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WW1 Thornycroft restoration

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Thanks for your kind comments Ed. I am very interested to hear your experiences of piston clearances. Are these for alloy pistons or iron? I am very keen on doing the job as closely to how it was when new as possible so we have made iron pistons for it and fitted the rings we could get. I really don’t want to go to aluminium pistons even though they are so much better! If memory serves, I gave them a five thou clearance along the length with 0.009” above the top ring and simply turned them circular.

Another of our forum friends in South Africa has very kindly sent me these experiences of a local restorer

‘He used Model T pistons as an example as those were the most recent pistons he had worked on. (3 3/4 bore.)

1. The pistons were machined perfectly round and 4 thou clearance with the bore size.

2. Pistons were then tapered 30 thou from the crown to the oil ring. (Crown expands more due to higher temperature)

3. He then machined a dolly to hold the piston and clamped it in a 4 jaw chuck.  It was then off-set 5 thou on the one side and then on the other side to allow 10 thou total ovality. The thrust side will expand more due to heat build up.

When you remove the first block on your engine, you should see scoring marks on the piston and possibly the bore. The scoring on the piston will indicate where the material will be removed to make the piston oval.’

These views are similar to yours. Also, I was reading the FWD Model B drivers manual, as one does, and found this:

‘The clearance between the cylinder and piston, at the skirt, is 0.003” to 0.004”, under bottom ring 0.006”, at the third land 0.008”, at the second land 0.009” and at the top land 0.012”.

They don’t write drivers’ manuals like that any more!

All of this suggests that I am going to have to set the pistons up again and skim them, a job I don’t want to do! Oh well.

Steve   :) 

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Posted (edited)

Steve, The earliest car we have serviced is 1897, and the newest is a current Top Fuel (nitromethane) dragster, so we have been all over the board when it comes to pistons, piston troubles, and attempted solutions. Most of what we have is 3.5 inch bore plus or minus, but we have done six inch down to just over two inch. My first paying job as a teenager was running a boreing bar machine after school and weekends. It’s best to just cover your current situation here to keep things short, and I will address what I think I would do. I’m not sure it will be very helpful, but I will try. 99 percent of the time we use custom pistons made for a application that the piston manufactures make all the decisions. We learned over time when we got in the middle of the engineering and decision making we ended up with problems! You have many issues in play on your Thorny, new sleeves, modern fuel, possible jetting issues, home made pistons, unknown alloy, etc. In the current situation I would do a tear down of the engine before I made any decisions. I expect that I  would probably increase ring gap, and depending on what you find, I would tend to leave the pistons alone and hone the cylinders. I’m quite sure your problem won’t run in and solve itself. I understand and respect the reason you made you own pistons.........it’s an impressive accomplishment and very few people I know have done it. The question now is, keep what you have and attempt to fix it..........and you may or may not be successful on the first try. I understand the challenge of getting it right after all you have done on the vehicle. I would tend to order modern pistons, as you know the bore and stroke, and have an old piston sample, it would be easy to just order a set of new modern pistons, drop them in and forget it. BUT after following along with you for years on both of you trucks, I’m sure you much rather deal with the problem and finish the job you started. Cam ground pistons were the norm for years, but if one doesn’t have access to a machine I think you can still move forward. As the engine turns quite slow, I think you can solve most of your problems by opening up the ring gap and piston clearance. I would go to .009 or .010 on the piston clearance. I would set the ring gap at .016 on the compression rings. You can turn them and taper them as mentioned in you post similar to what you friend mentioned on the Ford T, and I don’t think think you will risk anything. With such a low compression ratio, leak down and blow by won’t be an issue. Holding things tight on this particular job isn’t too your advantage. If you build it loose, the only downside will be piston slap when cold. Run the fuel mixture rich, and add as much timing as you can. Lean mixtures make the motor run hot, and can burn valves, melt pistons, and crack rings. Does the thorny have a thermostat? Most engines ran at 140 before the 1920’s, and went to 160 by the mid 1930’s. Also, with today’s modern fuel you want to run a hotter spark plug if available. We use a five gas exhaust analyzer when we set up the carb and ignition. I’m looking forward to see the pistons and cylinders when you take it down. Also, have you done a compression check on the engine yet? It’s a worthwhile exercise to see what they are wet and dry across the board. I’m sure others here will chime in. I look forward to their experiences and responses. Ed.

 

 

Edited by edinmass
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Make up 1 copy of the pistons being used in the thornycroft... 

Get a temperature on the engine of when it locks up... 

Mic the copy cold...then heat it up in an oven to same temperature...mic it again and compare it to the bore... 

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Flandersflyer. A lot of common sense in your suggestion. One wrinkle I can see is that they made their pistons round, but previous comments say they should be oval. Does that matter on such an ancient engine?

Is it just a case of proportional expansion or do they have to factor in the need for "ovality"?

Doug G.

Australia

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I think that pistons were generally machine circular in the time period when these vehicles were made, and one reference I found seems to suggest that this was the case right up to WW2. ( http://www.voc.uk.com/net/docs/3.1/3.1-653-15.pdf )

 

Also bear in mind that the clearance required for an aluminium piston will be much more than that for an iron piston, which might go some way to reconcile the disparate numbers suggested here (from 3 thou specified by Thornycroft to 12 thou specified by a poster here)

I suspect that the Goslings might even be able to fix their problem with a bit of judicious piston swapping. Maybe the biggest piston is in the smallest bore at the moment and things are otherwise within spec. 

 

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Having a DVD in the toolpost brings a whole new meaning to CNC ! I am a bit concerned about wear on the feedscrew  and nuts though.

On the question of pistons though, all will become much clearer if the engine is taken to pieces but my feeling is that a couple of thou extra clearance in the bores is all that is needed and that will have no detectable down side at all.

David

Edited by David Herbert

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32 minutes ago, David Herbert said:

Having a DVD in the toolpost brings a whole new meaning to CNC ! I am a bit concerned about wear on the feedscrew  and nuts though.

It runs with ballscrews on both axes. 

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The Perkins p series engine may give some your full info on piston size ,engines rated 1500 rpm and below had iron pistons industrial ,1500 up road engines ,alley pistons .

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After seeing the photo's of the steep drive I kinda remember that when I visited some years ago. Must be re-assuring to go downhill knowing the brakes work.

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On the subject of starting aids, I recently swooped on an original impulse coupling, still attached to a Simms SR4 X on eBay. The asking price was horrendous but after some serious haggling (and no other bids !) I managed to cop the lot for a realistic sum. Unfortunately, the mag armature has 'semi toasted' sometime in the past and the bearings are completely knackered but there are many good parts to be harvested. The prize find is the impulse unit, marked 'Type X' and apparently in good working order. I will add some pics.

One puzzling thing is the apparent lack of any means of lubrication for this item. Later versions show an oiling tower with a sprung lid, but there is nothing on this one.

I am now looking for a 19/20 tooth Simms rubber vernier coupling and a 19 tooth backing plate with clamp, to complete the ensemble. All this is required for a new Thornycroft J Type project, recently acquired from Australia of which more later.

Tomo

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On 5/14/2018 at 1:35 AM, edinmass said:

Steve, The earliest car we have serviced is 1897, .....

Thanks for all of your thoughts, Ed,  and everyone else's contributions. I think the next step is to tear it down, look for evidence and measure what we actually have. I don't really want to skim the pistons if I can help it but we shall see. It may be very obvious where the problem lies so we just have to have a look. That will be part of the task for the next visit.

An impulse starter is definitely on the cards. The difficulty is that very few magnetos have the holes to mount the trigger plate. I have a variety of impulse bits so I think I shall have to put my thinking cap on and make up a 'special' to suit the lorry. I have a similar problem with the Dennis but space is very tight in that case and another special will be needed but this time, anti-clockwise. I must go and get the bits out and study them.

Steve   :)

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Quote

I spotted these but the 19 tooth section needs a split clamp arrangement to attach to the shaft. I don't think there is enough meat on the casting to provide this ?

Tomo

 

A while back my students reverse engineered a magneto coupling used on early Wisconsin engines. Here is a rendering of the assembly. The disk is leather. The splined hub is keyed to the input shaft the eared coupling is keyed onto the magneto.

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I am not sure if this would help but its an alternative. We have a set of shop drawings and a set of patterns as well.

 

Best regards,

Terry

 

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Edited by Tharper
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