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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.

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...

Here's my planned solution. Basically we have a slot milled across the mounting point, and two hold down blocks with screws fit into it with their 'toes' over the mounting flange of the injector.

A bit more cylinder head work: This forms the seat for the injector. The comedy of errors continues, as I broke my tap while threading the holes for the injector hold down bolts. It's a tiny m1.6 tap, so I've no chance of getting it out of there. So I need a plan B. Currently I'm thinking of...

Not running a centre bearing?

What is often used industrially is nikasil. It's electroless nickel plating with embedded silicon carbide particles. I have looked into it, however there are quite a few issues with DIY hard coating of any aluminium parts: 1. Dangerous/difficult to get chemicals are needed. How bad this is...

I've long wondered about the issues of doing this with a multi cylinder crankshaft. I guess you'll have to make some kind of alignment jig for pressing the sections together?

It's Oscar, in fact.

We have valve guides and seats, and inlet and exhaust ports. I've made a goose of myself and drilled the exhaust port in the wrong place (it's not a Nerd Build without a comedy of errors!), but I think I can live with the result. I also drilled and tapped the block for the cylinder head bolts.

For cams and gears the main source of wear is often the Hertzian contact stresses, basically from the slight deformation of each part along the line of contact. Over time these stresses will fatigue the metal below the surface, eventually the resulting fatigue cracks reach the surface of the...

Very nice, and much more finely finished than the general butchery I did on mine.

Hmm yes I knew taking it out could cause issues even as i did it. Unfortunately my setup didn't have clearance for my 13mm drill so I had no choice :(

A bit more progress on the cylinder head. I've bored out the seat for the pre chamber cup, drilled the valve bowls, and counterbored the head bolt holes. I'm contemplating various ways I might cut the valve seats. Initially I thought I might make a piloted cutter, but now I wonder if I might...

I noticed that various types of brakes for go-karts can be had from Ebay for a fairly low price.

Are the clearances just a bit tight?

How will you make the crankshaft?

Work is now in progress on the cylinder head. For now that basically means I milled the stock to size and drilled some holes. The corners are for the M5 head bolts while the two small holes in the middle are reamed to 6mm for a press fit of the valve guides. The larger, offset hole forms the...

I suggest a tweak to your breather: make the top a barb fitting for a bit of tygon fuel line, and route that to your intake downstream of the carb. This way the intake vacuum will assist in pulling the blowby gases out of the crankcase, and you'll reduce the emissions of oil mist when you run...

Ok, as promised almost 2 months ago, here's the gear after milling, and the backplate on the engine! Not the prettiest outcome unfortunately, I had a few boo-boos on the way. But it assembles ok and won't be visible when the engine is finished. Speaking of boo boos, I fumbled and dropped my...

What grade of silver solder did you use? welding suppliers will usually offer several, so you might be able to get one with a lower melting point that you can use over the top of the existing solder without melting the joint.