I have two Yamaha AG175's, 1981 and 1990, both twin-shocks (I am from South Africa). They were fairly common bikes at one stage here but are becoming increasingly rare. I recently took both motors to an engineer to have them re-bored. After assembling and driving the one for a very short time, it busted the rings and did some damage to the cylinder. The barrel is basically a write off. I think the problem was that the ports were not properly chamfered. I guess I assumed that the engineer would take care of this. Anyway, now I am looking for another barrel for this bike but they are near impossible to find these days. I as hoping that I could get one of those Chinese made top end kits and just pop it on but I am concerned that I will end up buying something that does not work. Any suggestions? 

We are not that familiar with the AG bikes in the US...  does yours have the 4 long studs that clamp the head and cylinder together to the cases? 

Or short studs that clamp the cylinder to the case and then more short studs that clamp the head to the cylinder?

Hi, Mark
These ones: short studs that clamp the cylinder to the case and then more short studs that clamp the head to the cylinder.
I believe they are very similar to the 1975 DT175

"75 has the long studs old style. Your will be like later monoshock DT175 as you say--short studs at the base. Not sure if DT is a direct swap but need to find a local wrecker/dismantler to compare. DT cylinder could liven it up if it fits as porting will be less restrictive as in farm bike style restrictive porting. 100cc Ag's stayed long studs right to the end but not 175's. We get the same model Ag's in NZ as you guys do.
Talking about your damage, i really don't think lack of champfer would be the direct cause of your damage. Sorry to have to ask but i guess piston was in the right way--just thinking ring ends caught in exhaust port. Or a pirate piston with ring pegs in the wrong place & rings catching in transfer ports or inlet. All a wild guess on my part. Here's a link to "75 & later DT's to compare. Cheers from NZ.
www.partzilla.com/catalog/yamaha/motorcy...b/crankcase-cylinder
www.partzilla.com/catalog/yamaha/motorcy...1980/dt175g/cylinder

Here's a DT175 cylinder on Ebay. Copy & paste into the search bar as my computer won't add a live link with pic for some reason. I had a chuckle looking at the pic with the wrong base gasket. But reading the story i can see why they put that to show early type & later to compare--so good on them.

well that didn't work as link wouldn't post--ok try ebay number 272386367550

My bike is definitely a twin shock, not a mono shock. The mono-shock engines are not compatible. Here is a pick of the engine. The piston was definitely installed correctly, it has an arrow on it to show the direction. This actually happened twice. The first time, there was not much damage to the cylinder, so we just popped another piston in and ran with it. It broke again after a couple of rides. If the piston was backwards, it should have busted the rings on the first ride. 
I did not mention that we bored the engine to 4.0 oversize so maybe its an overheating problem, I was trying to get max out of the engine so maybe I just pushed it too hard. The bike was running hot but it is hard to say as I did not have a way of measuring engine temp.
I am a beginner at all this bike stuff and was experimenting. 
I don't think that barrel in the link will fit my bike

Holly hell is that mess or what. Ag's stayed twin shock right to the end but i can 'just' see it is a 6 stud head so separate sleeve nuts to hold the cylinder on down through the fins of accessible externally at the base--whatever, she's poohed. Not sure if its damage causing what in can see but piston looks like one of those Chinese ones with advanced port timing?. If so, i wonder if that goes partly to giving a reason for twice failing. I'll find a piston pic.
www.aliexpress.com/item/32502893040.html...%3Bsearch-mainSearch
Play the video to see the top. Was yours cutaway like that or just an ordinary piston
www.aliexpress.com/item/1005001743388680...%3Bsearch-mainSearch
 

Thanks, this is actually what has been bugging me for a while. I struggled to get pistons for this bike and had to do a measurement of the old piston to find what was the closest. The place I bought it from probably just sold me the cheapest junk they could get. I have two of these bikes and did the motors at the same time, when the one failed the first time, I used the other piston that I had bought for the other bike in the same motor. I still have one untouched barrel but the idea is to get both bikes going, sell one and keep the other. I have heard that the Chinese barrels are not 100% the same. That is why I have been hesitant to get one. Is it not possible to to buy the whole kit and fit it to the case without hassle. I just want to get rid of this bike but I also don't want to sell it and have it break down a short time after.

looking at your pic again it looks randomly eaten away & not inline with exhaust port or anywhere in particular [if what i 'think i can see is rats eating away] , might be just my old computer not clearly showing the pic.