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Edward Thomas. 5" gauge. Hackworth Valve Gear.

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Chinkoa26/04/2022 16:45:23
35 forum posts
9 photos

Has anybody ever buil this Loco. Or had experience of the Valve Gear.

I am trying to get my Edward Thomas running on air and I am struggling.

The building instructions say the "D" Valve should be 1.15/64" long. The engine would not even look like it would run. The drawing has this Dimension crossed out and replaced by 1.5/64" with the modified length it will somtime do the long stroke between TDC. but will not go over TDC.

I drew it out to scale and came to the conclusion that the overall length of this valve should be .875" So I made two new valves to the 0.875"

It will now on one side go over both TDC. on one side and on the other the it will do the long stroke between TDC. but will not go over TDC. but if rotate the wheels a little it will kick in to a long stroke but stop at just at the next TDC. The other side rotates around TDC without a problem but stops at half long stroke. I am presuming the other sides stop it because it won't go over TDC.

I am still thinking the Valve is still too long. I have now got Lap and Lead to start and full port opening at full stroke

Any usefull suggestion appreciated

Jon Lawes26/04/2022 17:31:38
927 forum posts

The most experienced engineer I know with regards to both this locomotive and valve gear is a chap called David Clarke of Atlantic Engineering Design in the South West. I think his company is advertised on Facebook.

Be aware he is one of those people who is run off his feet with work despite never needing to advertise, so he sometimes takes a while to get back to you (and like any small business his knowledge is what pays his bills).

You could also try Sweet Pea owners; as you may be aware Hackworth is common on those.

Chinkoa26/04/2022 19:54:03
35 forum posts
9 photos


Thanks for getting back. I have E Mailed David at Atlantic Design. I look forward to his reply. I guess at this stage the most im portant thing is the exact length of the "D" valve. With that it should be straight forward!

I cannot find any modern up to date drawings as I think they are only done as a 7.1/4" now by Reeves. The drawings I have are very similar to the old "blue print"

David-Clark 126/04/2022 20:08:50
220 forum posts

I ‘think this may have been serialised in Engineering In Miniature many years ago.

Suggestion. Get Martin Evans book about locomotive or valve gear design and try and work out the valve events yourself.

Alternatively find a loco with similar port sizes and valve travel and copy the dliDe valve events/ sizes.

Chinkoa26/04/2022 20:42:08
35 forum posts
9 photos


I have the serialised magazine pages by Ross Harrison. One of the problems is that in his articles it gives a length for the Valve but the Drawing gives another.

I've got Martin Evans book but while I have been active in engineering for over 60 years done nothing resembling valve gear design, may as well be in Chinese.

I drew it out to scale and worked out what I thought the length should be. Much shorter, made two and it seem to be just about ready to run.

I will work on your advice with an engine with an erngine with similar valve gear and see if that sheds more light.


Nick Clarke 326/04/2022 21:18:30
1427 forum posts
63 photos

Before making a series of valves have you tried one of the simulators available?

I hesitated to suggest this as I got tied in knots and gratefully accepted help from MECH to come toa solution, but many people find them useful.

Edited By Nick Clarke 3 on 26/04/2022 21:19:05

Redsetter27/04/2022 03:00:34
203 forum posts
3 photos

Hackworth gear is not as simple as it looks, but as said, the Sweet Pea design is worth looking at and will be similar to yours.

Without knowing exactly what you have, it is hard to be specific but a few general points:

How have you arrived at the valve length? It should be the length over the steam ports plus an addition for lap at each end. The lap is typically from 75 to 100 percent of the port width. 75 percent is probably adequate in this case. The exhaust cavity in the valve should be centrally placed, and its length equal to the length over the port bars.

Are the valves positioned correctly on the spindles? They should open the ports equally (but usually not fully) at each end of their travel when the engine is in full gear, in both forwards and reverse though there may be slight variation between the directions. If you can achieve this by adjustment of the valve spindle, then the valve is not too long.

Are the return cranks correctly set? They should be at 180 degrees to the crankpin, i.e. exactly opposite.






Edited By Redsetter on 27/04/2022 03:01:18

Chinkoa27/04/2022 07:57:51
35 forum posts
9 photos


Thats shone a light on the problem thanks.

1 The Return cranks are set at 180 Degs.

2 Exhaust cavity is exactly central.

3 Overall length over steam Ports is 0.8135".

4 Steam Ports are 0.125" wide.

5 I am ok. with positioning the the ports opening equally. Why not fully open?

6 Valve travel is 0.390".

7 If I go for 75% Lap then that will be.0.093per side. If I add that x2 to 0.813". Give me a Valve length of 1.000"

8 Length of Exhaust cavity is 0.640" Is that to long? Port bar between inlet and exhaust is 0.125" central exhaust opening is 0.3125" Do you mean 0.125" x 2 + 0.3125" giving 0.5625" Which says the Exhaust cavity of 0.640" is to long?

julian atkins27/04/2022 09:07:42
1254 forum posts
353 photos

Hello Colin,

The variable is the amount of 'lap', but starting from the 'lap' being 3/4 of port width, the 1 5/64" dimension for the over all length of the valve is only just a tad more and will be perfectly ok.

Exhaust port width 0.3125" is a bit less than I would use but doesn't affect the maths.

The exhaust cavity on the valve will be 0.5625" with no exhaust clearance (which I wouldn't add anyway).

0.125" between edge of exhaust port and start of steam port.

Steam ports 0.125" wide.

The steam ports do not need to be fully uncovered in full gear; the exhaust should.



Edited By julian atkins on 27/04/2022 09:09:58

Redsetter27/04/2022 09:33:03
203 forum posts
3 photos

Julian beat me to it!

There does not look to be much wrong with those figures - we could argue about the finer points all day - so I suspect the basic problem is elsewhere especially as each side is behaving differently. Are the wheels quartered properly? Slide shafts at the same angle? Both eccentric rods identical?  Axles supported in working position?

I would not have added exhaust clearance, but it may be deliberate and isn't enough to upset things.

With 0.39 valve travel the laps could be longer, so the original 1 5/64 valve dimension might be about right. If the ports are opening fully to steam, it suggests the valves are too short.

It would need all the valve gear dimensions to analyse it properly, and this is not a 5 minute job.



Edited By Redsetter on 27/04/2022 10:01:56

duncan webster28/04/2022 13:47:04
3987 forum posts
65 photos

The mid gear valve travel is determined by the return crank radius and the dimensions of the vibrating lever. If you just set the lap without reference to this you could get too much or too little lead. I don't want to open another can of worms, but 0.01" is plenty of lead in 5"g loco.

If this doesn't make sense, I'll post a sketch. Don't expect brilliant valve events, Hackworth is not a good valve gear.

Edited By duncan webster on 28/04/2022 13:48:01

julian atkins28/04/2022 22:44:29
1254 forum posts
353 photos

Hi Duncan,

"If you just set the lap" as you stated bemuses me.

Notwithstanding the above, I recall Simon Bowditch providing some very valuable input to miniature locomotive designs with Hackworth valve gear.

Of note is that Colin (the OP) has yet to respond.

duncan webster28/04/2022 22:56:08
3987 forum posts
65 photos

Set as in decide upon. Once the length of the valve is decided on the lap and lead are fixed as well

Chinkoa01/05/2022 08:00:37
35 forum posts
9 photos

Thanks to both of you,

I will have to remake the Valves to get back to 1.5/64" Do I make the exhaust cavitys to 0.640" or go with Julians suggestion of .05625

Chinkoa01/05/2022 08:01:53
35 forum posts
9 photos


Please post a sketch re. 0.010"

Chinkoa01/05/2022 08:42:22
35 forum posts
9 photos

Julian and Redsetter,

Sorry for late reply but I have been responding to the notifications on my PC. But apparently they do not get tp ME Forums.


Not all resolved but Loco going round nicely now. Solution.

Redsetters comment " So I suspect the basic problem is elsewhere as each side is behaveing differently" These were my thoghts too as no matter what I did with valve length I always got the same result. After staring at it for some time the problem beacame obvious. The RH. side was working OK.IE. full stroke which in turn moved the LH. side over TDC or BDC then nothing. This said no air was entering the cylinder neither at TDC or BDC to move the Piston.

So I took the gear apart, pulled the Piston out and it became very clear what was wrong. The steam port drillings were drilled out of position (not by me. I bought it partly complete) As you know these drillings usually come out in the step between corner of the register and the rest of the endcap. In this case the drillings actuall came out behind the step into the outer flange where tho bolt holes are effectively sealing off the air. So evenwith a large chamfer it would still be blocked. A bit of carefull milling solved the problem. When I took off the front endcap it had the identical problem. Same solution. Runs well now on air.

Onto next problem. With steam port open about 50% and with the valve length set at 1.5/64" there is no lap or lead which suggest that the valve is too long. I feel some compromise have to be accepted? shortening the valve will mean a much wider steam port opening. Probably meaning the valve opening 0.125" inches plus.


Redsetter01/05/2022 09:07:03
203 forum posts
3 photos


Glad to hear you are getting somewhere. If it runs well on air it is not too far off.

Your last paragraph I'm afraid doesn't make sense as you can't measure lap or lead with the steam port 50 percent open.

Lap is the amount by which the valve overlaps the steam port with the valve in mid position - usually on the steam side so in your example with a 0.125 steam port if you had 0.125 lap the valve face would be 0.25 wide.

Lead is the amount by which the valve is open to steam with the piston at top dead centre.

With the valve length of 1 5/64, what is your opening point relative to TDC in full gear? This is easy to observe.

If there is no lead the valve will start to open at TDC. If there is 10 thou lead the valve will already be open 10 thou at TDC.





Edited By Redsetter on 01/05/2022 09:10:21

Edited By Redsetter on 01/05/2022 09:11:14

Chinkoa01/05/2022 09:22:18
35 forum posts
9 photos

To clear up. There is no lead at tdc by a long way.

I cannot do the the check on opening point in full gear as Loco is packed away in car ready to go up to Thurso for a m4 week holiday. I am taking Loco to give me someting to do?

Chinkoa01/05/2022 09:34:47
35 forum posts
9 photos

I've just drawn out out to scale to give a 0.010" Lead I need to shorten the v/v. by 0.098 overall to 0.980 from 1.078

Do you agree?

Redsetter01/05/2022 10:04:24
203 forum posts
3 photos
Posted by Chinkoa on 01/05/2022 09:34:47:

I've just drawn out out to scale to give a 0.010" Lead I need to shorten the v/v. by 0.098 overall to 0.980 from 1.078

Do you agree?


Don't fiddle with it!

While the lead and the lap are related, i.e., you can effectively increase the lead by reducing the lap, altering the the lap will also affect the timing. The lead is a design quantity, controlled by the geometry of the valve gear. It may have been designed with no lead, so don't worry about it unless you are prepared to work through the valve gear dimensions and find out what the designer intended .

Check the opening point as said - it should be either exactly at TDC or slightly before, but not later than TDC. The valve should close at around 75 to 80 percent of the stroke. In mid gear you should get the same opening point but hardly any valve travel.

If that checks out and it is running well on air I suspect it is probably OK.


Edited By Redsetter on 01/05/2022 10:05:07

Edited By Redsetter on 01/05/2022 10:10:32

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