QT50 to 60 - bore or replace cylinder

Iain James /

May 16, 2009 12:47PM

My girlfriend complained her QT50 wasn't as fast as normal, and was stalling at idle, so I took a look at it - and she hadn't filled the oil injection tank. Attached is a picture of what I found when I pulled the cylinder off my girlfriend's QT50. You should see the cylinder wall...

Anyway, I figure if I'm rebuilding it anyway (and converting her to premix), I might as well go to 60cc. Looks like I can either try to find a YT60 top end with cylinder, head and all, or I can bore out the QT50 cylinder for a 60cc piston. Is one of these options better than the other? I can see how boring might change port sizes and timing... all you with QT60s, did you just bore your cylinder or replace it?

Also, does anyone know what the diameter of the YT60 piston is? I think the QT50 is 40mm, right?

Re: QT50 to 60 - bore or replace cylinder

Ryan /

ryandurepo

May 16, 2009 01:36PM

k the two ways you explained are reachable but one is more expensive than the other. you can bore the stock qt50 cylinder to 44 mm which is the diameter of the 60 cc piston and it will be cheaper, also you will need to rechamber the head or run 2 base gaskets if your going to run the stock 50cc head because they piston will hit the head. now the yt60 cylinder and head combo would be the best choice but also the more expensive. the yt60 has a higher exhaust port so it gets a little more power out the piston as it was made for it unlike the 50 cylinder. all are good choices, i bored my cylinder and rechambered my head and it ran great, one thing to check is your 50 cylinder and if it says 501cm near the base on the right side then it has had some mild porting done to the exhaust and intake.

Re: QT50 to 60 - bore or replace cylinder

Iain James /

f13ndus OP

May 16, 2009 04:30PM

Yeah, I've looked around a bit more, and I think I'll go with the bore/new piston approach. Got a machine shop just down the street so that's no big deal, I can order a piston now instead of waiting who knows how long to find the YT60 top end, and get this back together for next weekend hopefully.

I've noticed a couple PW50 "overbore" pistons on eBay, the largest is .080 over - 42mm diameter - if 40mm is stock, and 44mm is YT60, then I suppose 42mm would be 55cc? But that's about the same price as the 60cc piston so why not go big, right? But with the overbore piston, I don't think I'd have to rechamber the head - it should be the same dome profile, just wider.

This is for my girlfriend remember and she doesn't want to go faster - I figure this'll be an almost stock rebuild with more torque. Then again, maybe I can talk her into a jemco pipe as well...

Re: QT50 to 60 - bore or replace cylinder

Zeke /

ai

May 16, 2009 04:36PM

my piston in my fa 50 is just as bad. i had to rip out a ring because it was welded in, sand the piston so the rings went in, sand the cylinder because some piston was welded on it it runs fine now that i got new rings though :S

Re: QT50 to 60 - bore or replace cylinder

Iain James /

f13ndus OP

May 16, 2009 05:08PM

Yeah I'm kind of up in the air about just reringing it, but I don't think the cylinder wall is salvagable. I'll take some sandpaper to it and see. And hey I ride this thing too sometimes, I won't mind the extra displacement haha!

Re: QT50 to 60 - bore or replace cylinder

ottoman /

ottoman

May 16, 2009 05:21PM

If I remember correctly when they were boring the QT cyls to 44mm they were offsetting the bore to leave extra material on the thrust side of the cyl. The 44mm is the absolute max you can bore the QT50 cyl.

I would go with the 42mm overbore for better life or find the 60 cyl and head.

Re: QT50 to 60 - bore or replace cylinder

Iain James /

f13ndus OP

May 16, 2009 05:28PM

Sounds good to me. With the 42mm will I notice much difference? Some more torque? Of course, it'll have better compression too so I won't know what's from the extra displacement...

Re: QT50 to 60 - bore or replace cylinder

mat /

84_qt50

May 16, 2009 08:34PM

i would get the whole yt60 moter because its guts are a little more beefier, get her a jemco pipe to, she wont be complaining about goin to slow then lol

or find a yahama chappy (some have 2 speeds) put a yt60 jug on it and throw a jemco at it

that thing will hit like 55

Re: QT50 to 60 - bore or replace cylinder

Tate /

madtownmick

May 17, 2009 01:55AM

I would try and find a yt top end personally. They pop up on ebay every week or two and go for not a ton of money. Just seems like all the pieces were meant for each other, you know?

Re: QT50 to 60 - bore or replace cylinder

collin /

kickasscollin

May 19, 2009 12:08PM

im not sure if they are exactly the same piston but search "towny" in ebay and down twords the bottom of the page under international sellers there is a new piston, rings, clips, and wristpin for like $20 with shipping. That would be your cheapest option and you would end up with a brand new piston and everything instead of just new rings. I have the yz60 top end and it works great, never heard of anyone boring out the stock qt cylinder though

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