What is the thread size for O.S.engines?

[frank mccune]

     Hello:

     I would like to replace the head screws in my older O,S. 35S and .40S engines with SHCS.  Does anybody know the tread sizes and lengths of these screws?

                                                                                                                                               Tia,

                                                                                                                                                Frank

[Steve Helmick]

Well...the closest size that you can buy or order is 5-40 thread. No idea about the lengths. Most or all of the engine tuners put 5-40s in the OS .35S, as well as ST G.21 .46's and .60bb's. The OEM thread was 3m x .6 pitch, which seems to have been changed to .5 pitch. This subject is occasionally brought up, and a search should turn it up.    Steve

[RandySmith]

The only place I know you can get bolt assortment for the OS 30 - 35 S   is Micro Fastners , the have the  6 pitch  3mm  screws  that OS used

Randy

[frank mccune]

   Hi:

    A big thanks to all of you who replied!
                                                                                                           Stay well my friends,

                                                                                                           Frank

[Andrew Hathaway]

The 5-40s will generally thread right into the older SuperTigres, but the Max-S thread seems finer to me.  I didn't push it, but the 5-40s wouldn't thread in any of my OS engines.

[RandySmith]

The 5-40s will generally thread right into the older SuperTigres, but the Max-S thread seems finer to me.  I didn't push it, but the 5-40s wouldn't thread in any of my OS engines.

The 4-40 will  fit the 35S  fairly well, the 5-40 is too large, many people just use the 4-40 bolts

Randy

[RknRusty]

You can screw 4-40 screws into m3x6 holes?
I guess you wouldn't want to unscrew and rescrew it if you didn't have to.