Flat head max bore???


Just curious what is the maximum bore size you have ever seen a 5 horse flathead, a max effort type build? Around what size do the blocks start breaking?


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The fat blocks (135 series) give you some more room to go than the older blocks for sure..
An aluminum bore block that has had the center two head bolts moved out like the Honda flatheads or Blockzillas would be the first big step. We use to offset these holes and use 3/16" bolts in them to do it without welding the outside of the block. If the block is bored, honed, then Nikasil plated would be the largest you could go with one without serious welding.
When you go that thin on the cylinder wall you start to get a lot of distortion (which the plating won't like.) Using a thicker sleeve is good, but then you are more limited to bore size.
Keep in mind that going from standard bore to +.030" over only equals 1/3 of 1 cubic inch. T Cubic inches is only one way that bore size helps these engines. :)

🏁Thanks and God bless,
Brian Carlson
Carlson Racing Engines
Vector Cutz
Carlson Motorsports on Facebook
30 years of service to the karting industry
Linden, IN


Dawg 89
.192 was the standard large bore .
.100X.100 makes a killer engine .
My .155 has seen many seasons .
.060 is about it on a steel bore with out a sleeve, and you may bore through there .