6M/7M crankshafts

[gofastgeorge]

Don't use the 6M if your going to build a high rev 7M.
(I'm talking up to 8500 rpm)
I did, and constantly broke off flex plate bolts,
not to mention welding the harmonic balancer to the crank nose.

I have done in 2 sets of ARP bolts, and one set of stock.
ARP suggested a stronger bolt,
but they want $100 per bolt & 3 months to do up customs.

So out comes the light weight 6M crank,
(as soon as I get the damn ballancer off)
and I have a ballanced 7M crank ready to go in.
I also dowel pinned the flex plate to the crank, to take some of the load off the bolts.

I may even loosen up the accumulator springs in the trans,
so that it allows a little more slip when it shifts.

I am getting real tired of dealing with this one weak point,
and don't want to shake another flex plate off.

By the way, anybody running an automatic as I am,
ARP has a shorter bolt than the 4AG ones that most Supra places sell.
It is p/n 254-2901, and is .800" long.
The 4AG bolts, p/n 203-2802 are 1.050" long,
and can only be used by bottom tapping the crank holes, and running the ATI flex plate (which is much thicker than the stock one).

 

[IJ.]

I use a dual flex plate and arp bolts and haven't had an issue 6 or 7M cranks to sustained 8300 George.

 

[gofastgeorge]

I use a dual flex plate and arp bolts and haven't had an issue 6 or 7M cranks to sustained 8300 George.

Damn,
Then I may not have the problem figured out yet.......
I wonder if my converter is way out of ballance ?

 

[toy fanatic78]

not to mention welding the harmonic balancer to the crank nose.

I had this problem with the 6M crank also,balancer spun the key and transferred so much metal onto the front of the crank that I could hardly get the old one off,let alone the new one back on.And with slight rod knock(which may have been the balancer "wobbling" on the crank)I had pretty much decided to go with a crank in better condition.Pulled the motor from the parts car apart a couple weeks ago and was glad to see a 7M crank in it,probably send it out for lightening/balance next month.

 

[toy fanatic78]

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.

Would you happen to have the specs on said profile?Like maybe the CNC program coordinates,or what cosworth profile.

 

[gofastgeorge]

I had this problem with the 6M crank also,balancer spun the key and transferred so much metal onto the front of the crank that I could hardly get the old one off.

I am about to start cutting this one off........
It's loose, and moves around,
but will only pull out about .020" before it locks up.
Even with a big ass puller & heat.
I have had this happen on high reving 2-stroke bike engine flywheels too.
So much metal transfers from one piece to the other, it ends up acting like a key, keeping the two from coming apart.
I had to cut those off also.

 

[suprarich]

Damn,
Then I may not have the problem figured out yet.......
I wonder if my converter is way out of ballance ?

Bolts backing out? Worn out holes on the flexplate? Threads in the crank messed up? The converters are self balancing being full of fluid, but it could be off center. In most cases of off center, the transmission input shaft bearing will fail before the flexplate assembly.

 

[suprarich]

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.

IJ is right on with that. If you are just going to lighten the crank, take even amounts of weight off each counter weight going straight in towards the center. This will remove a good bit of weight from the counter weights, but will still leave mass on the outside edge of the counter weight. Like in the pic below, the crank on the left is lightened by 5 lbs, but was cut straight in without knife-edging the counter weights. Leaving the weight on the outside of the counterweights disturbs the balance less.

Now in the next two pics, this crank was lightened and knife-edged. Most of the weight was removed from the outside portion of the counter weights. This really played hell with the balance, which we fixed later with 100.00 dollars worth of 2 mallory metal plugs drilled into thhe counter weights. I eventualy got that crank back to within 1 gram of zero balance.

Long story short, take metal from inside the counter weight and less from the ouside edge and you will have less headaches later trying to balance it.

 

[IJ.]

Would you happen to have the specs on said profile?Like maybe the CNC program coordinates,or what cosworth profile.

Remind me in the new year and I'll see what I can get.

 

[TheNewRed]

Long story short, take metal from inside the counter weight and less from the ouside edge and you will have less headaches later trying to balance it.

Excelent tip, i will have a long talk with the person who does mine, about this.

 

[toy fanatic78]

Remind me in the new year and I'll see what I can get.

I will do that.Thanks.

 

[gofastgeorge]

Bolts backing out? Worn out holes on the flexplate? Threads in the crank messed up? The converters are self balancing being full of fluid, but it could be off center. In most cases of off center, the transmission input shaft bearing will fail before the flexplate assembly.

"Bolts backing out?"
Nope,
Clean breaks right under the heads.

"Worn out holes on the flexplate?"
Nope,
each time it was a different, but either new or excellent condition flex plate.

"Threads in the crank messed up?"
Nope, threads in great shape, even after the head failures.

But I am planning on having the converter checked out.

Everything points at a harmonic problem, or the fact that the 6-bolt crank just can not take 8500 bang shifts.
Hoping that the dowel pins will help take care of the extra shock loading.
And the 7M crank taking care of any vibration issues.

But it is still a weak point.
Toyota didn't go to 8 bolts on the 1JZ/2JZ for no reason.

Sent an e-mail to Crower to see if they can still make a 7M crank.
If so, I'll have them put in the 2JZ bolt pattern.

Anyway, it's back to the garage for me.
I finally had to split the harmonic balancer to get it off.
So hopefully I'll have the 7M crank in the block sometime tonight.
Maybe have the engine back in the car by Monday.

 

[BlackDynamite]

when i had my 88 black supra, i rebuilt the engine because of spun bearings and i came across the same issue. IJ gave those EXACT 2 pictures!

As i did research, I put in the 7m crank for durability and it was the only one i had available for cheap, but noticed a COMPLETELY different rev pattern than the 6m....The 6m in my supra now, revs more than twice as fast as the 7m in my old supra.

 

[suprarich]

"Bolts backing out?"
Nope,
Clean breaks right under the heads.

"Worn out holes on the flexplate?"
Nope,
each time it was a different, but either new or excellent condition flex plate.

"Threads in the crank messed up?"
Nope, threads in great shape, even after the head failures.

But I am planning on having the converter checked out.

Everything points at a harmonic problem, or the fact that the 6-bolt crank just can not take 8500 bang shifts.
Hoping that the dowel pins will help take care of the extra shock loading.
And the 7M crank taking care of any vibration issues.

But it is still a weak point.
Toyota didn't go to 8 bolts on the 1JZ/2JZ for no reason.

Sent an e-mail to Crower to see if they can still make a 7M crank.
If so, I'll have them put in the 2JZ bolt pattern.

Anyway, it's back to the garage for me.
I finally had to split the harmonic balancer to get it off.
So hopefully I'll have the 7M crank in the block sometime tonight.
Maybe have the engine back in the car by Monday.

You have to have some sort of other problem then. There really is no issue with the 6 bolt 6m or 7m cranks not being able to handle power or shock loads. The 4age uses 8 bolts also, but it was not for strenght issues, just a different design.

If you really feel you need a billet crank, try Moldex. I would not want to mess with the year long lead time from Crower or the Minimun 5 crank purchase from Brian Crower.

I don't feel anyone needs a billet crank under 1400 hp, as the stock crank has been proven to that level already with shift points over 9k rpm

 

[gofastgeorge]

You have to have some sort of other problem then.... try Moldex. I would not want to mess with the year long lead time from Crower or the Minimun 5 crank purchase from Brian Crower.

I'll check out Moldex, thanks.

As far as BC, I wouldn't use jack from him, even if he paid me......

 

[4U2QUIK]

Remind me in the new year and I'll see what I can get.

I'd be very interested also. a nc or cad/cam file would be even better.

 

[wiseco7mgt]

Asking for trade secrets guys? tsk tsk lol

 

[IJ.]

Asking for trade secrets guys? tsk tsk lol

LOL none of it is a secret I'm happy to share if I can get the files, only problem there is the code is usually generated for a particular machine so unless you have access it's not going to be of much use.

 

[wiseco7mgt]

LOL none of it is a secret I'm happy to share if I can get the files, only problem there is the code is usually generated for a particular machine so unless you have access it's not going to be of much use.

Ever thought of sourcing 7m cranks and selling a performance crank based on your own spec? Would this spec also be applicable on a 2jz crank.

 

[dan0311]

what size is the standard rod bearings? some places say its .25mm, but my rod bearings say .75mm. what does that mean?

it says .75mm.