4 Cylinder Inline Engine

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Spent the day working on the connecting rods.
First was drill the holes for the rod bolts.

Then cut the ends off and tap the threads.

The needle bearings I am using have a rather thin shell so not to squeeze them too tightly with the rod caps I used an expandable reamer and an extra bearing for fit.
Once I had the first rod cap size set the other three were a piece of cake.

Thanks for looking
I thought about it before I milled out the center section of the block but decided against it. The compression ratio will be 6 to 1 with a 1" stroke and don't expect the rpms to exceed more than 2500. I like slow running engines. If it proves necessary I can cut the crank in half and add another bearing and add a block to mount it to.
Thanks for the comment.

I machined 4 connecting rods on the CNC today but it took me twice as long as it should. There was a power glitch that spoiled one of the rods and I had a couple of senior moments but I got the job done.
View attachment 154696

Made some slots for the rod bolts to set in and then halted machining for the day.
View attachment 154697

One last thing I wanted to get some paint on the crankshaft before it started to rust.
Looks pretty nice.
View attachment 154698

Thanks for looking

You might be surprised by how well it holds up even at higher revs. The Austin 7 car has a 4-cylinder engine with only two bearings, but broken cranks are apparently only an issue if you race them. Having handled an Austin 7 crankcase and cylinder block, you'd be forgiven for mistaking one for a large model engine, they really are very small.
Rev limit will have zero bearing on how much your crank will bend.
I can see it being a problem running unsupported over that distance.
The crank is subject to 'whirling' motion independent of combustion forces. That will become a significant issue as RPM approaches its 'critical speed'. https://roymech.org/Useful_Tables/Drive/Shaft_Critical_Speed.html

Rdean will have to calculate this to be sure of his limits.
I doubt 2500 rpm would be considered anything critical,normally, in fact will be more like idle speed.
Built quite a few running and frequently used engines in my time(without needing any whirring motion calculation....what evere it might be) and this simply doesn't look right without any centre support.
I doubt 2500 rpm would be considered anything critical,normally, in fact will be more like idle speed.
Built quite a few running and frequently used engines in my time(without needing any whirring motion calculation....what evere it might be) and this simply doesn't look right without any centre support.
In essence the critical speed is the point where any out of balance weight (and there is always some) can overwhelm the crank's stiffness. The crank will begin to whirl around like a kid's skipping rope (or jump rope). So obviously this will happen at much lower speed with no center support, as the long unsupported crank is less stiff.

I don't believe this would be a matter of concern at all for most crankshafts because usually at most two crankpins are unsupported between main bearings, and thus each unsupported section is short enough to have a critical speed well above any practical RPM. However, with a very long and wobbly crank (such as an I-4 with no center bearing) it does become a concern.
My thoughts are: The crankshaft will be fine, as long as they are assembled to tight tolerances and the crank pins are made long - The longer the better- as shown


Furthermore, the engine is just for run . It is not like the crankshaft of a motorbike, car, truck.....there are many forces acting on the crankshaft
Currently I'm building a 4 cylinder engine and the crankshaft only has 2 bearings, but I'm not too worried about it. What I need to focus on is making everything as smooth as possible.
This is a 2-cylinder engine with a similar crankshaft, and it's made quite carelessly, but it still runs quite a long time and is fine.

Nerd 1000, Blue Jets, and Minh
Thank you all for the interesting comments and I do take them seriously and objectively but for now I will stay with two main bearings.

The crank looks so nice and shiny that I decided to paint the engine block.

I have this triangular piece of aluminum left over from my 5 cyl rotary build. It is too good for the scrap bin but it never seems to work for what I am making at the time.
So nothing ventured nothing gained.

In there somewhere are four motor mounts
I just need to get rid of the other bits.
This is fairly close to what I imagined.
Maybe a little more machining to do.

Thanks for looking
Been working on the pistons the last couple of days.
This shows cutting the o-ring groove in the piston.

And a group photo.

Slotting the piston for the connecting rod.

Drilling for the wrist pin.

Ready for the connecting rods.

I installed brass bushings in the piston end of the connecting rods but didn't get a picture.

Everything fits and turns smoothly.

Happy days.

Thanks for looking.
Cylinder head day.
Time to see if my calculations are correct.
First spot drill the hole locations.

Then on the drill press finish drilling through for the head bolts, intake, and exhaust holes.
Counter drill for the head bolt recesses and partially drill for the spark plug.

The spark plug hole gets finished later just before final assembly.

I am happy to say the 16 head bolt holes all lined up just fine.

Thanks for looking
I made 8 brass valve guides, these are longer than usual for me. I am planing an overhead cam for this engine and I noticed that on the other engines I had built with overhead cams the valve stems were subjected to some side loads. Making the guides longer should give the valve stems more stability.

Installed the guides.

Thanks for looking
Brass will last about as long as it took to install them.
Bronze on the other hand...............
I made up 10 valves today so I could get 8 that I liked.
This is the way I make them.

All valves cut to length and threaded. The intake valves are shorter by about 1/4".
Because I didn't want to make any more.


I have not lapped them in yet but that will have to wait for a few days as I will be resting aboard ship.

Thanks for looking
Got home yesterday all rested up and ready to go.

First item is to make a valve leakage test plate to fit this engine.
I can will work on one cylinder at a time lapping the valves until I get good results.

Testing Cylinder #3 valves.

All four done for now but I won't know just how good until the head is bolted to the engine block with a head gasket.

Thanks for looking