School me on 4-stroke engine rebuilds

1) You need to pull the valve cover, ignition cover or bolt (for timing alignment), undo the cam caps so the cams can come free so you can get the timing chain off.  Once you have the timing chain off the cams you can then unbolt and lift the head and cylinder off.

2) just pull the valve cover off the top, again align timing marks for TDC and then use feeler gauge under the cam lobes. If you need to adjust, unless something is way off, you can change shim thickness that sits in the cam bucket.

Service manual is highly recommended, they're full of great stuff.

If you handle it right….you sell them right before they exploded…

Service manual and YouTube videos of someone doing the top end on your bike will be the best place to start. There's lots of good how to videos since you have never done one that will let you see first hand what it looks like. 

I promise you one thing they are not nearly as hard as some people make it out to be. There are just more steps to do the work.

For tear down make sure you bag and label everything as it comes apart in groups. For example a separate bag for each valve, spring, bucket and shim set. Make sure to lable which valve guide that valve belong to. I always set it up as the head is sitting cam side up with the head pointing forward meaning im looking at it and the intake(exhaust on the yamah ) is pointing towards me. Valve 1 is always gonna be the top left and valve 4 is always gonna be the bottom right. That way you dont mix up shims and valves with the seats they have been run in on. That is if you dont have enough room to lay everything out on the table. Timing cams takes a little time to get use to moving the cams while the chain is on. You'll bloodie some knuckles doing it at first. 

If you dont get a valve springs compressor get this tool. 99% of heads i do i use a socket and magnet to remove the retainers and spring and this tool to put them back in. Spring compressor is slower but you can take you time making sure its right.

Feel free to message me if you have any questions or when you go to do one. Im more than happy to help walk you through it.

Buying used    buy w low hrs

I did my first 4 stroke top end on my 350 a few years back. Service manual and buying the right tools made things go smoothly. Highly recommend buying an ultrasonic cleaner if you don't have one. I used it to clean up valves and other internal parts. YouTube is your friend too.

Falcon : what has the young bloke been riding up to / including now?

If he's comparing a 250F to 85s / Superminis / 125s, or even many 250 2ts, well, you could apply the need to be a "hyper aggressive 2 stroke style rider" .

But heck, you can easily set up a 250 2T to be smooth and easy to ride, whilst still being fast and competitive. And, far easier to maintain that a 4T. By the way, I'm Not anti 4T - I raced / built them for decades, and own / ride both 'T' types.

Flywheels weights, correct jetting of a Carb'd bike, different / healthy reed valves, Pipes  - AND the actual use of some models so easily adjustable power valves ( I've encountered so many 2T riders , with PVs that can be adjusted in Minutes, if not Seconds, that have No Idea of that being  available  and Free to them) and so on. I've transformed quite a few riders view of their 'unmanageable' 2 Strokes, with just a turn of a PV adjuster, or, a Change of a PV spring.  

But, if the young bloke has his heart set on a 4T, well, you're up against that determination. 

See if you can find someone with say, a KTM 'Group' 250 2T ( most common of 2Ts nowadays, though there are Plenty of well sorted YZs out there, never forgetting the Betas, TMs, Sherco's and Rieju's),  that has gone the route of usability - he / you might get quite a surprise how easily, and competitively ridden such a set up is. 

I think Rocky Mountain posted videos on YouTube of some of this on a yz250f in the last couple of weeks. One tip is don’t let the timing chain fall down in the engine although if you are looking at a 2023 or older yz250f you should be putting a new chain in with the top end if not more often depending on how much your kid likes the rev limiter. 

If you're buying a used 250F and engine maintenance is your concern, get an Austrian bike and not a YZF.  The Austrian engines are much more durable with less maintenance.  On the Austrian bikes, you can pretty easily  go 100 hrs with oil changes every 5 hrs.  Most people would not advise that on a YZ250F.  They have a smaller oil volume and need oil changes every 2-3 hrs.  Austrian clutch is also bulletproof.

Of course you have to be an "absolute Monster to ride a 125 and beat regular Joes on 250Fs"

That it was ever made a class rule : 125 2ts against 250 4ts : is an Obscenity's / Act of Idiocy / a Monumental f**k up.

Just pointing out that a 250 2t, can be made easily useable, and, pretty bloody cheaply.

I'm assuming your boy is racing locally , and, hopefully, he's not racing in an organization that applies AMA Pro rules, of course. I may be wrong, but even if you are chasing Lorettas,  there's capacity equivalency in classes in DC's Domain?

And yes, 250Fs have become so good, in and of themselves. Especially going from a frantic 125, to one, it can / will  be quite revelation to a youngster - well, anyone, actually..  There's no surprise in your Lad going Oh Yeah, when trying a 250F.

Just trying to give you some ideas (for him and you) to try other 250 2T set ups. You've the YZ - perhaps try a few things - though, you may well have it set to more a 'civilized' set up, you being an Old Dad ( just stirring you up 😜) Conversely, you might be one of those Old 'Hard Men' that has set up your YZ to hit like a Heavyweight Boxer, and thus, perhaps freaking out your boy. I've seen so many riders go from a 125, and (try to ride) a 250 like a 125, and not enjoy life..........

Me, I largely ride an ancient 500, that's a Big, Cuddly Teddy Bear to ride - though, it could, at Any time, still launch me into space, if I was less than precise, careful , or just bloody stupid. And, I can be Bloody Stupid, at times!

Not being a dick so dont take it that way. The fact you are even asking this question means you should 1000% be buying a new four stroke. You go out and buy a used YZ 250F with this level of mechanical knowledge and your bank account and enthusiasm for this sport will drain quicker than you can regret your purchase. Buy new, buy a honda, end of story. Also, your son doesnt "need" the four stroke power curve. Youre band aiding technique issues that should be fixed with training, not by switching bikes. Youll see "improvement" in the beginning, but the end result will not change. 

Whenever I work on things I’m unfamiliar with I like to have a printed manual with me. It way easier to flip through pages with dirty hands then use my computer or phone. 

Videos are beneficial to watch beforehand 👍🏻

The service manual will have all the details you need.