Hey fellas, I have a 351M that is a pre March 2, 1977 from the Michigan Casting Center. I have read that these specific motors, from MCC that are pre 3/2, '77 are prone to the blocks cracking. I noticed my casting date when I pulled the motor in order to do a mild rebuild, but the motor has been running fine and I have not noticed any cracks in the block valley that are common for these. My question is, does every 351M from MCC that's pre '77 eventually crack? If there is no cracks in it now then should I not worry about it? Just trying to decide if I need to go ahead and change the block to a post 3/2 '77 MCC or CF block. Thanks!

 

If it isn't cracked now after 40 years of use it must be good I'd use it.

 

True story
Not all of them cracked
Pound the crap out of it and recheck in another 40 years

 

My '77 351M isn't cracked either, it is a MCC from January 07, 1977, my truck was built March 1977. If it has the smooth valley surfaces, it is not a given that it will ever crack, but some did. I have owned it since 1986, I have read a lot on it, I know it can happen, maybe easier than the later blocks, but I never heard or read anything indicating cracks were common.

 

Their rate of cracking isn't anywhere close to other crack prone castings like Dodge Magnum cylinder heads. I've only had one set of those come through our shop in the last 5 years that wasn't cracked. If yours isn't cracked it is probably fine.

 

Almost certainy a very small percentage. I don't know how many blocks they cast at once but probably only one position out of the six or ten or whatever that was affected. Just my speculation of course.

 

My '77 has the "bad" block with 150K+ miles and no cracks, and I checked closely. I'm not changing it out.

R.