Longboy's "SPLIT MONO" Model Engine!

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Well-Known Member
Nov 19, 2009
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Tucson, AZ
I have a pair of single cyl. models in my collection. Want to add another......and show something that most likely, you have never seen before! Can you power up two crankshafts with one cylinder? 🤔 The SPLIT MONO story coming soon!
The nature of SPLIT MONO requires a piston that is larger in diameter and also longer in length. I use my 1- 1/4 in. DOM cylinder liner for this engine along with same size piston stock.


Appreciating the difference in size and final displacement with this comparison to 1 in. piston stock.


The piston at B.D.C. needs to extend out from the cylinder for service and assembly reasons as well as broad enough by its diameter as a support structure.
I salvage a pair of cast iron wheels from my 40 yr old floor jack no longer working and scrapped. Square it up from its hub in the three jaw.


Press in a steel hub. Narrow the width by a half inch and bore 3/8 inch. Flywheel to crankshaft mounting via the collect lock method. Brass retainer with the drive for the cordless drill starter.


Cleaned up real nice and at 3-3/4in diameter, will idle down this big bore engine well.


The flywheel weighs in at 22oz. Lighter by around a third plus from making one from steel 4 inch round stock. Will do the same with the other for future use.

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Two and two point five inch round stock for the radiator and cyl. head.


Cylinder liner to the head with O-ring seal.


Static compression ratio under 3:1. Piston rises to .250in. from top of liner. Combustion chamber .375in. deep into head.


.250in. aluminum angle deck bored close to the apex for cylinder mounting.


On the vertical face of the angle piece , bearing carriers for the twin crankshafts and a slot milled across for a short height deck piece of aluminum stock.


Now I am committed to this twin crank, single cylinder concept. From here, reasonably but not absolutely sure this will come to be.



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I salvage a pair of cast iron wheels from my 40 yr old floor jack no longer working and scrapped. Square it up from its hub in the three jaw.

View attachment 145915

Press in a steel hub. Narrow the width by a half inch and bore 3/8 inch. Flywheel to crankshaft mounting via the collect lock method. Brass retainer with the drive for the cordless drill starter.

View attachment 145917

Cleaned up real nice and at 3-3/4in diameter, will idle down this big bore engine well.

View attachment 145916

The flywheel weighs in at 22oz. Lighter by around a third plus from making one from steel 4 inch round stock. Will do the same with the other for future use.

View attachment 145918
I recently bought a cast iron 750 pound caster to use as well. After stripping out the bearings and boring for concentricity, I inserted a cast iron piece and drilled and bored for the crankshaft end. VOILA’, a cheap and effective flywheel! You folks just keep adding more “tools” to my toolbox, thanks for the inspiration and I look forward to your latest.

John W
The twin crankshafts rotate in R1810ZZ bearings. Each of the brass carriers holds two.

bearing carriers.jpg

An issue I was having is the bearings were from two different venders and one set had to be a couple ten thousands undersized. Would not slide up on the shafts and not turn freely when installed in both front/rear carriers mounted in the frame. The solution: Remove one bearing from each carrier. On lathe, run some 400 grit carbon paper on the shafts and file the area where the bearing runs on shaft a little. A pair of bearings per shaft is adequate and they rotate easily now.

Crankshafts are geared together. One shaft the flywheel. Other shaft the cam drive.


Crank webs.jpg

Now the mystery part. One cylinder and two crankshafts. The pathway and connection to the crank.....is the "bifurcated reciprocating assembly"! 😲
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Time to withdraw the "B.R.A." item from the mystery box. :oops:
Mystery box..png
A pair of unconventional looking rods to the piston emerge.


Somewhat nervous here. My concerns are that the throw pin position in the webs have to be extra accurate in placement. So one web has the throw directly under a gear tooth valley, the other under a peak. The same in the big and small ends of the rods, which were stacked and drilled as being a single unit getting their centerlines even. Going by instinct rather than geometry in motion math, if one rod rises up at a position slightly ahead of the other....will it cock the piston in its bore and bind? I set up on my workbench a test buck.


With some sample rods in place, hold the cylinder over the mesh centerline and rotate the webs.......🧐

.........The motion works fine! From here, the remainder of the build is familiar territory.

Cylinder and webs mounted in the frame figuring out the shape of my production connecting rods.


Radius the rod small ends. The scallop out into the rod beam is there to clear the bottom of the cylinder in travel.


With the crank pins at the 3 and 9 O'clock position, those rods are at a 30deg. angle to the cylinder! Un-nerving to witness, but the result of offset cranks to the cylinder centerline gives this jumping jack spread appearance and does not affect wear or running. No side thrust upon the piston skirts! 👌

The production con rods are serviceable one at a time without pulling the piston from the bore. They will be drilled for some bushings fixed to the webs and piston for them to swing upon.


Doing primary balance. The piston/ rod assembly is heavy, about 2.7oz. vs. around 1.2 oz. for a conventional one inch bore set and same length rod. Shown is a preliminary with just one web
this photo. A couple of small neo magnets hold some steel to the web to find that neutral spot.



Add both webs to the bearing in vice and the correct weight brings the assembly to that 3-9 O'clock unbiased area.


The counterweights are fashioned in brass and bolted to the backside of the web over the gear. Shared between two webs, their size volume is the same as for single rod assemblies.


On the rod big end, a hybrid shoulder bolt. Delrin bushing is threaded to the 6-32 SS socket head
and with blue Loctite, into the web. Great idea! No lube needed here then for the rod to swing upon.


In trouble shooting SPLIT MONO, These big end screws would loosen up and back out! My best guess is even with Loctite, could not get proper torque due to distortion of the plastic bushing in this application area. Remade the bushings in brass.......that problem ended. 🙂
Building up the lower frame. A couple skid bars tie in the upper component support.


Cross wise to these rails, steel foot bars with the rubber bumper feet.


SPLIT MONO being a large single has a small footprint upon the table. Going to add a steel plate between the foot bars. The added weight to quell the combustion vibes.
Now the engine can stand on its own and will be looking for placement of the cam, radiator fan and their drives. Make fuel tank, fuel mixer, valve block and place ignition points.
I like to do forced air cooling if room allows.


For my scale, I have a two inch bulkhead connector in black PVC with 2in. aluminum fan from Ace hardware.


Back of cylinder drilled/tapped for a 3/ 16 in. shaft after PVC is bored and trimmed for fan and mounting to upper deck of frame. The fan can run direct on shaft or thru bearings. A Delrin pulley pressed over fan hub for the drive at near 2:1 ratio.


Some aluminum sheet stock encloses the PVC shroud to the radiator finning by some 2-56 screws into the hex. The fan runs as pull back thru the shroud being driven by the clockwise running crank side.


The lower deck gets slotted for the drive belt and the slot can be of shorter length with an idler pully on the underside. 1/16 in. O-rings used as drive belts not available at this length so elastic pony tail wrap bands from the dollar store are used and they hold up well in this purpose.


And the brass drive pully on the crankshaft!


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Hi Longboy !
Since I know and join the forum, I've watched the engines you build, I hate to say one thing: I don't know the shape, how it works... until your engine is almost complete , that makes me "angry" .
But I really like the way you build your engine : build your own brand engine : " Longboy "
👍👍 .
Thanks Minh. Kinda out of the mainstream from those machinists trained to journeyman standards. Can drift out from following allowable procedures and bring forward a few unexpected custom design, mechanics and innovations to my engines without reference to any plans or drawings. SPLIT MONO follows that footprint. It's story concludes this week! ......Dave
The fining done on the cylinder and head. A single port entry in side of head covers both intake and exhaust duties to the combustion chamber.


A combination valve block meets the head port transversely. A single specialized bolt with the dual purpose of a fastener and a port created.



Fashioned on 3/8 in. brass hex and through the top block piece threaded in head. O-ring seals under the hex and above the 1/4-28 threads. Bolt is drilled up shank about half way. Torqued down to the cyl head, a hex face is marked with center punch. Then through the valve block port access, drilled through the shank for a T- intersection of both in/ex valves to the engine. The marked spot on the hex is at noon hour for the correct line up of ports.


The valve block with one fastener to head must remain parallel to the deck surface. If it tilts when running, the valve clearances get messed up.
A flat machined in from the bottom of the head ends at where the top of valve block will seat as a wedge preventing any tilt off.


My fuel mixer. The plastic extension needs to be metal as it is too hot here. Now I can locate the camshaft on the frame, the last major component..........
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The lifter bores are located and drilled in the top deck now that the valve block has been placed. Brass lifter guides inserted and locked in with some set screws from the edge of the deck. Forward from the bores a bearing hangs from the underside of deck to support the end of the camshaft.


Sampling up a geared possibility at the drive end of cam. Not quite right looking. Default back to my usual MXL cogs.


A long brass bushing support at the drive end. The lower deck is cut in from it's edge for the cog flange with the belt on.


Waiting for that belt somewhere on a slow boat from China.


The belts for cam and fan could be lined up so the single guide pulley bracket is replaced for this dual belt guide.


Lifters are drilled for 4-40 socket heads and manage the valve action directly. From 3/8 in. leaded steel rounds come the cam lobes. Off set drilled for the shaft and the ramps milled across. #6 set screws lock lobe to shaft. The advantage.......variable valve timing to whatever the engine likes.


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Just a couple more items to go before the show here.

The reliable Tecumseh points set.


Cutting off my fuel filler neck.


A 2.5 OZ fuel tank on the frame.


Going with a vinyl wrap across the top surfaces of frame in satin grey. Undersides in satin black.


☺️ This is what five weeks in the garage looks like! Individual parts and subsets.


And you definitely don't want to miss the next posting from LONGBOY ENGINE. - Dave.

Always a great day to introduce a new engine! 🙂 Welcome to Longboy's 'SPLIT MONO" premier.

Another use of my geared together crankshafts successful in four previous builds. In those engines, each crank segment had its own piston.


And you can gear 2-3-4 shafts together with only one of those shafts having a connecting rod and piston driving the assembly. But could one piston drive two crank segments directly? 🤔


And the answer is yes as you see here.


SPLIT MONO build started 1st week Feb. and finished up last week of Mar.


SPLIT MONO I can say ( reasonably) is the 1st model built showing bifurcated rods on one piston. It would of been great to also say this is my original idea!


That is not so. Looking through web sources like Pinterest, YouTube and such, I have found only three examples in I/C with this arrangement. One being an educational discussion module on the concept. Another, a motorcycle racing development from the '90s. The third is a current production engine from a German manufacturer. IF YOU FIND THIS ONE, post it here! Then you'll have an ideal on how long I spend cruising the internet after shop time! 😉


Looked like it would fit my abilities and fit in with previous surprises around "something different". Maybe another machinist will try this out. It's a single. Sold in the real world. No mystery to the reciprocating assembly. Good luck with your projects and see the video now! .Dave!



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