you can just use the 88 piston with the 85-86 cylinder. you would need the spacer plate if you would use the 85-86 piston with your 88 crank. it's worth the money getting the plate they really rip that way.
since your using the 86 cylinder it wont run as good as if you has a 87-89 cylinder. the porting is better on the newer ones plus the bridge ports on the intake make it last longer.
JASON LISKEY
1985 atc 250r ---- finsh line axle,.fmf exhaust and rictor privateer silencer, prm grab bar.
2008 KTM 525xc quad ----- HMF exhaust, HRE chromoly stem,
86 trx 250r --- with after market chromoly frame and everything else done to it
01 400ex --- with 86 trx 250r motor
87 250x + 93 300ex both 350x powered
2 = 86 atc 350x's
86 atc 250r
83 atc 185s --- from retired guy not ridden since 87
Bandito framed 110 with 85 motor and yellow us90 fenders + tank