6M/7M crankshafts

[toy fanatic78]

Exactly when did they change the crankshaft?How do you tell the difference?I bought a hard wrecked 88 for the engine,my 86.5 was knocking lightly.The 88 runs good,car has 110k on it,but the PO said the motor had been changed,and he thought it was from an 87.Pros and cons of the different cranks.

 

[figgie]

6m is not counter balanced.
7m is counter balanced

7m weighs more than the 6m but does not "whip" as much as the 6m.

SupraRich has pictures of both.. look him up.

 

[TheNewRed]

6m cranks are labeled "6M" on the edge of one of the middle weights, they also have weird markings(they look like Japanese symbols?), that i did not see on my 7m crank. You can just tell, the 7m crank is a lot more beefeier!

 

[Nick M]

6M is counter balanced. All crankshafts are. Not all crankshafts are balanced to zero by drilling or weighting.

 

[IJ.]

6M
[thumb]http://ij.supramania.com/crank01.jpg[/thumb]

7M
[thumb]http://ij.supramania.com/crank02.jpg[/thumb]

 

[92nsx]

IJ in with the best post again!

 

[toy fanatic78]

6M
[thumb]http://ij.supramania.com/crank01.jpg[/thumb]

7M
[thumb]http://ij.supramania.com/crank02.jpg[/thumb]

:bowdown::bowdown::bowdown::bowdown::bowdown:
Thanks IJ.I like pictures:icon_bigg.Which crank would you prefer for an na that might eventually go na>t?

 

[Doward]

Lightened and knife edged 7M crank

 

[AJ'S 88NA]

Lightened and knife edged 7M crank

+2

 

[suprarich]

7m

6m

7m above, 6m below

 

[TheNewRed]

Nice! I have a question now, if you look closely at the 3-5 rod journals on the cranks in Suprarich's pictures, you can notice a slight discoloration right outside of the journals. Is this a sign of heat fatigue? i see a lot of cranks like this, 6&7M alike.

 

[suprarich]

The rainbow color you see on all stock cranks is from heat treatment at the factory.

 

[IJ.]

"Induction hardened"

 

[Tanya]

I'm using a normal, but balanced 6M crank with my 6M engine and have no idea what this "whip" is that you say IJ.... my 6M has 10:1 compression and is rev happy with the internals balanced and machined stock flywheel


*shrug*


My preference would be to use the 6M crank > 7M for an NA build but for a turbo build, 7M would probably be better.

.... but don't listen to me, Im just a girl

 

[IJ.]

Tanya: Once I got to 500rwhp I started seeing really odd wear patterns on the Main bearings that I can only put down to crank flex/whip when turned fast for extended periods.

ie: Long pulls above 8000 rpm at high boost.

Not the normal situation for these engines and might apply to .05% of members here but figured I'd report my findings as I'm just a guy :icon_razz

 

[Tanya]

Ah, gotcha. I really thought that the 6M crank "issue" was apparent at all specs, whether stock or not. I thought I had read somewhere that you can feel a difference between engines running the different cranks.

I only have experience driving one 7M car, with the 7M crank, and I didn't "feel" it acting any "smoother" than my 6M.

Or maybe I just didn't care to notice *shrug*

 

[IJ.]

I ran my motor with both cranks identically prepped within 2 weeks of each other (one of my Pauters tapped the Oil Pump AUX shaft and chattered the #1 and #2 crank pins during an 8500rpm experiment)

The 7M is smoother across the board from idle to 6000 once above 6000 I couldn't feel a difference, the 6M while not "bad" was harsher, the 6M accelerates MUCH quicker due to it's lower PMoI.

This is what prompted me to cut down the 7M to around the same weight as the 6M.

[thumb]http://ij.supramania.com/crank03.jpg[/thumb]

 

[wiseco7mgt]

I ran my motor with both cranks identically prepped within 2 weeks of each other (one of my Pauters tapped the Oil Pump AUX shaft and chattered the #1 and #2 crank pins during an 8500rpm experiment)

The 7M is smoother across the board from idle to 6000 once above 6000 I couldn't feel a difference, the 6M while not "bad" was harsher, the 6M accelerates MUCH quicker due to it's lower PMoI.

This is what prompted me to cut down the 7M to around the same weight as the 6M.

[thumb]http://ij.supramania.com/crank03.jpg[/thumb]

Did you cut down the counterweights? is there a weight to ask for with the machine shops? or is it done on an individual crank basis.?

 

[IJ.]

My Crank shop used their CNC to do the work based on a Cosworth counterweight profile, we removed as much as possible without upsetting the inherent balance of the I-6 crank.

Once you get to this point it's a bit of a juggling act weight wise, remove too much and you get a funky balance requiring heavy mallory metal inserts.

 

[TheNewRed]

There is a lot of good info in this thread, thanks guys!