64mm and 69mm valves

[ravenrock67]

Hello, I am new to scooters, this site, and the internet in general, so please tolerate my goofs. I have a question about a 49cc to 100cc big bore kit. I have noticed there seem to be two specifications relating to 64mm and 69mm valves. Is there a better method to determine which I have than to tear down my engine and measure? Also, If the kit comes with a cyl. head, why would the differant lengths matter? Thank you for your time.

[90GTVert]

:welcome2:

It's my understanding that most of the GY6 50ccs have the shorter valves. You would at least need to remove the valve cover to tell the difference. If you were looking at 64mm and 69mm valved heads side by side, I believe you might be able to tell just by looks. Not sure if you would be able to tell otherwise without measuring. You might be able to tell by measuring how much of the valve protrudes through the top of the head. Perhaps if you had a curved caliper you may be able to measure from the tip of the valve to the head of the valve without removal?

Even if the kit comes with a head, complete with valves, they usually don't include the rocker arm assembly. Without the proper rocker arm assembly, your valvetrain geometry will be off. Meaning, the valves and tappets aren't going to line up properly, or possibly at all.

[ravenrock67]

OK, the rocker arm geometry makes sense, feel stupid for not thinking of that. But this brings me to another thought. I haven't seen anything on changing the valve size for performance. On a V8, this would be something to be considered. Is there a reason I dont see this on the GY6 racing crowd? Or am I missing the obvious again? :lol:

[90GTVert]

There are big valve heads available. Otherwise you'd need to find valves that would work and have the head machined to fit them. That's more than most wanna get into I think. It may also be necessary to cut new or larger valve reliefs into the piston. That's not a big deal though.