Stihl AV031 chainsaw cap for diagnosing no start conditions 28MM BY 25MM BY 2.0 Pitch

if your cap is cracked this will serve as a temporary replacement. The OEM has a check-valve for expansion

Print it in pla, abs will swell in 5-6 days and be a bugger to remove!

The vented one should be easy to make a small orifice​ with a hot nail that should be about .020"