Old Bill

The time has come to start putting the engine back together! The first job, of course, was to find the bits which proved to be a bit of a challenge so it will be good when they are all bolted together again. We started by hanging all of the con-rods on the crank and nipping them up to see how they fitted. They were not excessively worn but all had a little sideways wobble and a longitudinal clearance could be felt when pushing them up and down. To remedy this, I filed 0.006 - 0.008" from each shim in stages until when we did them up again, they gripped the crank. We blued them and rotated them around the crank in turn to find the high spots in the white metal which were then scraped back and the process repeated until they could be rotated around the crank smoothly without wobble. One question I would like to ask is how tight should they be? I just keep going until I think they feel 'right' but I have no idea what that should be! I must say, though, that we were subject to official supervision throughout the process. Now I am going outside to do some more. Gasket fitting today! Steve :-)

Doug. When we first got the chassis, it had been protected underneath a shepherd's hut and there was one good patch of army green which a friend of mine brought up with Brasso. The third batch of paint that we bought for the Autocar and all subsequent batches have matched it exactly so we are confident about the colour of our British vehicles. We used the same paint on the FWD because we have no better information for a US lorry but suspect that it may have been lighter originally. Leaving the Thorny chassis outside under a sheet for twenty years had, unfortunatley, destroyed that patch of original paint and the sand blasting finally finished it off. Steve

Mind you, military vehicles were grey with black lettering up to about 1915. Steve

Yes, it certainly feels better to be able to put things together, if only to reduce the amount of stuff on the floor around us! Steve :cheesy:

That would be interesting to see. We had a slotter at work but it was scrapped a few years ago (it was marked 'War Finish'!). I never saw it in action. I worked at Aveling Barford for a while and they splined gears by pulling a single long broach through. Long means over ten feet! I never saw that in action but the tooling was all stored around it. Steve

Thanks for all the ideas Chaps. I will let you know how I get on! Steve

Wonderful job! Thanks Cel. Steve

Hi Cel! Great to see some positive progress again! Please can you tell me how you went about re-metalling the big ends? Which bits did you heat and did you 'tin' the shells? I did the Autocar years ago and they have survived, more by luck than judgement I think. Do I understand correctly that you built up the main crank shell by adding white metal a bit at a time with a soldering iron? That sounds quite a difficult procedure which I would never have thought of doing and I would love to hear how it went. Keep up the good work! Steve :thumbsup:

An interesting idea. What is the 'right consistency'? The cork has to hold together as part of the tap. I don't want it falling apart and go losing a tankful. We have to pay twice to fill it as it runs beyond the maximum spend! Steve

That's an interesting thought Andy. Laying my hands on liquid nitrogen might be a bit tricky though! I am currently thinking of grinding it by using the Dremel with a small wheel, mounted in the toolpost. I have managed to drill a hole through a reconstituted cork from a bottle so I might be able to mount it on a mandrel and attack the outside. A bit of experimentation is needed I think. Steve

What a wonderful piece of work! I am looking forward to seeing it in the flesh! Cheers! Steve :thumbsup:

I'm afraid we are rather taking some liberties with this wheel. It was originally a steel tube rim surrounding a cast steel centre, not aluminium, and coated with celluloid or similar all over including the hub. We are using aluminium just because it is the easy way out and it will be disguised by the powder coat. Of the ones we have seen, all are in poor condition except the gun lorry of course. Unfortunately, when I last saw that, I didn't think to take any pictures of the wheel! Steve

As Richard says, we plan to use glossy powder coating again, just like we did with the Dennis. That worked well at very reasonable cost. This picture shows it on its return but before cleaning and tidying the finish at the ends of the spokes. Steve

Hi Barry. I haven't actually seen it done but I think that the wheel is placed face down on a board with the drag around it. It is then packed with sand completely including under the spokes. When the drag is turned over and the board removed, the moulder cuts the sand away down to the centre line of the pattern using various spatula like tools. The cope is placed on top and a sprinkle of parting sand is applied before filling up with more sand. When that is lifted off, the pattern can be removed. This approach is fine for the one-offs that we do but I think he would probably want a backing piece made up to go underneath if he was doing very many. I remain to be corrected! Steve :-)

Hi Andy. I hadn't thought of those. Rather outside of my usual shopping experience! I did spend some time trying to find something that might do the job but the brass was the only solution that I could make work. It doesn't Araldite very well though so I may lose some in the moulding process. We shall see. Steve

How amazing. You can learn something new here every day! Steve

Rather than a chock, I suspect that it dropped a sprag. The Dennis has the same arrangement although we have not fitted one. The authorities grumble about vehicles which drive spikes into the tarmac! Steve :-D

The M series engines were introduced in 1912. The first J types had the smaller L4 engine and must have been sedate to say the least. They must have spent a lot of time in lower gears when loaded as well. I guess it must have been a lot easier when the few other vehicles around ran at the same speeds. Driving them today can definately be a bit hairy! Steve

I have found a bit more about the engine with the results of a bench test. It also quotes the compression ratio as 3.56:1 which is the lowest I have ever seen! It is certainly not a racing machine! Steve :-)

I have been having some discussions about my piston weights and what effect the increase might have on the crankshaft. The acceleration force will be directly proportional to the mass, of course, but I had no idea of the sizes of the forces involved. I therefore thought I should do a few sums just to convince myself that I understood what was going on. These are my thoughts so far and I would appreciate any views or comments. For the sake of argument, I am assuming that the piston moves with simple harmonic motion which is not quite true due to the crank angle but good enough for a first stab. Under that condition: a= -ω²x Where: a= acceleration in m/sec ω= angular velocity in rad/sec x= displacement from the neutral point which is mid-stroke. Now, Force =Mass x Acceleration: F=ma The maximum force occurs when the acceleration is maximum which will be at the top or bottom of the stroke. x = ± half the stroke =±3” or ±0.076m for this 6” stroke engine. The maximum engine revolutions shown on a power curve in ‘The Book of the Thornycroft’ are 1400rpm making the angular velocity: ω=(2000/60) x 2π =147rad/sec From the above, maximum acceleration =1633m/sec² The original piston that I have here weighs 1932g so the maximum axial inertial force due to the mass of the piston: F=1.932x1633 =3115N =708lbf For our new pistons of about 2400g: F=3869N =869lbf The question now is ‘Do I need to be concerned’? The book quotes a mean effective pressure of 55psi at this speed climbing to 75psi at 800rpm. For the original 4 ½” diameter piston this would give combustion forces of 874lbf and 1192lbf respectively. Does this matter? I don’t know! Trying to do a fatigue analysis on a crankshaft would really have me stretching for the text books! All thoughts and comments gratefully received. Steve :confused:

Yes, wonderful pics. I spotted the steering wheels too, after having had to make one! Steve

Thanks for that. I must admit that I haven’t read the ‘Auriga Book’ for some years. Not much in it really but there are a few gems. Balancing to me means that the pistons are all the same weight. Mother has kindly presented me with a digital scale so I weighed them all this afternoon. They are: 2397g 2399g 2393g 2407g To my mind, they are remarkably consistent but considerably heavier than the original 1932g! I think that the extra 1/8” on the crown is about 165g so I have still won an extra 300g somewhere. I put this down to my poor quality pattern making as all of my castings seem to come out heavier than the originals. The question now is do I remove the 1/8” from the crown or not? I really don’t want to try machining the inside of the piston as the gudgeon pin bosses make it very difficult and now that I have machined the outside, they are quite difficult to hold. I have just been letting the fingers wander the calculator buttons and I think that if we assume that the original compression ratio was 4 then reducing the bore but leaving the pistons the same height would reduce the compression ratio to 3.75. However, increasing their height by 1/8” pushes it back up to 3.92. Which is more important? The compression ratio or the piston weight? As a bit of light relief, here are a couple of pictures of the rings. The oil control ring will go in the skirt with the three stepped rings at the top. Steve (More to follow!) :-)

Nice job! Please can we see some more? Cheers! Steve:cheesy:

That was my original thinking in adding an extra 1/8" to the crown although it only adds an extra 120cc. I hadn't thought that would let me balance them as well! Steve

Thanks Stef and Wally. I guess I should have been more specific in what I ordered. I seem to have wandered into another minefield of whose existence I was unaware! The oil control ring has a continuous groove with intermittent slots to let the oil through which I think is what is shown in your book, Wally. I must drill some radial holes in the piston skirt to meet them. Another pal has just suggested that I balance the pistons. How important is this? I haven't weighed them but I did leave an extra 1/8" on the crowns to allow me to do this so it won't be too hard. Steve:)