WW1 Thornycroft restoration

[Gordon_M]

Ta, I suspected a number down there somewhere.

 

Given fuel volatility and octane rating, would you not be tempted to try and up it a little with marginally taller pistons or the like?

[Old Bill]

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

[Barney]

Steve,

 

If you fit the stepped rings with the step downwards they won’t work as scraper rings unless you machine your pistons with the oil relief groove and drain holes which are necessary to return the oil to the inside of the piston. Without the oil relief they will just act as compression rings.

 

Please see Wally Dugan’s post #1022 for diagram of the oil relief grove and drain hole positions. (Centre picture, top row)

 

As you still have some work to do on the pistons it would certainly be worth the small amount of extra time taken to balance them.

 

John

Edited October 16, 2013 by Barney

[wally dugan]

BARRY

The book in question is called fundmentals of motor vehicle technology by V W HILLIER and F PITTUCK l got the book in

1967 the second edition because it was compulsory reading but it was so usefull i got the updated 3 edition it was published by HUTCHINSON EDUCATION but do not know if its still in print

 

REGARDS WALLY

[David Herbert]

[quote=Old Bill: 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

 

Steve,

 

If you added 1/8" to the top of the crown but kept the inside where it was your pistons will be considerably heavier than the originals. I would have thought that it was critical that the new pistons were no heavier than the originals and even better if they were lighter. I am thinking about the loads on the con rods and bearings which would not have been exactly over engineered when new. Obviously it is good if all the pistons weigh the same and that the individual compression ratios are the same but this is not exactly a high performance engine and given its low revs and C/Rs I very much doubt that you would be able to tell if they wern't exactly the same. If you must remove metal from the pistons I would very strongly advise removing it from the insides.

 

I would definately keep the higher C/R as modern petrol is a much higher octane rating than the engine was designed for so at the same power output it should run much cleaner and cooler. Similarly you should be able to run with the ignition further advanced with the same benifits, as long as you can still retard it enough to start it.

 

David

[mammoth]

Looking at my copy of the the J Type spares list dated 1924 there appear to be 3 rings at the top (all part No 66217) and the ring half way down is described as 'locking ring for gudgeon pin' (60958). "The Book of the Thornycroft" by 'Auriga' dated 1923 is interesting as it describes design revisions and corresponding new part numbers. In respect of pistons Feb 1915 saw the height above gudgeon pin increased 1/16" and depth below increased by 11/16"; oiling groove omitted, six 9/16" holes added below gudgeon pin. June 1916 skirt thickened up. Further in, there is a section on over hauling and over/under size parts. It would seem that there was a later type piston referred to as a 'scraper ring type' (71713) which used different compression rings (70920) and a scraper ring (71714). The gudgeon pin is of the floating type.

 

As far as "balancing" goes, what is balanced with what? In performance engines the weight of the reciprocating mass is balanced against the rotating mass of the crankshaft using counterweights on the crankshaft. It is more easily understood with single cylinder motorcycle engines, and the choices between balancing primary and secondry forces are more obvious. The Norton Commando was a good example of these choices at play as the engine was designed with a lower primary balance so the the motor would oscillate in the vertical plane on rubber cushions so the the secondaries could be more refined so your fingers didn't tingle as in the Triumph.

[Old Bill]

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!) :-)

[mammoth]

Elswhere in the Auriga book it notes that in June 1917 the piston rings had a stepped joint instead of diagonal, so you have the earlier pattern rings.

For my money I would be looking for higher compression, and if she seems to be vibrationg excessively you still have the option of maching weight off.

[Great War truck]

On our chassis, there is one little shaft, about 2 1/4” long and 1 1/2” in diameter sticking out from the sub-chassis, near side, which is the remains of the original engine mounting on that side of the vehicle. The shaft goes through the chassis rail and is held in position with four 1/2” countersunk machine screws through the chassis rail and mounting plate – nutted on the other side of the chassis rail. The engine has a three point mounting arrangement. The “mushroom” between the four nuts is the remains of the original Greaser. The third picture shows part of the complete set-up on the Carlton Colville “Thorny”. The angle iron takes the casting for the single engine mount which is on that side of the lorry and is designed to move or give slightly on the “prong”.

A little wire brushing helps and also shows the full extent of the corrosion. The thread for the retaining nut is 1” Whitworth.

After the usual treatment of heat on the nuts and screws and then persistence, all four came loose. The pivot pin with its mounting plate were stuck in the chassis rail and would not come free until further extreme heat treatment on it and then one sharp tap with a heavy hammer released it. It seems that the pivot pin was pressed into the mounting plate and we shall have to replicate that when the new pin is made.

[Asciidv]

Although there must be a 1001 ways to resolve the piston problem, here is another, which resolves both the weight problem and also gives the required increase in compression. Quite simple as much of the crown is turned off as possible leaving a small spigot in the centre which is then screw cut. An aluminium disk is then screwed on to replace the removed material.

 

 

Barry.

 

Edited October 21, 2013 by Asciidv

[Great War truck]

Steve has now finished the pistons. He first machined the gudgeon bosses to length and then drilled the oil control ring slots to allow the oil to return. The pistons vary by only 14g max to min so we will leave them as they are and put them away ready to fit. We have decided that our Christmas project this year will be to reassemble the engine, if only to get all of the boxes of bits off the floor!

 

 

 

 

[Old Bill]

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:

[David Herbert]

It seems to me that any combustion pressure will reduce the tension on the con rod at TDC and will not greatly add to the compressive load at BDC as the exhaust valve is already open but the TDC between the exhaust stroke and the inlet stroke is almost devoid of pressure on the piston and likewise at the end of the intake stroke there is virtualy no pressure at BDC so at all dead centres the acceleration of the piston is the vast majority of the load.

 

Therefore it seems that as long as the engine is not over revved the load on the big end will not exceed the load at peak combustion pressure which should occur at about mid stroke. I must say that 75psi as max MEP sounds unbelievably low, any modern engine will make that just on compression with no combustion!

 

Since it is almost impossible for you to get engine oil that is not better than original this would sugest that the bearings should cope ok.

 

That leaves the question of the tension load on the con rod at TDC at max revs. Is there any evidence that this engine tended to throw rods or that they never did? I think that I would still like to remove some metal from the inside of the crown and possibly inside below the gudgeon pin but your calculations certainly show less of a problem than I expected.

 

Nice bit of applied physics.

 

David

[Old Bill]

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 :-)

[ajmac]

Looking at that curve really brings home that despite the boom in technology during the first world war, said technology was still in it's very early days. Having an internal combustion engine rev limiting at 1400RPM and only producing 30-40BHP to haul your Lorry shows industry was still only just getting to grips with these new fangled machines! How did they cope with gradients when loaded?

 

When was the engine introduced? I guess the new designs in engine improvements didn't make there way down from Aeros until after the war, and even then probably only in high end products. After all the second world war Ford Flathead V8 only made 85 BHP, 20 years later.

Edited October 26, 2013 by ajmac

[Old Bill]

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

[monty2]

see the amount of graft mans ship in this project a specially and some others on HMVF. I think we should apply for an extra category at "Britain got talent" program. Very impressvie

 

[Barney]

Steve,

 

From my days in motor engineering I remember the instructor explaining the forces on the crankshaft, some of which were caused by balance issues.

 

It’s a long time ago so some of my terminology may be incorrect and I have probably forgotten some of the finer points but I hope this will give an idea of the forces involved; please accept my apologies if you are already aware of the following.

 

The crankshaft balance is affected by rotational and longitudinal forces. Rotational forces are akin to front wheel balance problems where an out of balance wheel will shake the steering wheel at say 40mph but at 50mph the shake will disappear. I’m sure older drivers have experienced this at some time and if I remember correctly the maximum imbalance occurred at what, I think, is termed ‘The Critical Whirling Speed’. Engine manufacturers fitted the crankshafts with counterbalance weights in order to raise or lower the Critical Whirling Speed out of harms way because it is not possible to get perfect balance in a piston driven engine. In order to minimise the shake even further some manufacturers of luxury cars fitted one or more unbalanced countershafts geared and timed to the crankshaft; mass produced cars, by comparison, just had a rubber damped crankshaft front pulley.

Presumably, with the Thornycroft crankshaft running at only 1400 rpm it didn’t cause any problems so they didn’t consider it necessary to fit balance weights to the crankshaft although they may have drilled the flywheel to achieve some degree of balance from there. I would suggest that you attempt to get the piston weights as near as possible to that of the originals as heavier pistons may move the Critical Whirling Speed.

 

Longitudinal forces are akin to two children on a sea-saw; the heavier one will go down faster and take more effort to raise. If the piston weight on the crankshaft is heavier at one end than the other then an out of balance force will arise along the length of the crankshaft and this is why the pistons are all kept to the same weight in each engine. Of course the weight of all the con-rods should also be the same as they contribute to the rotating mass and balance.

 

I don’t remember being given any data regarding what is an acceptable tolerance but whatever it might be pails into insignificance in the following true account.

 

A chap brought an early six cylinder ohv Humber Super Snipe into the garage and explained that he had brought it at a local auction where a recently deceased farmer’s effects were being sold off.

The car was in lovely condition but it wasn’t running too well and sounded as if one cylinder of the six cylinder engine was ‘missing’. We traced the fault to no5 cylinder and removed the spark plug, which was soaked in oil; a compression test revealed no compression so we suspected a hole in the piston. We removed the cylinder head to find that there was no hole in the piston; in fact, there was no piston or con-rod and the crankshaft journal had a length of leather belt wound around it secured with a jubilee clip to block up the journal’s oil feed. I should have said that we already suspected some foul play as the push rods for this cylinder were also missing.

Shortly after one of our fitters met the deceased farmer’s mechanic who explained that the old boy had not checked the engine oil for some time and it had got very low. His son had borrowed the car, tore it off to Yarmouth and knocked the big end out. The mechanic said that as the old boy only drove the car to Norwich City football matches he didn’t want to run to the expense of a crank regrind so had instructed him to remove the ‘rod and piston; it had been running like this for about five years!!!

 

Your new pistons look great.

 

John

Edited October 27, 2013 by Barney

[Redherring]

John; that is a story I can relate to. At risk of destroying the focus of this thread I would like to relate a short story in return. My dad ex farmer and ex RAAF pilot, bought us a tired, probably third-hand, Humber snipe way back when. One day he was driving said Humber, low-flying was the term the family used, over a local hump bridge. We kids hit the roof and out from behind the back seat arose a big cloud of small white moths... I'd like to think that moth colony survives to this day... Cheers. Robert

[andypugh]

I have been having some discussions about my piston weights and what effect the increase might have on the crankshaft.

 

To put things in some perspective, the White and Poppe 4 cylinder in LP8389 ran for decades with one piston weighing 2lbs more than the others. It didn't seem to cause any real problems.

When we nibbled away as much excess metal as we could (using a boring bar made from a road-breaker chisel) we got the imbalance down to a few hundred grams. And some people managed to persuade themselves that the engine ran more smoothly.

 

I thought that White and Poppe were the only ones crazy enough to run piston rings through the gudgeon pins, but clearly not. On LP8389 the gudgeon pins were a (very) tight shrink fit, with assembly requiring the use of heat and liquid nitrogen. Not a great combination as getting the pins in to just the right depth and with the ring-grooves properly aligned was extremely difficult.

 

On one piston the gudgeon pin lost its fit for some reason. The effect of this was to twist out a chunk of ring (which caused some bore damage). I would be very nervous of your arrangement of rings through the pins + free-floating pins.

 

On LP8389 we gave up on the original arrangement, and now simply omit the middle piston ring, and have floating pins with bronze buttons in the end. This makes life a little less interesting, but the white-metalling guys smash fewer of our pistons when pressing pins out.

[Great War truck]

There is a fifth ring in the middle of each piston which is sprung inwards to locate the gudgeon pin. This is simply a piece of rolled spring steel. Of course, with our smaller pistons, the originals no longer fit so Steve opted to reduce them to suit. First, he softened them by heating to red hot and then allowing them to air-cool.

 

 

He then cut 9/16” from the end using a Dremel with cut-off wheel.

 

 

 

Once this was complete, he had to tighten the radius and he did this with a press tool. He machined a radius on a block of ash.

 

 

Using a bit of tube from the stock drawer, he pressed the rings in steps around their circumference until the ends met once more.

After hardening them once more by heating to red heat and dunking them in a bucket of water, he tempered them back by polishing them and then gently warming them until they went blue. They are now ready for use again.

[andypugh]

It seems that Thornycroft were a bit less mad than White and Poppe then, the spring retainer seems relatively sensible.

 

I would like to point out as a general observation that heat-treating spring steel doesn't get you back to a spring temper. That is typically the result of cold-rolling or wire-drawing. But it is typically eutectic steel (0.8% carbon) so will harden well too. Having said that, I am pretty sure that leaf-springs are typically heat-treated rather than cold-rolled.

 

I probably should have introduced myself, though you have probably guessed by now that I am one of the IC motor club folk. I was at college at the same time as Steve (I think) then went on to do some postdoc research on spring steels.

 

I don't actually play with old vehicles very much any more. (Other than Jez, the 1916 Dennis N type). I do own a 1922 Ner-a-car kit, but haven't touched it since I bought it.

 

I mainly experiment with machine tools for a hobby. My Youtube channel gives a flavour of what interests me (finding ways to make things, though not necessarily doing so) http://www.youtube.com/user/blyndpew/videos

[Great War truck]

With all of the smaller parts now removed from the chassis, the only parts left to remove are the wheels, springs and axles. These are a bit much for Tony to remove on his own so we hope to have the “heavy gang” there within the month to help and to finish the job so that the frame can go for sand blasting. As much preparatory work as possible will be done prior to the arrival of the “help”. Every part taken off so far has been a challenge because of heavy rusting and we expected the same problem with the wheels. The off-side front hub cap has been missing for many years by the looks of the wheel hub – the wheel is held on by a collar with a good solid pin inserted through it and held in place by a split pin on the end. The split pin was surprisingly and pleasingly easy to remove and one sharp blow with a big hammer loosened the retaining pin. The other three hub caps are still on the wheels and fortunately, we do have a spare front one in the collection, obtained sometime ago which will replace the missing one.

This is the other front hub – the hub caps are held on with nuts and bolts where the bolt is inserted through the outside face of the wheel from the inside and nutted on the outside – so very easy to replace. No nasty rusted studs to deal with for a change! Removal of the hub cap revealed that the hub was still heavily greased – once the grease was removed, the split pin came out easily and the retaining pin was easily loosened.

 

These pictures show the rear hubs – again, the hub caps were held in place with nuts and bolts – but only three were put back in when the fitter removed the half shafts all those years ago when the chassis was converted for use as a Shepherd’s Hut. Again, the hubs remain nicely oily and the big retaining nuts look as if they will undo quite easily.

 

Edited November 3, 2013 by Great War truck

[Great War truck]

Steve has picked up the steering wheel pattern. We don’t actually need it yet but our friends at the Coventry museum need a four spoke wheel for their Maudslay which looks very similar so he has taken the opportunity to get on with it.

 

The first task was to assess the information that we have. Steve measured up the one on the Portsmouth bus many years ago but it was wrapped with tape so the detail could not be seen. Similarly the one at Carlton Coalville is also wrapped.

 

 

However, Dad took this picture of the tipper at the Tucker sale twenty years ago and ribs can just be seen on the underside.

 

 

These are confirmed by a picture in the parts book. Finally, we had had a look at the wheel on the one in the scrapyard in Taunton. The rim has disappeared but it can be seen that the spokes were coated as well as the rim.

 

 

Based on all of this, we have enough information to do the job.

 

We decided that the wheel should be cast in the same manner as we did with the Dennis wheel so Steve laid out the profile which a friend had laser cut for us in 3mm steel. Steve then put the set in the spokes by pushing them whilst the rim was held in the vice.

 

 

 

 

Next, he proceeded to build up the shape using MDF. Quadrants were screwed to a plywood disc. This was then rotated by hand on the milling machine against a 3/4” radius router bit.

 

 

 

[Great War truck]

The rim was attached using contact adhesive as were the spokes. The spokes were simple flat strips which were then dressed to an elliptical section with glass paper.

Then fillets were created by adding car body filler and dressing them back with the Dremel tool and glass paper.