Tony Webster, we have come up with some clarification and corrections.

The boiler barrel is eight inches long, with 3/8in. cut off at the firebox end for the lower one third of the circumference.

There are five bushes in the boiler, there is one each side of the barrel at the front so there is a spare for a hand pump. The hole sizes for the bushes can be obtained from the bush drawings.

The holes for the eight corner stays for fixing the horns should be 7/32in. dia. i.e. the 1/4in. dia. is reduced slightly in diameter to provide a 'head' to stop them falling through when silver soldering. The head is on the outside which makes it a two operation job. Reduce to 7/32in. dia. part off, chuck the other way round to centre, drill and tap 4BA. (It also cleans the bronze surface for silver soldering.)

The overall width of the firebox is 4 3/16 inch. The inside width of the horn plates is 4 3/8.

The stays go right through the inner and outer firebox side with the inner side holes being 7/32 and the outside being 1/4 with the outside of the stays protruding 3/32 inch from the outer firebox? The stays are stepped to fit into the 7/32 inch holes.

In the end section to the right of the drawing on page 364, the 11/16in. dimension to the first row of stays should be from the top of the foundation ring as shown in the door plate drawing to the left. Also the 1/4in. should be for the foundation ring only and not include the firebox wrapper plate.

The front to back location of the water gauge bush and manifold will be detailed in a further instalment of the series. There is no harm in having a static connection to the water gauge, i.e. a pressure gauge where there is no movement of steam or water through the manifold. I agree that there must not be a connection to the blower or injector etc. from the water gauge. This would have a misleading effect on the water level in the water gauge.

The 1/2in. hexagon for the top water glass connection should not have the 3/16in x 40 tpi hole drilled and tapped yet. Drill 6BA clear and tap the hole in the boiler 6BA. Fix with a 6BA brass screw.
After the boiler has been silver soldered the screw is drilled out and the outer part tapped 3/16 x 40. This is for a banjo pipe connection to the pressure gauge.

Boiler: The height of the backhead is 6 inches.
The height of the firebox is 4 1/4 inches, neither is critical.
The wrapper plates are probably overlong and will need trimming to match the flanged plates.

The 7/16in. boiler tubes should be 20 swg; a standard thickness that should be available from your friendly model engineer's stockist. The tubes rise towards the front of the boiler.

Before fixing the skirt length (see below) check that the lower tubes will fit the lower holes in the firebox tubeplate, clear of the inside of the barrel.
The important dimension, when setting out the tube positions on the firebox tubeplate, is that the top row of tubes should be 11/32in. from the inside of the flange, (or 15/32in. from the outside of the flange if marking out on the other side of the tubeplate). Do not drill the 7/16in. dia. holes until the flanges have been made.
When the drawing was redrawn for publication the firebox tubeplate thickness of 3/32 inch was not transferred.
The 'skirt' i.e. the firebox inner wrapper should be 5/16 inch below the outer wrapper. The foundation ring should be 1/4 inch square all round but you may be only able to obtain 6mm square which will be fine.

The outer door plate (backhead/front plate) and the firebox doorplate should be parallel to each other and vertical.
The firedoor and the water gauge bush should be on the same horizontal centre line and the stays should be equally above and below this centre line and maintain the 11/16in vertical centres. They are offset 1/8in. from the lower ones to place them nearer the centre of the area alongside the firehole.
Get the blowdown bush as low down as you can, i.e. 7/32 inch above the foundation ring.

The drawing says two off regulator bushes, one fits the top of the barrel and the other goes on the backhead.

Q. Confirmation of an available thickness for copper in substitution for the 3/32 inch and 1/8 inch thickness shown on drawing. A. Go slightly thicker not thinner.

Q. For the cylinder drawing the text says machine the valve face back to lines 1 and 2 but drawing shows it at least 1/32 inch away from them. A. Machine to the line as the text says.

Q. The length of cylinder and protrusion of end faces are not shown on the drawing?

A. They can be found in the text.

Q. There is no depth for the steam port slots.

A. They must not break into the cylinder bore or any of the steam holes.

Q. Width of valve chest given as 3/8 inch?

A. This should be 1 3/8 inches.

Q. Crosshead has no thickness given.

A. 3/8 inch is fine.

Q. No depth for piston ring groove.

A. Make deep enough to take packing or alternatively use O ring charts to fit an O ring.

Q. No length is given for the piston rod.

A. My drawing says approx 2 3/4 long. Check on job before finalising.

Q. No length of 1/4 x 40 thread on piston rod.

A. Make to a length to screw into the crosshead as far as the slot, approximately 5/16 inch. The 1/4in piston rod dia. should be stepped down to 3/16 inch and threaded 3/16 x 40. To save creating a stress point at such a step, taper at 60 degrees inclusive and 'countersink' the piston with a combination centre drill before drilling tapping size for 3/16 x 40.

Q. No depth for cut out in crosshead.

A. 9/16 inch will be OK to clear little end.

Q. The valve chest is shown with a cavity 3/4 inch wide yet the drawing of the valve has the 13/16 inch dimension fitting into this 3/4 inch space.

A. The slide valve should be 13/16 long and 3/4 inch maximum wide to fit in the steam chest. The dimensions have been accidentally reversed.