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Thread: 1970's air head cylinders

  1. #1

    1970's air head cylinders

    Just curious ,does anyone know if there is a difference other than bore between a 1971 r75/5 and a 1975 r90 cylinder? I have 2 stock bore pistons (r90) and there are a pair of 1975 R75/5 cylinders on e-bay , is it possible to bore the r75's out and use them on a r90 ?

  2. #2
    Liaison 20774's Avatar
    Join Date
    Sep 2005
    San Antonio, TX
    It appears that the difference in bore is 6mm (82 for the R75/5 and 90 for the R90S). That means approximately 3mm had to be removed from around the circumference. Seems a bit much...not sure how much meat is there on the cylinders. I just read a post from several years ago by Matt Parkhouse on the Airheads site and he said you'd run out of iron lining. Apparently Reg Pridmore made an 840 kit which was about as far as anyone can take the 750 barrels.

    Plus there's the issue of opening in the case... At some point, this opening changed dimensions and the cylinders changed to fit.
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  3. #3
    Registered User PHMARVIN's Avatar
    Join Date
    Oct 2003
    El Paso, TX
    To answer your question, NO, you cannot bore the R75 cylinder to take the R90 piston. You can bore it to take R80 pistons, but not R90. A 1975 cylinder will fit on a 1971 block and vice versa; not so with 1976 and later.
    Ride Safe,
    Phil Marvin - El Paso, TX
    '94 K75A/3
    '95 K75RTP

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