Old Bill

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:

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:

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:

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:

For our new pistons of about 2400g:

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:

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:)

Gordon M " You are right - you might be over-solving this one.

What's the compression ratio on the engine, then? Nothing too high, I assume. "

I think it is about 4:1 or at least, it was before we lined the bores. I think we have lost 500cc of swept volume so the ratio will have dropped a bit.

Steve

Very many thanks for all of your thoughts, Chaps. I bought the rings from a ring specialist after telling him what sort of engine it is and the dimensions of the current rings. I am simply planning to fit what he supplied, relying on his expertise. Unfortunately, he didn't provide any guidance notes!

Looking at Chris' website, it appears that if the rebate is upwards, it is used as the top ring for dodging the step at the top of a worn bore and if it is downwards, it is a scraper ring. As we have new bores I think I might assemble them all with the rebate downwards. That way, if ever we need to replace them without reboring, we can put the new top ring in the other way up to dodge the step.

Will I get any lubrication problems if I assemble them all as scraper rings?

The originals are just plain compression rings. Perhaps I am being too clever!

Steve

I reckon this is all a hoax and your buying these bits from Halfords, unwrapping them and having us believe you make them.....!!!

I wish! I have just remembered that I need to machine the ends of the gudgeon pin bosses as well. There is a lot of work in a piston and these are relatively simple.

Now, please may I have some advice? The top rings are all 1/8" thick but they are stepped such that the contact surface is only 1/16". The question is, which way up should they go or does it even matter? Your thoughts would be appreciated.

Cheers!

Steve

How will you transfer the logo to the hub cap? Photo etching or engraving perhaps? It does look good!

Cheers!

Steve

Well, that's interesting. Doesn't look like a Thorny to me but I can't Identify it. The driving position looks very well forward from the hand brake position and I have never seen anything like those wheels before. None of the fittings have a Thorny look about them so I don't think it is. We need Doug or Tim or Runflat to have a look!

Steve

Thanks Richard.

Punching is an interesting thought. The sleeve, if I remember correctly, is 5/8" outside diameter and 1 1/2" long with a wall thickness of 1/16". The brass centre has a piece of tube pressed through it crossways to secure the sleeve. I drilled the PTFE through, fitted it to the spindle and then turned the outside afterwards. It hasn't leaked at all which I count as some success but not being able to turn it on is a bit of a downer!

Steve

Over the bank holiday and when we put the lorry into store, it has become apparent that the fuel tap has become very stiff, to the point where I am concerned that I will either break off the handle or worse, break it out of the bottom of the tank. You may remember that I made it up to represent the one in the fuel tank drawing but I took the liberty of using PTFE for the liner sleeve instead of cork. This may have been a mistake! Well, I had the great good fortune to meet the delightful Sarah at the rally at Old Warden over the weekend and she worked for some years in an organo-fluorine chemical laboratory. I asked about the sleeve and she told me that petrol will not corrode or degrade it. However, she did explain that the octane molecule, a major constituent of petrol, is of a similar length to the ethylene molecule such that inter-molecular Van-der-Waals forces will securely attach it to the PTFE. (Gosh, I wish I had paid more attention to my chemistry at school!). The net effect of this is that the PTFE grows which explains why the valve has tightened up.

Now that I understand the problem (more or less...) I must find a solution. At the moment, I think that a cork sleeve may provide the answer but does anyone have any suggestions how I should make one? Do I use a lump of whole cork or some reconstituted crumb material? How do I turn it? Any advice would be gratefully received!

Steve :embarrassed:

I think the centre lorry might be an FWD. The others are certainly Quads though.

Steve

Nice pic again Tom.

Difficult to identify the lorries but I would suggest that they are Peerless. They certainly have the look about them. Did the RFC have any?

Cheers!

Steve

You've stolen my thunder Barry! Yes, that is part of the plan.

My intention, at the moment, is that the right hand lump will be turned to the outside dimensions of the onion with the big core print at the bottom and the little one at the top. The centre lump will be turned to the inside dimensions to become the shape of the core. The left hand lump is to be turned to the outside dimensions of the main pattern in the area of the hole. This, I plan to cut to the profile of the hole and glue to the core block giving a section, raised by about 1/4", in the area of the hole. That will support the core in the hole in the sand.

Now, the clever bit. I shall make up two core boxes as empty wooden boxes. The wooden core will be supported half into the box and the box filled with resin around it. It will then be reversed and the same done for the other side. This should give me two core boxes, one for each half of the core. The moulder can glue the core halves together before placing them in the mould.

Well, that's the theory anyway. I have not done this before but will keep you posted with progress!

Steve

Thanks Chaps. Just what I wanted!

Cast iron is lovely to machine but the swarf is filthy. I shall have rusty handkerchiefs for a week!

Steve

I am just getting prepared to machine them now, just as Barry described with the draw bar through the headstock of the lathe. I will face both ends and clean up the inside ready for the gudgeon holes to be bored by a friend with a bigger mill. One particular question arises, though, and that is how much clearance should I give them on the diameter? I have asked around and been given figures from 0.005" to 0.010". The bore is 4 5/16" and it is an iron piston. I plan to make the land above the top ring about 0.004" smaller in diameter than the rest of it which will remain parallel.

I would value your thoughts please!

Cheers!

Steve

take a look at the beardmore engine then here:

 

these were fitted to the FE.2b aircraft....which was an artillary flyer/observation aircraft used by the RFC during WW1.

 

to the O/P:

how do you find the general construction & design of the Thornycroft in comparison to the Dennis?

/QUOTE]

 

Hi Glenn.

 

Where is that Beardmore? Looks like a nice bundle of tricks but I wouldn't want to have to keep it clean!

 

The Thorny does not seem to be as refined as the Dennis although there is not a lot in it. They are both true Subsidy lorries and generally very similar.

 

The American lorries are much more distinct. The Autocars are a very nice job indeed with bushes at every wearing surface so refurbishing them was relatively straightforward. They are even bushed for the shackle pins at the non-moving surfaces. The FWD by comparison is quite crude. The casting and machining quality is 'just good enough' and I get the general impression that the engineers did a good job of the transmission and driveline but then lost interest and threw the rest together. It can't be that bad though, as all of them have lasted 90 years.

 

Steve

Details details details and how to accomplish them is what keeps these projects stimulating.

bob

Quite right. The challenge is everything. If it was too easy, we wouldn't do it!

I hadn't thought of Dental resin. That has worked well with a lovely result. How did you obtain the materials? Perhaps I am just being dim and should ask Mr Google.

One other thing intrigues me and that was how you set all of those rivets? I have found using such large rivets in thin plate troublesome as they tend to roll over in the snap and twist the plate locally.

Nice job. Keep it up!

Steve

Wing nuts can be clearly seen in the drivers' manual so we have continued their use. Those I found twenty years ago are of a modern pattern. Since then, the internet has come along and Dad has found the correct pattern quite easily so we will replace them in due course. The Dennis box was secured with nuts and washers (Again visible in the manual). However, it also has a filler on the front of the case with a cover. That is fixed using wing nuts. I would suggest that this would allow the driver to top up the box with no tools to hand.

Regarding the 'gate' this is just the same as the Dennis with reverse through first and a trigger to allow the lever to move to that position. I'm not sure how common this was but I have seen it elsewhere.

Cheers!

Steve

Trailer wheels I should think.

What do you think Hedd? You are the Eagle trailer expert!

Steve

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