Building a twin cylinder inline i.c. engine.

I guess the bottom line is you need to design the engine (crank and cam) to suit what is possible / required for the ignition you want to use. Turn it over in your CAD program and watch the timing of the whole assembly and make a plan..

Fourty three dollars worth of bearings from Canadian Bearings, some semi finished connecting rods and some semi finished pistons.

Brian Rupnow said:

I just talked to my cast iron suppliers, and they want $40 for a piece of round cast iron 2" diameter x 4 1/2"long. That would be enough to make my two cylinders, but man, that price just kills me. Aluminum cylinders don't work well, they constantly shed a sludge made up of lubricant and the inside of the aluminum cylinder wearing away. I normally make my cylinders from cast iron, but with the price of cast iron, I'm looking for a cheaper alternative. I've been wondering about a cold rolled steel cylinder running with a cast iron piston and Viton o-ring. Any advice would be a good thing, but don't tell me unless you've done it yourself please.

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Brian, I am too late on my reply - but yes, the "Steel Webster" that I made a few years ago uses a steel liner* with a cast iron piston and viton o-ring. I've never tried for an endurance test, but it runs fine any time I get the urge to start it up.

*not a totally steel cylinder, but rather a steel liner in an aluminum cylinder - hopefully to provide better cooling, though I don't know how much difference it is really making with the low run time ...

Okay Awake, thank you for the answer.---Brian

Best look at the counter balance weight design as those heavier iron pistons will make the engine a bit jumpy.

Jason---Good point that I never thought of. All of my engines are a "bit jumpy".---Brian

Today I finished the connecting rods. I had to make up a fixture to hold them in place on the lathe so I could turn the relief of 1/16" on each side. They seem to have turned out well, and now I can finish my pistons.

Looking back through the files I keep on all the engines that I build, I see that I used a double ended coil from a two cylinder snowmobile on the opposed twin that I built about ten years ago. I also see that I paid $10 for it. I called one place today and was quoted $50 for a used double ended coil. Tomorrow I will call around and try to get a lower price.

Harley motorcycles use dual output coils too. Onan twin engines and their generators also use dual output coils. Just a couple more sources.

You don't still have the coil from the engine you built 10 years ago?

Regards ignition timing, for an inline twin there are three popular crankshaft designs:

360 degree crank, the crankpins are in line with each other and cylinder 2 fires 360 degrees after cyl #1. This results in even firing with 360 degree intervals, but the crank has the same dynamics as a single leading to lots of vibration.

180 degree crank, where the crankpins are 180 degrees apart. Less vibration than the 360 crank, but very 'lumpy' uneven firing due to #2 being only 180 degrees after #1.

270 degree crank, the two crankpins are 90 degrees apart and #2 fires 270 degrees after #1. Practically a compromise between the 360 and 180 cranks in terms of vibration and smoothness, it also has the same firing intervals as a 90 degree V-twin. Very popular on motorcycles due to the exhaust note.

Hi Nerd, There are a number of reasons the 90degree crank is selected for high performance bikes. A key point is the performance of "rubber on road". The somewhat uneven spacing of pulsations eliminates some resonances in the hysteresis of the rubber tyre walls as the back tyre is in torsion - which cause tyre wall heating and shorten the fatigue life of the tyre wall. Also the odd pulsations give riders "better feel" as the pulse-pulse - gap - pulse-pulse - gap causes the second pulse to develop slip on the limit of tyre-road contact, followed by enough gap to restore grip. Hence a partial slip condition that helps the rider retain "maximum" grip on acceleration and when powering-on in a cornering. - One reason Ducatis are popular as they have the 90degree within the Vee-twin configuration.
But I think it is simply trying to get all the advantages of a Vee-twin in terms of balance, torsional power variation, etc. from a cheaper to manufacture vertical twin (compared to a Vee-twin).
I did read about a guy who built a 75degree offset crank for a Triumph vertical twin, which was very smooth for primary and secondary dynamic balance, as the "vertical" imbalance of one cylinder was off-set by the "horizontal" imbalance of the sideways motion of counterweight versus big-end and con-rod of the other cylinder... The power imbalance was a torsional issue that was insignificant compared to the mechanical imbalances, or so I understood? - Something to do with out-of-phase sine waves?
Haven't seen it in 40 years though so if anyone knows what's what, I am curious.
P.S. SORRY if this detracts from Brian's excellent build thread.
K2

I've went back in and changed the heads so they are happy with 10mm threaded sparkplugs. This removed the "slope" that was originally on the back half of the heads, and now the sparkplug sets vertically. This necessitated a number of other changes, and now my pushrods have an offset bend in them. The red angles added to the main crankcase prevent the pushrods from turning out of line.

Wy not just have straight rods running at an angle, works for plenty of other model engines. If you use a ball ended cutter to do the hole in the tappet it will be fine.

Jason--I might do that.

Jason's idea is used on many motorcycle engines. Spherical ball-sockets at each end accommodate the misalignment. Straight rods don't buckle as readily as anything bent - so are lighter and permit more revs. (tube steel or solid aluminium). This model uses piano wire push-rods that are about 1 1/2 in. long and at a Vee, from cam followers only about 1/4in apart to rockers over 1/2 in apart.
The cam followers are not aligned with the rocker ends. But this engine has run, so must be OK...
K2

Lets face it the top of the rod is always at a changing angle anyway as the rocker moves in an arc. Provided you don't get silly with the angles a few degrees won't hurt.

Not a whole lot accomplished today. I did finish off the pistons, make the wrist pins, and assemble them with the connecting rods this afternoon. This morning was spent redesigning the cylinder head so I could use a 10mm thread size sparkplug. The pistons have not been lapped into the cylinders yet. Any scuff marks on the pistons will disappear when I do that. I am in a lot of pain this week, and am going in for knee replacements at the end of this month.

Brian Rupnow said:

Not a whole lot accomplished today. I did finish off the pistons, make the wrist pins, and assemble them with the connecting rods this afternoon. This morning was spent redesigning the cylinder head so I could use a 10mm thread size sparkplug. The pistons have not been lapped into the cylinders yet. Any scuff marks on the pistons will disappear when I do that. I am in a lot of pain this week, and am going in for knee replacements at the end of this month.

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Rotten luck with the bad knees. A lot of my family members are afflicted with a genetic disease that means some of them need artificial joints (ankles, knees and hips) by the time they are in their 20s, some others last a bit longer but they all go wrong well before their time (thankfully i don't have that mutation, so my joints are safe for now). I joke with my uncle that he's more metal than man at this point. As a silver lining, I suppose you'll have a bit more in common with your engines after the surgery...

Have you decided what kind of crank you'll use yet? Also I notice that there are no shells in the conrod bearings, so will the aluminium be running directly on the crank?

Nerd1000 The aluminum I use is 6061 T6 and it stands up very well without bearing shells. I've never had one fail. As far as the crankshaft goes, I haven't decided yet.---Brian

Hi Brian, Sorry to hear about the knees. Thought you may have knocked-up and fitted some new ones in a spare hour? My knees are still managing - despite having hit roads and cars when I felt my motorcycle should not be carrying me along the road so painlessly.... I do have moments in life when all is running so sweetly I have to balance that with a bit of pain.... but from friends I know that knees needing replacement are VERY painful. Just hoping mine last a couple of decades yet, as I have no plans to stop using them...
Hope you can keep on modelling..
K2