should i go with 6m or 7m crank?

[corewitz]

ok heres the low down....
my 90 supra spun a rod bearing and i have torn everything apart for a rebuild. My crank got pretty messed up and the machine shop said it would cost around 300 dollars to regrind, resurface, and balance. A buddy of mine has 3 motors from pre 89 supras that were all in good running condition and hes willing to sell one of his cranks to me for 50 bucks, and i could take his in and just get it balanced and polished. I know there is a difference between the 6m and 7m cranks, but i am wondering if the 6m crank is really that much worse. Wich one would you guys go with? Should I go with my beaten up 7m crank or should i take my buddy's decent condition 6m crank.

 

[Stretch]

It's a nobraner dude. Take the good condition crank, and even if it costs another $50 to have your machine shop polish the journals to make it nice, it's still cheaper than the $300 for your old one. The lighter 6M crank will let you rev a bit faster, but the 7m is supposedly smoother due to the extra counterweights. Regardless, if I were in your position, i would pay the $50 for the good crank, and have my machine shop clean it up. You could take it to them first also, so you know it's good before the money changes hands.
Just for the record i have a 6m crank, not by choice, but I couldn't care less which one I had.
eric

 

[IJ.]

Eric: Not "supposedly" they're much smoother

 

[Jeff Lange]

I dunno, I never felt the 7M crank was much smoother compared to my 88 w/6M crank. They seem very similar to me, but I guess my 88 could just be running very well, I dunno, or the couple 7M-equipped cars I was in were rough, but everything seemed very similar.

 

[IJ.]

Jeff: The 6M "felt" square above 6000 whereas the 7M is silky smooth all the way to 7700.

 

[Stretch]

IJ: We "get" your point.
eric

 

[IJ.]

Eric: LOL I agree with you though if it came to a choice between a shagged 7M and a good 6M I'd go the good crank as it cost me plenty to have my Cranks prepped correctly as they're both 2nd undersize so are through the factory hardening!