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    🙂 A little help please….I need some 1/2″ to 3/4 ” flat head machine screws to backplate mount an OS .28 & .32. I’m not sure what size they are or where to get them. 😥 Ideas please.

    Dean McGinnes

    The bad news then the good news.

    Depending on how old the engines are, it may be an odd thread. On my MAX-III engines, .19, .29, & .35, they are an odd metric thread. On the .19 those screws are not even made anywhere. I am not sure of the larger engines.

    The good news is that it is not too difficult to convert. According to Bob Mattes, the larger engines are so close to 5-40 that they can be substituted directly. You will have to slowly screw in the 5-40s with a lot of oil but the threads will move around enough. You want at least one diameter, more is better, of thread engagement.

    I think your engines are later so they may have standard metric screws, available from Micro Mark or others.

    Thread pitch gauges are cheap or you may know a machinist type that will let you use theirs. You want the thread pitch and diameter in metric sizes, and look on Micro Mark’s web site. If they are standard metric, you will be in luck.

    I have converted my OS .06 to radial mount, converting the 2.5 X .8mm screw holes to 4-40, using a thread forming tap. This is not the normal tap that cuts threads. It rolls the metal into the grooves. These taps are available from someone like They are also called “roll” taps. You need a plug tap to get the threads started, then a bottoming tap to finish the threads to the bottom of the hole (hence the name). I think these taps are around $12.00 each. You just have to go slowly with lots of oil. 1/8 turn into the metal then backing out 2 turns. You will have to drill out the backplate to clear the 4-40 screws, but the holes in the crankcase are fine like they are.

    Incidently, .8mm is identical to 32 pitch!

    On my .19, I did it the hard way. Not knowing about the thread forming taps, I machined a new combination backplate and radial mount from aluminum round stock. Really, really tedious. It took three tries to get one that worked. The thread forming taps are the way to go.

    You may have to go to 5-40 on the .28 & .32.

    Here is the URL for the page on website

    Copy this into your browser window, go to the image saying “True Lead Taps, then click on “View all SKU’s” That will give you a list of all they stock, and you can click and order to your heart’s content. Again, you will need a plug and a bottoming tap of each size.

    Hope this helps 🙂


    Thanks Dean for the help…I will be contacting my machinest friend tonight.
    DD 😀

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