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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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Posted (edited)

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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Posted (edited)
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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We are still pottering on but only slowly. There are other things which need doing at the moment! In the mean time, Adrian has kindly welded up the rear wing brackets and I have been filing some shape into them. That is proving to be very tedious and time consuming but I have three down now. I have only to make the front ones to go with them!

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For a bit of light releif, I have cut the pedal slots in the floor.

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I am a bit concerned about the strength of the item though so I have cut and rebated a strip of steel along the front to brace it.

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Looking through my notes, I made a decent sketch of the original pistons in 2013.

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Apologies for the drawing clarity or lack thereof. However, assuming that the bore was 4.500" originally and that there was no wear on the piston, the clearance above the top ring is 0.017", between the rings, 0.012" and for the remainder of the piston, 0.005". Once we get the top off, I will have a measure and see what I actually did!

I am sure that I gapped the rings in accordance with what was on the packet.  I would quite like to do some sums to check so can anyone give me a feel for how much hotter the rings could be than the bores? I am guessing at 150°C but I would value your views.

Steve    :)

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Hi Steve

I have always set the ring gaps to those that I was taught during my training and have never had any problems.

For bores 2 to 3" - 2 thou per inch of bore pro rata

For bores 3 to 4" - 3 thou       Ditto

For bores 4 to 5" - 4 thou       Ditto

Sorry but I can't give any info on ring temps. 

Hope this helps.

John

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Good Grief !

I think this might be a little too sophisticated for a WW1 engine. The major variable in the case here is "does it work?"

When they pull a cylinder off I think it will be very obvious where it has been getting tight and simply giving it a few thou extra clearence should make a huge difference. My inclination with the ring gaps is to make them deliberatly rather wide. This engine runs with much lower cylinder pressures than even a WW2 one and will do such a low annual mileage that fuel efficency and oil contamination are not important within reason. It just needs to start and run reliably. Making it a bit loose might make it easier to crank too...

David

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Posted (edited)

Sorry, re-thought this post and decided it might cause more problems than it would solve so decided to delete it.

Edited by Barney

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Posted (edited)

Well, I have found my way to Devon again with the intention of putting the lorry away and then making some more progress. Since I crocked my arm, the lorry has been sitting outside as I have been unable to start it  to back it into the shed. Three weeks later, I am mostly recovered but still cannot start the beast due to the damage. Our pal couldn't make it to help out ( he is moving house!) and our neighbours both have bad backs! The only solution was to push it back. Whilst we were coming to this conclusion I fitted some more bits, starting with the reinforcing strip around the tailboard.

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Then the capping strips along the body sides

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And the floor strip just inside the tailboard. It was a nasty wet day as you can see.

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The we decided to have a go at manhandling the thing back inside. At four tons, this is a bit of a performance. Dad pulled it back with the car to start with and then I steered it forward, downhill on full lock.

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I managed to misjudge it and Father had to shift some of Mother's plants.

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Then we worked it back in using a pinch bar, 1" at a time. We got there in the end but it was hard work. After that we carried on with some more bits. I trial fitted my floorboard and, much to my surprise, it was right first time.

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Then I set in a flush ring to allow us to get the board up.

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That was all satisfactory so Father now has the board in the paint shop.

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Then, this morning, we decided that the time had come to pull the engine down and have a look.

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Now the lorry was in its usual position, the chain block could be installed.

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Not immediately too bad.

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Some scuffing along number two piston and all of the rings were stuck.

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A shiny patch on the wall of number two.

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Interestingly, there is a ring towards the top of the bore which can be felt with a finger nail. I can't see how it was formed unless it is just where the piston ring stopped and dragged up the surface, the next time it moved.

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Interestingly number four piston is clean where the others are well sooted.

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A close up of the damage to number 2 piston

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Number four piston was very clean and all of the rings were free.

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Number 3 piston is scuffed as well but not quite so badly.

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I had a good measure of all of the bores and pistons and find that I machined them all straight with a nominal 0.005" clearance from end to end. Cylinders two and three appear to be a thou or possibly two smaller than one and four and I think that must be the trouble. My plan now is to remove all four pistons and un-stick the rings from two and three. Then I want to set them up in the Myford and take 0.006" off the top lands in each case as well as giving them a polish to tidy up the scuffing. I also plan to get 0.003" honed out of each bore to tidy them up and give a greater clearance. Hopefully, that will sort it.

Then I went on to the front wing brackets. This is a horrible job and I am not enjoying it at all.

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More wing brackets tomorrow.

Steve :)

 

 

 

 

 

 

Edited by Old Bill
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Well that's bad luck and good luck, isn't it?  Bad luck it didn't work first time, and good luck that nothing got ruined.

I'm not seeing anything on there that won't respond to a re-size, clean, and polish, but maybe some of this rings might not be safe to re-use?

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Gordon,

I'll second the ring problem.

I wouldn't like to use any rings out of cylinder 2 and probably 3 as well.

For the cost of a couple of sets of rings it could save a lot of heartache and work.

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Posted (edited)

Well, it’s not too difficult to fix. Replace ALL of the rings, hone ALL the cylinders, .009 minimum. On such a low use vehicle, and low speed engine I think I would go to .010 or .011 for ease and safety. Remember you have NOT run that engine hard yet, just for a short time at idle or above, under maximum load for longer than thirty minutes I think you need more clearance.  Just .003 isn’t enough in my opinion. I spent the last week at Lake Como for the Concours D’ Elegance with our 7.8 Litre Rolls Phantom II, with a 4.25 inch bore six cylinder engine. The motor was built “tight” and we had the cast iron pistons at .010, while it runs and starts fine, with everything new including the radiator and all internal parts and bearings, it’s still running just a bit warm if we idle it for twenty minutes at a standstill in any temp over 20c. I’m sure it will be fine in a few hundred more miles. I would change the oil on the Thorny two or three times, right away after you reassemble it. I would recommend a five weight oil for the first two changes, then I would go to a straight 30 or 40. After five hundred miles of running I would then go to 100 percent synthetic. Photo enclosed of the 7.8 Litre P II with me in front of it yesterday at Como. You gentlemen do great work, and I look forward to visiting you with a car to take you for a ride in the future! My best, Ed

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Edited by edinmass

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3 hours ago, edinmass said:

Just .003 isn’t enough in my opinion.

The opinion of Messers Thornycroft seems to differ, though. If I recall correctly the goslings used 3 thou because that is what it said to use in the Thornycroft manual. 

I wonder if it is possible that they were using a special grade of iron, though? 

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