minneapolis moline big mo 400M

rcflyer74

New User
I have a big mo 400M. when I took the heads off and checked the
piston size it is 3.756 but the book size is 3.625 (3 5/8).
when i went on line for parts all I can find is the 3.625 size for head gaskets sets.
Any ideas
 
Looks like the engine was rebuilt at
some point using overbore piston.
Most of the time a "standard " gasket
allows for overbore. Otherwise,
use gasket for a 220 engine (U302)
which is 3.750.
 
A BIG MO 400M (military) should have 3.7518-3.7522 cylinder bores, the same as a U302.
A BIG MO 400 should have 3.6255-3.6265 cylinder bores the same as other OHV 206's.
Head gaskets for a U302 should work fine on the big mo 400M.
You should be aware that the bearings and I think the crankshaft used in the 400M are different than those used in the non-military engines.
 
Can anyone tell me why the engine runs and dies, just can't seem to keep the thing running, I cleaned the carb but still the same thing. at a loss.
 
rust, dirt crude in the fuel tank and lines?? doing a winter refit on my mm m5 and tank is coming out to go to rad shop for cleaning, had same issues when we got it running this summer (after sitting in the trees for 20 years) flushed it out the best I could to keep going but I know it needs a proper clean up.
 
Check the flow rate from the tank, you may have an obstruction. Do you have an add on inline filter? Some of them will not flow enough for a gravity system.
 

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