Napier Lion Piston – seeking engineering and photographic confirmation

Hi gentlemen,

I am sorry for my delay in replying, I lost my old racer2uk account, I have the oldest running Napier Lion engine and the piston on the wooden plinth looks very original, I am not sure how rare these pistons are or if that many people would want to have one.

The attached photo is a spare replacement piston with a set of original valves showing how they were positioned on a Lion head.

Andy

This is a standard Lion piston

Hope this helps a bit

The 1927 racing Lion was built in two forms VIIA, ungeared, and VIIB, geared. Bore and stroke was 5.5 x 5.125in, as in the standard engine.  Several were built to power both the three Supermarine S5s and three Gloster IVs, so a reasonable number given development engines and spares.  One also made its way into speed record car Golden Arrow.  There are examples at Solent Sky and Beaulieu, probably others in the Science Museum and/or RAF museum.

The pistons are not the same as those in the standard engines. Napier chief engineer Wilkinson wrote…..

In obtaining a smaller frontal area the height of the cylinder blocks had to be reduced to a minimum, and this was obtained by taking 1in off the length of the connecting rods, reducing the height of the piston from the gudgeon pin to the crown of the piston, and from the piston to the crown of the cylinder through higher compression.

I cannot locate precise measurements but if you can get them for a standard Lion then the above should allow you to determine whether the one you have is truly from a racing engine.

Boothman had no connection with the High Speed Flight until the very end of 1930. He did have an unexpected bath when the Gloster IVA used for training ‘tripped over’ on landing so I guess there is a vague possibility that the piston came from the engine that was dunked along with him.

A Google search for ‘Napier Lion Piston’ brings up this image from the (British) Science Museum (Group).

I’m certain this piston is cast but it is also clearly a very different design from the ‘Schneider Trophy’ piston in the original post.

Agreed, but no piston I’ve ever seen is just cast or just forged; all pistons are machined from a ‘blank’ and that blank is either cast or forged.

Even with a forged piston any ‘unnecessary’ material, that adds weight but only ‘excess’ strength, can be machined away during the (many) other machining operations.

The Napier Lion piston is an incredibly easy shape to machine compared with, say, a Napier Dagger piston (which I know are made from forged blanks).

What makes me think the piston in the first post is cast are the ‘undercuts’ on the circumference of the inside skirt and the ‘meat’ around the yoke of the gudgeon pin being ‘undercut’, ie a forging tool could not withdraw and provide an ‘undercut’ result. 

I’m pretty sure the Lion piston I have is forged so was working on the assumption (dangerous!) that all Napier Lion pistons were forged? That said, the Lion was effectively a First World War engine so that makes me think (now) that the early pistons, at least, must have been cast?

I’ll bet you a virtual (or a real?) pint all the ‘race’ Lion engines had forged pistons!

One question: if a cast piston is ‘machined all over’ is it possible to tell it from a ‘machined all over’ forged piston without resorting to material analysis or density checks? 

Pistons of that era were generally cast from Y alloy. Making a casting pattern from wood not so expensive. Forged pistons did come into use in the early 30’s, Eg Bristol Jupiter ‘F’ for Forged piston. Perhaps racing engines in the 20’s did use forged pistons but I doubt it, as they were a ‘one shot’ consumable, given exotic, corrosive methanol, TEL fuel blends. The piston looks to have ‘meaty’ gudgeon pin yoke, which you could cast. Not sure, as I was not alive in 1928. 

An interesting article and very interesting sectional drawings of the Lion VIIB racing engine.

The modifications to the ‘standard’ piston to cope with the reduced cylinder height (the higher engine revolutions) and the much higher compression-ratio are clearly shown: reduced overall piston height, recesses cut into the piston crown to clear valves (when open), reduced distance from piston crown to gudgeon-pin centre-line, recess in underside of crown to clear connecting-rod, sections removed from lower piston skirt (under gudgeon-pin) and chamfer at lower end of piston thrust-face.

Unfortunately I don’t see much evidence of these in the ‘Schneider Trophy’ piston in the original post although one very interesting feature that I have noticed is the way that the inner edges of the cut-outs on either side of the gudgeon-pin are curved (not straight) to follow the curve of the gudgeon-pin and that the gudgeon-pin seems to align centrally between the cut-outs (rather than lie higher than the cut-outs)?

It would be an expensive business to produce separate tooling to forge the, very few, pistons needed for the racing engines (although the overall amount of redesign for these engines is quite staggering)! It may have been possible to machine the ‘race’ pistons from the standard piston forgings as, generally speaking, a huge amount of the forged blank (about 60%?) would be machined away to produce a standard piston anyway?

You may like this article from my good friends at the AEHS:

https://www.enginehistory.org/Piston/Napier/NapierLionHx/NapierLionHx.s…

I took some measurements from the piston and got a ‘piston height’ figure of 47.25mm from the centre-line of the gudgeon-pin to the (flat) piston crown (based on a gudgeon-pin hole with a diameter measured at 28.55mm). I wonder how this compares to the ‘Schneider Trophy’ piston?

The design of the Napier Lion, a perfectly flat piston crown and a flat-topped (?) clearance-volume with vertical valves, makes for some easy comparative calculations. None of the references make any mention of any change to engine capacity (although the 1928 Flight article provided by Oracal does mention a shortening of the connecting-rods in an effort to reduce the height of the ‘cylinder banks’ but that alone would not alter the swept-volume of the engine). A quick calculation reveals that a ‘standard’ Napier Lion engine with a compression-ratio of 5.8 to 1 would need a clearance-volume approximately 27mm high; to raise the compression-ratio to 10.0 to 1 would mean reducing this clearance-volume to approximately 14.5mm high (imagining a ‘perfect’ cylindrical clearance-volume). This could have been achieved by increasing the distance between the centre-line of the gudgeon-pin and the piston crown, effectively increasing the piston height, or actually increasing the piston height (or a combination of both), but (as the 1928 Flight article states) this was more effectively done by reducing the height of the cylinder and therefore the frontal-area of the whole engine with the primary aim of reducing drag (although there is scope for all three methods to be employed together).

Some requested ‘other photographic evidence’.

Please ignore the ‘extra’ holes in the crown and skirt; somebody (in 1930 / 1940 judging from the wiring and switch) turned this into a table-lamp!

I will add some dimensions (piston height) shortly.

No, these are new to me.

Thanks for posting.

CD – I suspect you will already have seen these…

CD – Napier we’re only able to achieve the power increase by using special ‘doped’ fuel that permitted the compression ratio to be increased from that of the standard Lion engine.

A quick look at Wikipedia this morning (I am away and had no better references) lists the supercharged Napier Lion VIID (racing) of 1929 at 1350 horsepower on only 24 litres; figures the Rolls-Royce Merlin (27 litres) would struggle to match a decade later!

An 875 HP Lion is not a ‘normal’ 500 HP Lion, probably running exotic fuel and a higher compression ratio. Is the piston height dimension different to a standard piston height? 

Flt Lt John Boothman won the 1931 Schneider Trophy, and was at CFS in 1926. The piston is inscribed with a 1926 date and ‘Booth’. Was this a piston from test flights in 1926 and was Boothman one of the test pilots? Race engines would have been stripped and investigated and the piston may have been marked for subsequent inspection, matching a pilot’s flight report in 1926. Was there something about the hidden top face that related to compression, perhaps a dimensional difference to a standard piston? When the Trophy was won in 1927, did a fitter grab an old 1926 retention sample piston and make a keepsake? If genuine, it would by Y Alloy, which could be determined by metallurgical analysis. It’s a great looking piece, sometimes treasures come up. 

Going back to my earlier comments about what you would need if you were to try to convincingly ‘fake’ such an artefact (as this is my usual approach to assessing something with a provenance that is impossible to verify).

What if it were a time when Napier Lion pistons were much more common? Such as 1927?

So what else do we need after we have the right piston? Timber for the base and a big lathe to turn it on. Wood stain or varnish and a piece of green baize. And then there is that brass plaque.

Something else that occurred to me is the shape of the plaque; looks like it was made to fit the lettering (or the base)? Notice also the ‘economy’ of the wording (not Napier Lion piston); this was made by, or for, somebody who would know exactly what this object was.

So who would have access to all these raw materials, machine-tools and also have the skills (particularly the hand-lettering on brass) and the considerable time necessary to produce such an item?

To me there is a obvious candidate: a member of the Air Force or Fleet Air Arm who had everything necessary and the (enforced?) time available?

Of course, these are exactly the same conditions required to produce the ‘genuine’ article!

(I would also like to point out I am not making any accusations toward the seller or any member of their family!)

You make a good point about commemorating a less well known Schneider Trophy win; wouldn’t a piston from the later win that secured the Schneider Trophy be more desirable (especially to a potential ‘faker’)?

This does however mind me of something I didn’t consider last night; what if this wasn’t a modern ‘fake’ but a ‘fake’ made in 1927?