MAX III porting

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    Greetings all:
    I just bought an OS MAX III .15 which I plan to use in nostalgia gas. However, it’s an RC version and I plan to convert it. Does anyone know if the porting is different on the RC versions and if so what modifications I need to make so that it is the same as the CL/FF version?

    Dean McGinnes

    As far as I know, all the MAX-III variants had the same porting for all versions. I could easily be mistaken. However, if you send it to Bob Mattes, he will optimize the porting and otherwise blueprint the engine. He will also convert it to Nelson plugs. He has done several engines for me and I find his work to be exceptional and prices reasonable.

    His website is listed elsewhere on this site. Go to the home page and look under the dropdown menu labeled “community”.


    Thanks Dean — I just looked at Bob’s site and he has a few paragraphs on port timing which points out that the MAX III engines typically have very conservative exhaust port timing, something he usually fixes by shimming up the cylinder. His site makes for interesting reading!


    I have worked on a few .15s and totally agree that the timing is mild. I tend to keep the liner in the normal spot but machine the ports to increase the timing- that way the head clearance remains the same- Plus, if the engine has already been run, it keeps the piston and liner fit the same- if you jack up the liner the piston is further down the bore which tends to be slightly bigger and thus (sometimes) changes the fit.

    If I remember right I opened up the exhaust to 155 degrees or so and the intake to 135 or so. I never really found the point where it was optimized. Crank timing can also be opened up to about 55 degrees ATDC. Put as big a venturi as possible on it. Plus, I converted it over to a K&B style needle valve.

    Going to a Nelson plug will definately be an improvement. Plus I would set the head clearance to about .007-.010″.

    I did run on on a 7 x 2.75 FAI prop and got over 23000 on 10%….

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