Longboy's "TWIN REDEMPTION" Model Engine.

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Well-Known Member
Nov 19, 2009
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Tucson, AZ
Last year's "SPLIT MONO" engine was a unique effort and a successful build. What would a two cylinder version look like? There are two ways to go here......and inline twin with both cylinders on one side of the crankshaft or a cross cylinder design, like an opposed engine with one cylinder each side of the crankshaft. I got to play with a lot of gears in the process.....and it ended up in a surprise finish! :eek: The road to the T.R. engine starting this week!

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This is a couple months into the build. 4x4 aluminum angle, 4 in. long time 2 pieces. The cylinder hole on the horizontal is offset on the length from center to accommodate the camshafts. Two screw attachment. A pair of valve lifter holes at the front top.


Brass lifter body, steel lifter with 4-40 socket head adjuster screw lock in with 6-32 set screws coming in at the frame edge.


Two bearing carriers per cylinder are necessary for a split rod engine for the crankshaft stubs and are mounted in their bore throughs from equal off set to the center of the cylinder mounting in the vertical leg of the 4x4.



And mounted in place on frame.

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Using a different part number for the Traxxas spur gears and less $$$ at the time ordered. Milling the raised areas on the hub off for flush fit to the webs.

4T less and an eighth inch shorter in diameter. Cutting back the diameter of the web round stock for the fit.

Fitted to the backside of webs with three 6-32 machine screws.


Piggy backed to the backside of gear and web to balance the piston/ rod assembly, a half moon shaped brass counterweight.


One machine screw with blue Loctite. Additional weight reduction of the steel web will complete the primary balance.

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Making up a pair of cylinders and their heads. Cyl. get some DOM one inch bore liners for that size piston.


Matching one cyl. head to the other. Central spark plug location.
Twin port valve blocks are made up from quarter inch bar stock.

Self contained units with brass guides, leaded steel valves, o-ring seals. Four corner bolt together. A single fine thread 1/4 in. screw mounts blocks to head.

A flat is machined to lower side of heads. Drilled for both ex. and in. ports at 5/32 in. Brass tube inserted with o-ring seal between head and valve bock. Upper half of valve block drilled for mounting bolt to head.


And as it appears projecting out from head/ cylinder assembly!
The pistons for a dual rod engine are somewhat weird. They are longer and have a more solid core as the rods are attached to their lower third, therefore heavier. A flat is machined across the bottom for the rod attachment. The depth of cut determined by the rod centerline to match piston centerline.

A couple of parallel 4-40 threaded holes for the small end attachment. A standard plumbers O-ring for the combustion sealing.


Rods have the same strangeness to them. With the crank webs offset to the side of the cylinder, you get steep rod angles at their 3 and 9 O'clock positions. The outside cut aways in the rods are to clear the bottom of the cylinder liner in motion.

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The two halves of the engine mockup. Have to square up with end plates to the frame. Run a 5/16 in. transfer punch across through web to web to come up with a critical shaft centerline.


With the lower crank endplates in place, how the bottom end will look with six bevel gears. I did not particularly like the vendor's gears here. They are dark end to end with the heat treat. I order 6mm hole thru and re-drill to 5/16in. A couple would not drill cuzz of the hardening. Ordered others from a different provider after the experience.

The cams come into the frame at 90 degrees to the crank/output shaft. My lobes crafted in brass this time, the easiest to cut and file a profile to. Slipped onto a transfer punch and by line of sight location to the vertical leg of the 4x4 pieces.

The marking drilled and bushed and the lifter adjust screw range will take up the valve timing needs.

The upper end plates have the distributor and cam drive shaft. MXL sprockets will mate the crank and cams.
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The cams driven by some .5Mod, 24T brass bevel gears. Three up front and two more for the distributor.

The bottom shaft will get a shaft stop collar to keep the gears in mesh.

The ign. points will sit on the vertical leg of the 4x4.

And its cam will be the shaft stop collar for the other side.

Thinking about distributor being in center of the shaft valley........but no. Needs another crossmember to support the dist. body and further limiting access this area.

Cam drive shaft will head to the back of engine and mount to top end plate. Some relief for the gear teeth milled in the end plate.

And the assembly ready to go.


An aluminum bracket for the insulated stud on the points bolt down with a couple of 2-56 screws to the top deck.


And from the top side......the geartrain connected.
Between the gear and bearing carrier, some Delrin/ stainless washers for thrust control on the shafts.

The brass bushing sits in a roller bearing and is tightened up against the face of the web.

I wrap the spockets with an unused belt cut in half and take a tooth count. Match that up with the belt length calculator and place an order. Best to go oversize on the belt by a tooth or so. If I need an idler...... I can fashion one up for the slack.
A little reduction on the engine width cutting back the horizontal on the 4x4 frame members.

About 3/4 inch.

A better proportioned component now.

Setting up the frame base. Either the parallel steel bars along side with the output shaft.....

.......Or running along side of the web shafts.
Outside vertical face and top deck of the 4x4 angle piece get the satin black.

My fuel mixer and manifold at the front top of the engine between the valve blocks.

🤔How do you join the carb to the valves? Fuel line tubing!

My sparkplug caps now home made, work well.

Ign. in place as well as fuel tank.

The button up for first runs.


Come back for a look/see this week now!
Here is the only video of TWIN REDEMPTION in its original configuration running...............

It has problems and I cannot prove the engine at this time. :eek: Gear clashes and broken roller bearings. My troubleshooting, possible remedies and alternatives to save the T.R. engine coming up!
Sometimes the near surrender of building models comes not during its construction but in its troubleshooting afterwards. The weak link in the T.R. engine is the bevel gears. When the death clash occurs the engine goes outta time between the two cylinders and time to take it apart. After about two clashes, the gear is destroyed, the teeth folded over. When two peaks meet, the shafts they mount on spread and the roller bearing in the carrier on the gear side grenades.

The solution: more bearing area. Add a third bearing in a longer bearing carrier.

I like to have some thrust movement in the shafts on my engines to relieve any tight spots in rotation. That plays on the backlash on the gears. Maybe that was enough to get them outta mesh. With roller bearings the end to end movement in the shafts are gone now, the spacers ride on the inner race of bearing and the lash is tight but the engine free wheels O.K. yet. And the clash continues.
This is what TWIN REDEMPTION looks like today!

To change a gear set takes me around 30-40 minutes. I have two timing factors to work with. The geared together webs and the tooth match on the bevel gears.....right about the time you wish you had three hands the size of a nine yr old to get in there with! Sometimes I get lost in the counter rotation of the crank with the engine upside down accessing the lower end getting it back in timing. I abandon the dual con rod arrangement so I have just three bevel gears instead of six to sort out with a conventional piston/rod set up.
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Conversion pretty straight forward over three days.

Going to overlay a 3/16in plate on the web side of the vertical.
Center the new bearing carriers in the middle under the cylinders.

Matching up each side.


And black it out again. These are the original carriers shown with Oilite bushings which I did not care for being a center mount shaft support.

Maybe have better luck on the sort out now.

Original pistons where reworked with new con rods made.
Another weak point was in the frame that would flex on compression and ignition. To box in the overhang, the upper end plate on the flywheel side was extended to the ends of the 4x4 horizontal and attached with eight bolts to end the flexing. It helped somewhat...........


.........but the real problem and solution is the gears. These 20T straight cut bevel gears are not right for the application. The gear lash was too shallow. They couldn't resist the sudden fire up of ignition even with retarded spark for too long. Used my spares up and have ordered some larger 25T spiral cut bevel gears. This should end the clashing and damage and TWIN REDEMPTION will live up to its name. Meanwhile I run it on one cylinder. The two straight cut bevels needed seem to carry the load no problem. The spirals should be here last week of May and we'll give it a go with further results then! 🙂

My spiral bevel gears showed up this week. A little larger, more teeth and better capacity for the duty.


Looks like the last step for this twin to prove the engine and it is working that way. ..........the sun came out! 😊 The birds are chirping and I have another engine to enjoy.


......you do too! Dave.


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