The 139QMB scooter engine is a popular small engine used in many scooters and mopeds. In this detailed video, we break down ...

According to Air Flow Research, bronze guides have a longer life expectancy than OEM guides, and will work better in ...

For the purposes of this article, we'll be focusing on valvetrain stability rather than valve-guide wear, which is another story entirely. See All 15 Photos. Implications of a Valve Change.

The guide is also machined at the top to mount a positive-style, 0.560-inch-diameter valve-guide seal. All of this conspires to limit total valve lift to a safe figure between 0.420 and 0.430 inch.

Concentricity between valve guide bore and valve seat face a major problem. Also, on my daily driver, the rocker arms started pissing out pin bearings at 3500 miles. Replaced those in 2008 ...