What's the VIN on your frame/engine?  Definitely not an AT1 (piston port) cylinder...  AT2/AT3 most likely.   Head has a 248 casting which was the AT1 prefix...  does head outside shape match the cylinder outside fins?   The combustion chamber part will be fine but I think they changed the shape of the head and cylinder fins when going to reeds.

I don't really see an issue with your cylinder, might be looking at wrong place. 

(Only the 250 changed the rod length when they went to reed valve.  125 and 175 used the same rod from 69 to 81 so you can swap to reed cylinder without rod change)

I might have some used studs but it would be a few days to look for them.

 

As Mark was eluding to, it looks like a 1973 AT3MX (reed valve model) to me . . . . .  but with an earlier AT1mx head.

I'd probably be looking for another 72 or 73 MX cylinder if it were mine ,,,, because the bottom of the cylinder sleeves have been cut off.

Measure the total height of the cylinder sleeve ... it should be at least 109mm to properly support the piston
Anything less, and I'd be concerned that the piston might not have enough support as it swings through the bottom of the stroke.

If you decide to use it, you don't necessarily need to taper the bottom edge, as the original taper is just there to aid installation of the piston, and especially rings, during assembly.  I'm just saying, that putting the tapered edge back on the sleeves is going to shorten the supporting surface even more
I would however, (barely) roll the sharp edge over with a file or sand paper .... but imho, best to just replace it. 

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Be interesting to know the part number on the cylinder base.
Not 123cc etc but should have another number or part of a number i hope.
Mark & Enduronut are onto it though & i'm giving "fake news" lol.
Good to know there "wasn't" a rod change between early & late non reed & reed models.
Gotta update my memory as think i've done that bum steer info before sorry. Yep gotta be "72 at least, that cylinder in your link, as reed inlet started in "72.
Have i missed a pic of your original cylinder or am i just confused!! & this is what your using?.
I think your head is just std compression not high comp. See what the others think. Possibly the cylinder is also enduro not MX go fast!!.

Might be worth a look. Can crank up the quality but maybe its only my old computer that plays low quality automatically & modern just go hard & clear straight off. I just went to 720p as max.

Are there more differences between an enduro cylinder and an MX cylinder than the port sizes and the fins being cut to accommodate a pipe?

Is the head in the pictures I linked to an MX head?

I found a piston ported MX jug on eBay, but it is $210. After watching Dale Sweger’s videos, I’m confident I could layout and grind the ports properly.

If the head I have is an MX head, would it work with a modified enduro head?

Am I headed in the right direction?

The hacked reed valve jug seems like it’s not worth using.

Thanks for reading.
Jamie Galvin

VIN number?

I shoulda did this first of course, the vin on both the engine and steering tube is:

AT1-336004, I believe that makes it a 1973 ATMX. Sorry for the confusion fellas.  I didn’t find a number on the jug yet, I’m gonna clean it and look again. I’ll report back if I find something.

I’m gonna look for a cylinder, head and piston, I’ll report back.

Thanks as usual
 

If you don't have a strict class displacement restriction, you could use a CT2/CT3/early DT175 top end and make it a 175.

175 top end is probably cheaper and easier to find... much smoother power and almost the same maximum power.

I've seen a couple YZ125's with the 175 top end swap, makes for a nice bike.

Thanks Mark, I’m prob gonna go that route. Many more choices, and better prices.

Is the small end of the 125 connecting rod the same for the 175?

Thanks
Jamie 
 

Rod is the same, bearing is wider to fit the 175 piston. Easy way to find the right one is use the 1977-79 IT175 part number for the little end bearing as the 125/175 parts lists are shared and lost info when they converted to digital.