Note: Use certified calibration instruments with valid calibration certificates.
This is the tricky part. Dealers rarely include a detailed shop manual. Here is where to look: 253fmm service manual
This is where DIYers fail. The manual shows the exact offset of the ring gaps (usually 120 degrees apart). It also specifies that the "R" mark on the ring faces up. This engine is a clone of the legendary Honda CB/XL 250
Before we dive into the manual, let's decode the stamp. On the 253FMM: 253fmm service manual
This engine is a clone of the legendary Honda CB/XL 250. While Honda manuals exist, the Chinese manufacturing tolerances differ slightly. You want a manual tailored to the 253FMM specifically (usually published by Loncin or Zongshen).
Because the 253FMM is a clone engine, finding an official "Lifan" binder can be difficult. However, you have three excellent options:
The 253FMM has 4 or 6 head nuts. The manual contains a criss-cross loosening sequence. Loosen them in the reverse order of tightening. Failure to follow this warps the head. The tightening torque is usually 32-38 Nm.