Hello everyone, 

I just got a  ‘69 AT-1 MX, not running, pretty much all there with some vintage MX “upgrades”. Preston Petty fenders, a plastic gas tank I haven’t Identified yet, a 21” front wheel, a Hooker Header pipe, and some other stuff.

After becoming “State Champion” in his first race a few weeks ago, my son wants to keep racing, and I would prefer to keep my street legal AT-1, that I just got running, in one piece. The ‘69 showed up on Maketplace so we went and got it.

The cylinder studs and head nuts had been replaced with generic 8mm hex bolts. I found studs, nuts and washers from Babbitts Yamaha Partshouse, about $100 for everything. I looked around on ebay, but it seemed like I wasn’t gonna do much better price wise, and I would’ve had to use multiple vendors. If anybody has a better source for those things Id appreciate knowing about it.

Next is the cylinder and piston. I didn’t have the right tools at home, but the cylinder bore is 57mm, 1mm over the stock bore. The bore looks fairly new, no scoring, no  shiny spots. The piston measures 56.5 mm, clearly the wrong piston, but it, and the rings look nearly new. I will get better measurements tonight. 

Next is the base of the the cylinder. The machined portion of the base of the cylinder is about 8mm proud of the gasket surface. It fits nicely into the base, no burrs or damage that I can see. Beyond the machined part is obviously cut off unevenly. The least material left is prob 15mm from the gasket surface, and maybe 25mm at the biggest spot. All of the chamfer on the inside of the bore is gone. If I milled the cut part so it was even, and cut a new chamfer, would it be ok? Or is the cylinder junk?

I found a cylinder on eBay that looked pretty good for about $60, the description implied that it was bored .5mm over. I could have it bored to match the piston that came with the bike.

Thanks for reading
Jamie Galvin

 

It would be extremely helpful if you could upload pics to Imgur or Google or someplace and link them here.

Not quite sure what is going on with the cylinder.

Oh and congrats on the win! 

Is the cylinder you found for $60 a true "MX" cylinder?  Is that what you have?  Or is it an AT1 Enduro that has been made into an MX with the original cylinder and head?  If you need an Enduro cylinder, I probably have a few. 

The engine doesn’t have a starter/generator, and the controls look original and are not equipped with electrics. I’m pretty sure both engine and frame are MX. I don’t know how to differentiate either the enduro head or the cylinder from the MX versions, so I can’t say for sure.

I’ll figure out how to post links to pics tonight.

Thanks Mark

Link to pics of the cylinder

imgur.com/a/LIfLSec

Maybe your local Yamaha dealer will give you a discount on the studs they are available from Yamaha 

Maybe, the guy at the parts counter at one of the places I go had an at-1 as his first bike. He likes the fact that I have one running and registered. I’ll call them and see. I’ll report back.

If you're going from genuine "69-"70 AT1 which should be piston port then you 'should' need a rod change due to gudgeon height change in pistons between older & reed design.

Link to pics of head

i.imgur.com/C5z7lSP.jpg

Hey RT, do you think the cylinder in the pics is not from a ‘69?