I'm tearing down a 351M and want to get it bored out, but how big can I go? I know that I can make it a 400 but can I go over that? For this project bigger is better. Also I've heard that the 351M is less desirable than the W's and C's is this true?

Thanks for any help or insight in advance.

 

The 351M and 400 have the same bore size (as do the 351W and C). You're not going to get that much extra displacement out of excessive overboring of the engine, in fact boring too much can be a VBT (Very Bad Thing). In order to make a 351M into a 400 you need to "stroke" it not bore it. You'd need to obtain a 400 crank & pistons. The 400 has an extra .5" of stroke.

Less desirable is a relative term. The 351M has a a much taller deck than the 351W or C. Since the stroke is the same for all 3 motors (3.5") the extra 1" of deck height on the M is taken up by taller (heavier) pistons. This is not the ideal setup for a serious performance motor. The M heads (which are basically 2V C heads) however are good pieces and will outflow the W heads hands down. If you add the 400 crank & pistons you get the best of both worlds, extra displacement, high flowing heads and beaucoup torque from the additional .5" of stroke.

In general the less overbore the better since you run less risk of ending up with an engine which overheats at the drop of a hat. I'd have the machine shop measure it up and see what the least amount you can get away with. I'd say anything up to .040" is fine. If you get up around .060" then get it sonic checked.

 

Other that putting in a 400 crank and pistons, and advancing the timing to zero what else can I do to get the most out of my 351m now turned 400?

 

Get a dual pattern camshaft with higher lift & duration on the exhaust side, 4V intake & carb and long tube, small primary headers. Make sure you get the distributor timing recurved to match the camshaft and take the engine to get dyno tested & tuned.

 

As long as you rebuilding and buying piston bump up the compression to match the cam. Don't go over 9.5:1 max.