The Internet Craftsmanship Museum Presents:

Clennell "Clen" Tomlinson

Added to museum: 10/31/03

CNC-machined miniature engine masterpiece

Clen is seen here at the 2003 Pacific Rim International Model Engineering show in Oregon with his nearly-completed Napier Deltic 18-cylinder engine. (Click on photo for larger image.)


When I had Craig add Clen to our Craftsmanship Museum I did so with the express idea to answer the question, “Is a craftsman still a craftsman if he uses CNC tools along with CAD/CAM programs?” Just a glance at the magnificent project that is “still in the works” by Clen and one should have the answer, and it should be a definite “yes.” In the hands of a great craftsman CNC is just another tool, and Clen deserves the same amount of respect as any great craftsman.

(To read Clen's take on CNC vs. Manual Craftsmanship, CLICK HERE.)

What especially impressed me about Clen and his project was he wasn't a youngster who grew up in the computer age; he was a craftsman who was willing to take on a seemingly impossible project and use every tool at his disposal to accomplish his goal. I'm looking forward to the day when Clen’s engine roars to life. —Joe Martin

Impressive engine attracts attention at American model engineering show

While looking at the engines on display at the 2003 PRIME show in Oregon, Joe Martin was particularly impressed with the work of Clen Tomlinson. Taking on a model engine project of this magnitude is no small task, and the finishes on this model are simply superb. Although it is not yet complete, we felt that visitors to this museum would want to see what has taken place so far and will follow the progress of this project to its completion along with us. Another aspect where this project varies from others in this museum is that it is machined almost completely using modern CNC technology. In the past we have excluded CNC work in favor of the hands-on craftsmanship of non-computer controlled tools. In this case we made an exception for two reasons: 1) The level of complication of this model meant that even with the help of Computer-Aided Drafting (CAD) and Computer Numeric Control (CNC) machines to make the part, the combination of knowledge and skill required to actually make the parts is in some ways more demanding than making them by hand, and 2) CNC machining is the way machine shops now work and the way in which craftsmen in the future will be making parts. As projects get more highly technical, some parts such as turbine blades simply cannot be made any other way than with the help of computer control. What you see here is a preview of the direction model engineering will be heading in the next decade or two.--Craig Libuse


Technical training lays the background

Clennel "Clen" Tomlinson is a 71-year old retired Electro/Mechanical engineer, living in West Sussex England. He started his working life as an apprentice in the motor vehicle industry and is qualified as a Motor Vehicle Technician. His experience includes cars, commercial vehicles, agricultural tractors and equipment, civil engineering plant and equipment and motorcycles.

Clen has designed and built many special-purpose machines, including road and race cars and bikes. He has also managed sports and racing car departments. In middle of this time he spent two years National Service (Draft) in Royal Air Force. He spent 13 months—5 days a week, 8 hours a day in the classroom being trained as an aircraft flight and navigation instrument technician. He says, "I learned more in that period of my life than in my life to that point!"

In the mid 1960's he made a change into scientific manufacturing and spent three years making small, high-speed (500,000 rpm) turbines and free piston engines used in cryogenics for liquifying Helium. He then took training in education and joined a large manufacturing company to set up and run an engineering training school. He progressed to Group Training Manager and finally to Group Personnel and Training Manager. The company designed and built large electro-magnets for atomic particle physics research. He also specialized in new manufacture and refurbishment of underwater weapons and designed and built ion implanters for the electronics manufacturing industry.

In the early 1970's, Clen joined with three other senior managers and set up new company operating in similar areas. The company developed into one of  the leading magnet manufactures in the world. As Engineering Director was responsible for tendering, design and manufacture of tooling. He designed and built large resistive and super-conducting  magnets and associated equipment for atomic particle and fusion physics research projects throughout the world.

Clen has been involved in many projects in the USA including the Brookhaven National Laboratory, Long Island New York; NAL, Chicago; the super-conducting super collider in Texas and Boeing, Seattle—Starwars program.

Hobbies with a technical slant

Clen has always been making or “improving” things including many relatively simple model engines, both steam and IC. In retirement he had been slowly developing his workshop and enjoying life. Then his wife died very suddenly some four years ago. He says, " I needed something to totally occupy my head for some time, and that is where the Deltic came in. I had been thinking about it for some time!

He is currently completing the restoration/improvement of a 30-year old BMW 30Csi coupe he has owned for 20 years. He has another BMW car and 2 motor-cycles to look after. He also has 2 grandsons to “train.” The next model engine will be another Napier; The Sabre. This is an “H” 24-cylinder, sleeve valve unit. He is working on the design, and it is some challenge!

Learning new technologies

Recently Clen brought us up to date on some more details of how he got into CNC machining. Here is what he had to say:

“I spent most of my working life with pencil and paper, “T” square, log and trig tables and a slide rule. I progressed easily to the calculator but a little more reluctantly to the computer. Towards the end of my working life I couldn't get an engineer to work for me unless I gave him/her a PC on the desk. I was still writing with a pencil and thinking “All this technology has arrived too late for me." Then I thought, “Hang on a minute, non of them are not that bright. It can`t be that difficult.” Surreptitiously I put a PC on MY desk. Word processing; no problem! Then there are was draughting. This CAD is really impressive, but that's beyond my ancient brain. Then I think, “Hang on a minute----”. It didn`t take too long and I was doing that too.

On the shop floor the handles have disappeared from the machines to be replaced by control panels and screens. My God, these kids today are bright! After retirement, I found two Denford educational CNC machines. They had no computers or software, and it became a challenge. I think, “Hang on, etc.” I had to learn the hard way, but the rest is history as they say.”

Software and Hardware used

Clen uses AutoCAD 2000 and tends to draw toolpaths. He then prints off co-ordinates and manually programs them into the Heidenhain 151 controller of the Bridgeport Interact 1. This is a relatively recent addition to his shop. Most of the engine was machined on a Denford Starmill benchtop educational 2.5 axis machine. It had only 170 mm in X travel and 90 mm Y travel, and the tooling was a nightmare according to Clen.

Making the Napier "Deltic" engine

by Clen Tomlinson

Prototype History

The prototype for this model is the NAPIER “DELTIC” opposed piston, 2-stroke cycle, Diesel engine range, used primarily in Naval fast patrol boats and minesweepers today, but also to power two classes of English Electric Diesel-electric locomotives. The engines were produced as 9-cylinder units (3 banks of 3 cylinders) or, as the subject of this model, as an 18-cylinder engine, (3 banks of 6cylinders). The principle has its roots in a German Junkers aero Diesel engine of the late 1920`s-1940`s. This engine was an opposed piston, 2-stroke Diesel with 1 bank of 6 cylinders arranged vertically with two 6-throw crankshafts;1 at each end of the cylinders.


D. Napier & Son Ltd. had a license to develop this unit as the “Culverin” aero engine in 1935 but it was not until after the 2nd world war that a use was found. The project was by then in the hands of the English Electric Group, who were looking for a light weight, high speed marine Diesel engine. This application required a couple of inspirational design leaps. The first was the realization that if one more crankshaft were added, then two more banks of cylinders could be added in the configuration of an equilateral triangle. The second was that if you arranged to rotate one crankshaft in the opposite direction to the other two, then the relative phasing of the whole assembly for port timing  “fell into place” This design produced an extremely compact, strong but lightweight unit with almost perfect natural balance.


With 18 cylinders having a bore of 5.125" and a stroke of 7.25" per piston or 14.5" per cylinder, this engine had a swept volume capacity of 5,384 cubic inches or 88.3 liters. There were 3 stages of development from the initial design with engine driven blower for scavenging then being turbo-charged, turbo-charging with charge cooling and finally “Compounding” using an axial turbo-compressor unit within the triangular central void.


The performance ranged from 1,650 bhp at 1,500 rpm for locomotives, to 2,500 bhp at 2,200 rpm “sprint rating” for FTB`s (bmep 92.lbf/in2) both mechanically blown, to 3,700 bhp at 2,200 rpm (bmep 130 lbf/in2) when turbo-charged and charge cooled. Note that the complex “Deltic Compound” prototype reached 5,600 bhp output on test during 1956.

The Model

The working model is an 18-cylinder, opposed piston, 2-stroke, spark ignition engine. The model is based on, that is follows, the design principles, of the NAPIER DELTIC Diesel engine to a 1/8 linear scale. The design was produced from a single cross section drawing of the marine version as published in The Model Engineer plus the bore and stroke dimensions. This drawing was photocopied and enlarged onto several A4 sheets each with a different scaling factor.

Here is the drawing upon which Clen based the model. Click on the image above to see a larger version. (File is 251 Kb.)

The internals of the engine are to scale, that is the bore, stroke, connecting rod length, porting positions and dimensions are accurate to within 5%. ( rounding off metric equivalents of inch dimensions) This gives a model capacity of  approx 160cc; hence the “Deltic 160” logo on the crank covers.

The following is a list of the major deviations from the prototype design:

1.The cylinder centers have been increased to allow greater cooling volume around each liner.

2.The connecting rods do not have split big ends due to scale space/strength restrictions. This has involved the design of a built-up crankshaft with ball bearing main and big end bearings.

Externally the design has changed to accommodate ignition distributors, spark plugs and multiple oil pumps (1 pressure and 3 scavenge). A great deal of external decoration has also been added.

The engine has 3 banks of 6 cylinders arranged on the sides of an equilateral triangle. There are 3 “V12” crankcase assemblies, one at each corner of the triangle. There are 36 connecting rods and pistons with 6 pistons operating in one set of cylinders and opposed by pistons from the crank at the opposite end of that cylinder block. The exhaust and inlet ports in the cylinder walls are “opened and closed” by the pistons.

Current state of the building program

The model is approximately 90% complete with the major structures in place including; crankshafts, connecting rods, cylinder liners, timing/phasing gearing, scavenge blower, exhaust manifolds, spark plugs, distributors, oil and water pumps.

There are no castings used. The entire model is machined from solid bar stock and “made to look like” castings.

The engine is currently assembled with tight slide fit pistons (without rings) installed to check the accuracy of machining and assembly and to prove that it is possible to assemble the engine with all piston assemblies fitted to the rods. I am happy to report that it does go together and that it rotates freely.

The oil pumps together with the full flow filtration system are complete together with the manifolds for the oil spray to the crankshafts and scavenge return from the two top crank cases. The water pump, with feed and return plumbing is also complete. The scavenge blower together with pressure relief and control regulators is now in place. The current project is the ignition equipment. The 18 spark plugs are fitted and if the engine is to run at the modest speed of 5,000 rpm it will require 90,000 sparks/min. If it were ever to get to 20,000 rpm (it is tiny inside) that would be 360,000 sparks/min! I am building a six-element infra-red optical trigger unit to mount on the end of the bottom crank. Each of these triggers will control one of six CDI modules. The final major project will be  the production pistons with the 144 rings; 2 compression and 1 oil control. In practice I may not populate all of the grooves.

Cylinder Block and Liner Assemblies

The cylinder blocks are machined in 3 parts; the centre section, the exhaust and inlet ends. Annular rings around the ports are machined into the mating faces, the exhausts exiting to the outside as per the prototype but the inlets to the inside only.

The cylinder liners are machined from grade 17 cast iron and are of the same design as the prototype with 4 annular grooves machined over the length for cooling water. The two central grooves are inter-connected by a series of axial grooves. At each end of the liners and between each annular groove there is an “O” ring.

Each cylinder liner has a discrete cooling arrangement with water entering via a drilled hole through the block and into the ring to the outer end of the exhaust ports. Water then passes through drillings in the block to the first of the central grooves, and into the second central groove via the axial grooves. From here it has to pass out of the block on the inside face through external “banjo” fittings to pass over the inlet porting and back into the final annular groove then out via a final drilling.

The liners are located in position by threaded bushes through the blocks which also receive the spark plugs. The block assembly is held together by 28 M4 cap head screws threaded into the centre section.

Crankshaft Assemblies

Due to the relatively small scale and lack of space it was not felt safe to have split big ends to the connecting rods. The crankshaft is therefore of the “built-up” design. This has enabled an all ball bearing assembly to be designed. During this process it was not considered practical to press the components together with the required degree of accuracy. The assembly is, therefore, bonded together from slide location fit components followed by threaded and bonded pins at each joint. There are three fitted axially at the main bearing to web joints and two radially at the big ends. Each of the 3 crankshafts is assembled from 118 components comprising:

7 main bearing shafts
6 big end shafts<
12 crank webs
9 main bearing races
24 big end bearing races
36 main bearing locking pins
24 big end locking pins

A relatively simple but accurate indexing assembly fixture was used to sequentially bond the components starting from the drive end and followed by a big end complete with con-rods and the next main from six down to one. On completion the assemblies spin freely in the fixture and the main bearing frames.

A single test assembly was first constructed to test the bond strength. This failed at one big end at a static torque of 17 ft/lb. This is more than adequate to achieve my wildest dreams with regard to power output. With the subsequent addition of the threaded locking pins I am confident about the static strength of the assembly but remain apprehensive regarding the high frequency torsional stresses applied to the relatively long and thin six cylinder shafts.

Timing/Phasing Arrangements

The timing and phasing of the crankshafts was designed to be identical to the prototype. The entire design was taken from the single cross section drawing shown at the beginning of this folder. When viewing the engine from the front (non-drive, blower or free end) the cylinder blocks are identified A,B and C clockwise with A to the left, B at the top and C to the right. The bottom crankshaft is therefore designated C/A and runs in the clockwise direction with A/B and B/C running counter-clockwise at the top left and right respectively. Number 1 big end of each shaft is at this end. All of the pistons are of identical design but are required to perform the secondary function of opening and closing either the exhaust or inlet ports in the liners. Again, when viewed from the front, the exhaust ports of block A are at the left bottom, those of B left top and C right top.

The firing order for each shaft in my engine follows standard 6-cylinder practice at 1,5,3,6,2,4. They are, however, 600, not 1200 shafts as this is a 2-stroke. Each of the 18 cylinders fires once in every revolution of the engine or a power impulse every 20°. I have subsequently discovered that the prototype engine has non-standard crank sequences  with the order of numbers 6 and 4 reversed giving the crank order of 1,5,3,4,2,6.

The firing sequence in each triangular bank of cylinders is identical, cylinders B,C and A firing at 40 degree intervals. The firing intervals per bank are therefore: 0°, 40°, 40°, 280°. The firing order for my complete engine is;

C1, A4, B1, C5, A1, B5, C3, A5, B3, C6, A3, B6, C2, A6, B2, C4, A2, B4.

For the prototype all the 4’s should be changed for 6’s and visa versa.

As with all modern internal combustion engines, the valve timing is designed with “lead and lap”. To this end the exhaust piston leads the inlet piston of the relevant cylinder by 200 of crankshaft revolution. This means that the effective TDC position; i.e., when the crowns of the two pistons in a cylinder are at their closest, does not occur when either of the big ends is at it’s TDC position. The exhaust is 10° after TDC and the inlet 10° before. It is easier to express all of the port timing in relation to BDC of the piston related crank.

The port timing for my engine equates to:

Ex opens 71°  BBDC.  (68°)
In opens 54°  BBDC.  (53.5°)
Ex closes 71°  ABDC.  (68°)
In closes  54°  ABDC.  (53.5°)

If this corrected in relation to effective TDC, the port timing becomes;

Ex opens 81°  BBDC.
In opens 44°  BBDC.
Ex closes  61°  ABDC.
In closes 64°  ABDC>

The exhaust period is, therefore, 142° (136°) and the inlet period 108° (107°) The exhaust lead is 37° (34.5°) and inlet lag 3° (5.5°).

The figures in parenthesis above are the actual values from the prototype which are published in relation to exhaust TDC and have been adjusted to the same base as my model.

Finding the right materials

It is always a challenge sourcing materials for engine components without spending a fortune. For the Deltic pistons Clen wanted the correct high Silicon/low expansion alloy and went looking for some scrap pistons from big diesel engines that he could cut up and make the model pistons from the crowns. His searches took him to a marine breakers in Portsmouth where he had been told they had dismantled some Deltic engines. No luck, but he did find racks of dozens of brand new pistons for large Paxman Marine Engines. These are approx 11" in diameter and 13" long. The skirts are 7/8" thick. Stamped into the inside are the material specs and all QA data. Perfect for what he needed. They cost £25, but what a bargain! A Paxman piston can be seen in some of the photos below. It is shown for size scale because all 36 model pistons will fit around the skirt of one of these large pistons.

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Clen is also an associate member of the Bay Area Engine Modelers of San Francisco, CA. This is how he happened to be displaying his engine at the show in Oregon along with the club and how the model came to the attention of the museum.

(My apologies to Clen for "Americanizing" his English spellings to conform to the conventions on this American web site.--Craig)

Clen Tomlinson's Napier Sabre—A Work in Progress

It can never be said that Clen Tomlinson takes the easy path. His current project is a model of another very complicated engine by Napier, the 24-cylinder, 2-cycle Sabre. It has two crankshafts, one above the other, each driving 12 pistons in arranged banks of six. Therefore, each cylinder head is six cylinders long and two high. It is a sleeve valve design, where the cylinder sleeves slide up and down while rotating to expose the valve openings and the piston cycles within the moving sleeve. The complicated cast cylinder head of the original includes all the passages for intake air, exhaust and cooling water. To duplicate this arrangement, Clen machined the passages in layers of billet aluminum using CNC technology. The layers are then fastened together virtually seamlessly to make up the head. A complicated gear driven mechanism rotates arms at each cylinder that have a ball universal joint to both rotate and move the cylinder liners in and out at the same time. The motion almost has to be observed to be understood. Shown below is a "spare" head that Clen loaned us for display at the museum that has now been returned to him. We hope you got to see it while it was here, but if not, here are some photos. We look forward to seeing the completed engine one of these days.

Overall views of the cylinder head from various angles. (Click on any photo to view a larger image.)

Details of the gear-driven sleeve valve actuating mechanism.

(Left) Valve openings can be seen in the cylinder sleeve. (Right) The head was machined in layers to be able to duplicate the complicated intake, exhaust and water passages in the original casting. Some of the tight-fitting joints between the layers can be seen here.

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Here are photos of Clen Tomlinson's Deltic engine:

(Click photos to view larger images.)

Napier Deltic engine as seen at the PRIME show in Oregon, September, 2003.
Left side view of the assembled engine

Right side view of the assembled engine

Three crankshaft main bearing frames—The main bearing frames are the center of the crankcases which form the “corners” of the triangle and bolt to the three cylinder block assemblies to form the main structure.

Three sets of cylinder block components—Each cylinder block is assembled from three sections to allow the machining of the internal water, inlet and exhaust passages.
36 connecting rods—There are 18 inlet and 18 exhaust connecting rods of the fork and blade design.  They are only 48 mm centers and are machined from Titanium.  These had to be machined on a conventional milling machine as the spindle motor of the Denford benchtop CNC could not drive the cutter  and the stepper motors could not drive the feeds. Each big end carries two miniature ball races; two together in the inlet and one on each side of the exhaust. The rods are so small that split big ends were out of the question, hence the built-up crank design.
36 pistons sit atop a Paxman piston skirt—The running pistons are machined from a Wellworthy high Silicon/low expansion alloy. They are only 16mm diameter and the Paxman skirt was 22mm thick.
A single model piston sits atop the skirt of a Paxman piston. This big Diesel piston had exactly the material composition Clen was looking for to duplicate the metal used in the original Deltic pistons.
Blower drive one-way centrifugal clutch—The blower is driven from the rear of the output shaft via a long flexible shaft through a six times step-up gearbox. This engine carries no flywheel and accelerates/decelerates very quickly. It was felt necessary to provide extra protection.
Cast iron cylinder liner—The liners are of the wet type and carry a series of  circumferential “O” rings to separate the water and gasses. They are located in the blocks by a Bronze bush screwed into the centre section which the houses the spark plug.
Liner with dummy piston
Liners in block 2
Crankshaft assembly fixture—This shows the relatively simple but very accurate assembly system.
Crankshaft assembly index plate (counter-clockwise)—See the crankshaft assembly fixture 4 photos above.
Crankshaft assembly index plate (clockwise)
Crankshaft assembly in case
Crankshaft assembly with dummy pistons in case—This and the following picture show the crankcase assemblies ready for assembly to the blocks.
Crankshaft in assembly fixture
Cylinder block center section 2—This and the following 7 photos show the machining for the various gas passages within the blocks. They are secured together by means of cap head screws threaded into the centre section.
Cylinder block exhaust end 1
Cylinder block exhaust end 2
Cylinder block exhaust end 3
Cylinder block inlet end 1
Cylinder block center section 1
Cylinder block inlet end 2
Cylinder block inlet end 3
Engine assembly as of August 1, 2003
Exhaust and inlet connecting rods
Pair of connecting rods
Phasing gear assembly as of August 1, 2003—This shows the method of coupling the three cranks and taking the power out.
Pipe "bends" in manufacture—This illustrates the method used to form the very tight radius bends of the external water plumbing. Rather than bend pipes, the shapes were machined in two halves and then silver-soldered together to form a pipe.
Single counter-clockwise crankshaft assembly CA
Top of cylinder assembly 2
Two clockwise crankshaft assemblies, AB and BC

Miniature Engineering Craftsmanship Museum, Carlsbad, CA

The model Napier Deltic engine was shipped from England to the USA in March, 2011 for display for one year at the Craftsmanship Museum in Carlsbad. This gave us the opportunity to take some additional photos of the assembled engine. The first and second photos show the engine with its right valve cover removed so the workings of the pistons, rods and crankshaft can be seen. A 6" scale and US Quarter can be seen in the foreground for size reference in the first photo. The second to last photo shows the three Bakelite distributors, while the last photo shows some of the extra parts, some of which were machined using computer-controlled equipment. Many of the tubing parts were created as CAD drawings which were split down the middle and machined in two halves. Then the brass "tubes" were soldered together to complete the round cross-section while assuring that all the complicated plumbing would fit perfectly when assembled. In the lower right of the last photo is part of the delicately welded sheet metal exhaust system.

This engine will be on display in the Craftsmanship museum for one year starting in March, 2011. It was also taken to the WEME show in Pleasanton, CA in August, 2011, and Clen was there for a visit to talk about the engine in person.

To demonstrate the complicated motion of the Deltic's three crankshafts and 3 banks of cylinders Clen created this demonstration model. It features the original Junkers two opposed pistons in one cylinder on the left, the Deltic's triangular configuration (center) and the diamond shaped 4-crankshaft design (right) also tried by Junkers. The secret to getting the 3-crank motor to move  pistons in the correct directions was to reverse the direction of rotation of one of the three crankshafts. The model is driven by sets of gears on the back and an electric motors powered by a model train transformer.
Clen flew out from England and spent a few days at the Craftsmanship Museum in Carlsbad before heading up to the WEME (Western Engine Model Exhibition) show held in conjunction with the GoodGuys Nationals hotrod show at the Alameda County Fairgrounds in Pleasanton, CA August 26-28, 2011. Here a video crew hired by the show promoters interviews clen for a video on the hotrod show. Getting the tiny engines in front of a new audience was part of the reason for holding the model engine show inside the huge hotrod show. According to the GoodGuys staff, they want the model engines back again next year. The display was very popular with the crowd.

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