372xp Big Bore w/ Tillotson carb (a true 80cc)

Hyway 372xp big bore cylinder which would be for a Husqvarna 372xp chainsaw (52mm x 38mm). The crank bought by Don Butler initially claimed to be balanced to 52%, turned out to be only 35%. To rectify this, I bored out the original holes and drilled them deeper, achieving 50% balance. The reciprocating mass was 156g, and I removed 23.4g to reach 50% balance.

I inserted brass bushings (10x12x8) in the piston to accommodate a 10mm wrist pin so I could still use a wrist pin needle bearing and not a 12mm wrist pin and a bushing on the conrod instead. The case is an undrilled YD100 case (super special), and I modified the transfers because the original transfers are horrific. The 66cc/69cc has better transfers. Additionally, I added an M3 to M5 fuel fitting to run the impulse line on a tillotson carb.

After building an MS440 last year, I've developed a preference for hybrids over "china dolls." I'm excited about this build, especially with my first experience with diaphragm carbs, which offer easy jet adjustment and can run in any position without flooding.

For those interested, here are the port timing and durations for the Hyway 372xp big bore cylinder on a 38.5mm stroke with a long rod.

Exhaust - 94° (172°)
Trans - 116° ATDC (128°)
Intake - 73° BTDC (146°)
Blowdown - 24°


  • 372xp final.jpg
    372xp final.jpg
    167.9 KB · Views: 317
  • IMG_20240116_220853066.jpg
    155.6 KB · Views: 272
  • IMG_20240116_220427061.jpg
    126.2 KB · Views: 70
  • IMG_20240119_174249103.jpg
    168.1 KB · Views: 40
  • IMG_20240228_034645834.jpg
    129.1 KB · Views: 33
  • IMG_20240217_161749603.jpg
    188.7 KB · Views: 32
  • IMG_20240217_161832635.jpg
    183.6 KB · Views: 31
  • IMG_20240228_033546413.jpg
    202.9 KB · Views: 34
  • 372xp balanced crank.jpg
    372xp balanced crank.jpg
    189.3 KB · Views: 34
  • IMG_20240410_061741642.jpg
    143.7 KB · Views: 32
  • IMG_20240410_045853868_HDR.jpg
    195 KB · Views: 33
  • IMG_20240118_191400706.jpg
    139.7 KB · Views: 50