WW1 Thornycroft restoration

[Hoseman]

you know what.... i recon given a sand pit big enough and lump of wood these guys would cast a full sized thornycroft in one go!!!! amazing the lengths you guys need to go to get things done. beautiful as ever gents (the workmanship that is!!!) lol

[Gordon_M]

You'd want the core to hang down into the cavity, as when you put the iron in the core would tend to float under the hydrostatic ( ferrostatic ? ) pressure. The sand core has reasonable strength in compression but very little in tension.

[joeferret]

Great thread.........

[GeePig]

So that is how it is done! Proper colours too!

 

I see that the core hangs down into the cavity whereas mine has the crown at the top. That way, the dross will end up in the skirt rather than the crown. I think I will get away with it as any dross will end up in the chucking piece on mine.

 

 

 

I suppose that rather begs the question of where you would want the dross to end up - in the hot, thick part at the top of the piston that takes the thumps, or the thinner, lower parts that are more lightly loaded? I have no idea which would be the better choice.

 

Trevor

[Gordon_M]

I suppose that rather begs the question of where you would want the dross to end up - in the hot, thick part at the top of the piston that takes the thumps, or the thinner, lower parts that are more lightly loaded? I have no idea which would be the better choice.

 

Trevor

 

If you were doing quantity production, you'd put them head down in the mould and vent off the thin top edge, allowing any slag to come up into the vent and overflow slightly, then get ground off.

[Old Bill]

When I made our first piston patterns for the Autocar many years ago, I did them like this and they worked well. It was only afterwards that I thought about the cores floating off! I hadn't thought about putting a large flange on the end of the core to trap it but it is an obvious solution now. Never mind. Father can take these to the foundry and we can hear what they have to say. Mother took the last ones in and she ended up with a guided tour of the foundry!

 

If I have to remake them, sobeit but I hope they can manage. We shall see.

 

I still have the onion pattern to make and then the water outlet as well so there is still plenty of opportunity to practice.

 

Steve

[Gordon_M]

The lift effect is primarily due to the volume of the iron, so as your volume is relatively low you might be OK, worth remembering for next time though.

 

The foundry will advise on what material they have available to cast them in. If given the choice I'd go for SG Iron ( Spheroidal Graphitic ) which is half way between cast iron and steel in its properties. If it has to be 'ordinary' cast iron you'd want as high a strength as you could get, grade-wise.

[mtskull]

Thinking outside the box, is the decision to go for new cast iron pistons for practical reasons or simply for authenticity? Do the pistons have to be cast iron at all? Wouldn't it be easier to do them in aluminium alloy, or even machine an existing piston to fit? -just asking out of curiousity; this is in no way intended as criticism of the fantastic work being done here.

[Old Bill]

What I really wanted to do was to just skim the original pistons to fit. However, we have lost 3/16" on the bore and the piston walls are just too thin to allow us to take enough off, hence the new ones.

 

Our aim is to have a 100 year old lorry that looks to have been cared for using factory parts. The factory would have used iron so we are doing the same. Now, we bend the rules a bit with our silver soldered fabrications but we generally try to follow the original process wherever we can.

 

I am still pondering as to how the factory would have machined the pistons. Possibly, they may have started with the bottom of the skirt, facing and boring and then by cutting the gudgeon-pin holes. Next, using a dummy gudgeon pin and a draw bolt through the lathe headstock, the piston could be pulled back onto the faceplate or a locating disc to turn the outside and face the crown. They would not have had a chucking piece like ours as it is a wasted piece of metal. We don't have the facility to bore the gudgeon pin holes and that job will have to be put out. Using the chucking piece, I can complete all of the other machining processes before having the hole bored.

 

Steve

[Asciidv]

I am still pondering as to how the factory would have machined the pistons. Possibly, they may have started with the bottom of the skirt, facing and boring and then by cutting the gudgeon-pin holes. Next, using a dummy gudgeon pin and a draw bolt through the lathe headstock, the piston could be pulled back onto the faceplate or a locating disc to turn the outside and face the crown. They would not have had a chucking piece like ours as it is a wasted piece of metal. We don't have the facility to bore the gudgeon pin holes and that job will have to be put out. Using the chucking piece, I can complete all of the other machining processes before having the hole bored.

 

Steve

 

Here are some pictures showing the process that Steve has just described.

 

The piston skirt has been faced and bored so that a spigot the diameter of the piston may be fitted. A gudgeon pin has been fitted with a block which will have a threaded rod screwed to it. This will pass through a hole in the spigot and pulled up tight against the end of the lathe headstock.

 

Here the spigot is held in the chuck of the lathe and machining of the outer diameter is taking place. In this instance we are turning down an oversize piston. It was quite easy to make the original piston run true to a 'thou' before machining took place.

 

Finally a comparison with the new and old. Pistons are never constant diameter, always smaller around the upper ring grooves to allow for expansion. Many pistons are taper turned. Pistons fitted to my 1930's Leyland engine were both tapered and oval. I am sure that would be a challenge that Tony would takei in his stride!

 

 

Barry.

Edited April 28, 2013 by Asciidv

[Richard Farrant]

hi Steve,

Pistons are generally turned oval on the skirt, with the thrust sides the larger diameter. Maybe this is only on alloy pistons as cast iron do not tend to expand as much?

 

regards, Richard

Edited April 27, 2013 by Richard Farrant
cross posted with Barry - oval skirt !!

[Gordon_M]

A bit of trouble-avoidance here then. I'm assuming at least one of the original pistons, in one of the engines, came out in bits, or is otherwise completely past it.

 

Take it with you to the foundry and get them to break it on the spot and do a visual check of iron grade & type on the clean break. If the originals were SG iron and you replace then with plain iron it"ll be a problem - best to check.

[David Herbert]

I have an idea that SG iron was invented / first used just after WW1 but I probably should have looked it up before writing this!

 

David

[David Herbert]

I have just looked it up on Wikepedia (the font of all knowledge) and was suprised to read that SG iron was invented in 1943.

 

Steve, you say that you would have to put out the boring of the gudgeon pin holes but I thought that you had access to a Bridgeport or similar which is what I would use. The key is getting the axis of the gudgeon pin exactly square to the axis of the piston and the right distance from the top and on centreline.

 

David

[Gordon_M]

I have just looked it up on Wikepedia (the font of all knowledge) and was suprised to read that SG iron was invented in 1943.

 

David

 

That's that then. On the other hand, if the foundry was casting SG, it would be a nice WW2 upgrade ...:cool2:

[Great War truck]

Whilst the “Leicester Division” has been pattern making, the “Devon Branch” has been working on the Steering Box. The actual “Steering Screw” is still in Leicester and awaiting straightening, as you may recall that we found that it was bent when it was removed from the “Box”. The objective in Devon was to get everything ready for reassembly as soon as the straightened “Steering Screw” came south again. These three pictures show the whole “Box” again before it was taken to bits, just as it came off the chassis.

 

 

 

 

The actual “Box” came apart comparatively easily and after the usual procedure of washing the two halves in a paraffin bath and then wire bushing them, they look quite reasonable.

 

 

 

 

[Great War truck]

The cleaned bits were then washed down with cellulose thinners before priming with Bondaprimer – a cellulose based primer.

There are two big ball races in the mechanism – one of them is at present in Leicester but the other one still in Devon has cleaned up beautifully and can be used again. The one in Leicester is yet to be cleaned but we are hopeful that it is in similar good condition

 

 

 

 

 

When the quadrant lever and shaft which is fitted in the “Box” was removed from the “Box”, it brought the two bronze bushes in which it runs, with it, as they had become firmly attached. It appeared that all that was holding them on the shaft was congealed oil. They could not be levered or slid off – some gentle heat was applied to get them to budge – but each one had to be heated several times as they would only move a short distance before they cooled and became stuck again.

If they had come off easily, they might have been fit to be used again, but sadly, they became damaged during that removal process. Fortunately, we already had a small length of suitable shell bronze in stock – bought many years ago as “might come in handy, one day” and two new bushes were machined up.

 

 

 

[Great War truck]

The bushes are just an interference fit in the two sides of the case and they were easily pulled into position with a draw Bar. A quick initial trial fit looked good.

 

 

 

 

 

Loosely bolted together – all it needs now is the Steering Screw and Steering Nut

 

 

[Great War truck]

Steve joined Tony in Devon over the weekend as they intended to go to Brighton together on the Sunday to see the completion of the London to Brighton Vintage Commercial Run, but they had the Saturday free and wanted to do something to the Dennis. It has been “bugging” Steve for sometime that there was a stiff or tight spot in the reverse gear of the gearbox which we re-built about 20 years ago and put to one side then until it was required and he was determined to look at this again before the gearbox was fitted into the lorry. The original re-build of the gearbox is covered briefly on page 2 of this “blog”.

 

The gearbox had been stored on the bottom shelf in the Garage and has picked up some dust and muck on the outside over the years! It was lifted out and the strip-down was easy and a real change from the dirty and rusty items that we usually have to deal with as that part had all been done before!

 

 

 

 

 

The following sequence of pictures show the strip down again are all really quite self explanatory and show the sequence in which it was done.

 

 

 

 

 

The first of these two pictures show the wear on the teeth of one of the gears – we put this down to “heavy” gear changing by the drivers over the years! It should not cause us any concern.

 

 

[Great War truck]

The strip down continues – the second and third pictures in this sequence of three show the reverse gear shaft – just to the left of the big bearing – being pushed out.

 

 

 

 

The Reverse Shaft is out and can be seen standing on end by the “4” in the last of this sequence of three photos. The shaft was re-measured and it was found that there was the slightest of tapers of one just one “thou” on this shaft and that was causing some of the trouble. So the shaft was put back in the lathe and the excess was taken off with some emery tape. There were also a couple of high spots in its bronze bush and these were scraped out. This took care of the problem and the stiffness has gone.

 

 

 

 

Then it was just a case of re-assembling the box – just reversing the strip down procedure previously shown. Everything is now turning freely in the gearbox and the casing can now be properly cleaned again and finish-painted ready for placing in the chassis. Paper Gaskets have been made to go under the covers. The gearbox has been placed upside-down on a mobile bench for ease of painting.

 

 

[fesm_ndt]

Are the wingnuts on the top cover just for your assembly or is this how it was done?

[Minesweeper]

Are the wingnuts on the top cover just for your assembly or is this how it was done?

 

This, apparently, was how it was done. There is no separate oil-filler hole in the top, so it seems that the whole top had to come off for filling the gearbox with oil! The wing nuts on it at present are the more modern type but we have just found some older style ones on E-Bay with the correct for the period, "Mickey Mouse" ears!

 

Tony

Edited May 9, 2013 by Minesweeper

[GeePig]

Such a huge box, with so much space between the gears and the sides, so much extra oil required....

 

Maybe the condition of the gear teeth had something to do with the necessity to remove the lid to check the oil level, meaning it did not get done as frequently as required.

[David Herbert]

[quote name=GeePig

 

Maybe the condition of the gear teeth had something to do with the necessity to remove the lid to check the oil level, meaning it did not get done as frequently as required.

 

Or a driver who couldn't change gear without loud crunching noises!

 

Many years ago I had to dismantle a car gearbox and found that the reverse idler gear had lost about half the length of its teeth. Not knowing any better I built them up with weld and ground and filed them to shape by eye using the unworn part as a guide. I was always a bit gentle putting it into reverse but it lasted about 40,000 miles and was still fine when I sold the car!

 

Interesting that on the Thornycroft the 'gate' is a simple H with the four forward gears as you would expect but reverse is an extension of the movement into first, so there is a neutral both sides of first. Was this common in trucks of that period?

 

David

Edited May 9, 2013 by David Herbert

[Asciidv]

This, apparently, was how it was done. There is no separate oil-filler hole in the top, so it seems that the whole top had to come off for filling the gearbox with oil! The wing nuts on it at present are the more modern type but we have just found some older style ones on E-Bay with the correct for the period, "Mickey Mouse" ears!

 

Tony

 

Tony, surely your Dennis gearbox was just like this with a cover plate and wing nuts?

 

Barry.