Crankshaft ID: LBZ vs. LLY

[McRat]

Best info I have is that the LB7 and LLY cranks are the same, and the LBZ and LMM cranks are the same.
But the two cranks are not interchangeable. The LLY/LB7 crank has ~ .075" larger counterweight radius which will hit LBZ/LMM pistons or be really, really close to hitting.
And the LBZ/LMM crank needs more external balancing weight, the flexplate has 4 large holes to balance, and the LLY/LB7 has only 3. The harmonic balancer on the LBZ is about 180° worth of weight, and the LLY is only 120°.

If the flex plate or harmonic balancer is still in place, it's easy, but sometimes it isn't.

You can measure the counterweight radius with calipers, but that's a bit iffy if you aren't really confortable with calipers. From the top of the rod journal to outside of the weight at BDC, it's 4.110" on the LBZ and 4.185" on the LLY.

But there is an easier way. There is a lump on the casting of LBZ cranks on the counterweight closest to #2 cylinder (second counterweight from the front). See pics:

 

[Smokin LB7]

I know 2 LB7's running LBZ cranks...

 

[skintback]

So how goes putting lb7 pistons on a lbz crank and rods?

Just balancing right?

 

[x MadMAX DIESEL]

Best info I have is that the LB7 and LLY cranks are the same, and the LBZ and LMM cranks are the same.
But the two cranks are not interchangeable. The LLY/LB7 crank has ~ .075" larger counterweight radius which will hit LLY/LB7 pistons or be really, really close to hitting.
And the LBZ/LMM crank needs more external balancing weight, the flexplate has 4 large holes to balance, and the LLY/LB7 has only 3. The harmonic balancer on the LBZ is about 180° worth of weight, and the LLY is only 120°.

If the flex plate or harmonic balancer is still in place, it's easy, but sometimes it isn't.

You can measure the counterweight radius with calipers, but that's a bit iffy if you aren't really confortable with calipers. From the top of the rod journal to outside of the weight at BDC, it's 4.110" on the LBZ and 4.185" on the LLY.

But there is an easier way. There is a lump on the casting of LBZ cranks on the counterweight closest to #2 cylinder (second counterweight from the front). See pics:

found that small typo

 

[x MadMAX DIESEL]

I know 2 LB7's running LBZ cranks...

They are probably machined to lose that extra area with a different balancer, or they have pistons to match the Z crank?

 

[ripmf666]

I know when they balanaced my motor with the LBZ crack with Crower rods and LB7 piston they had to take alot of weight out of the crackshaft.

 

[TheBac]

All kind of makes sense.

Say, dont LBZ pistons have shorter skirts than LB7s?

 

[McRat]

found that small typo

Thanks!

Yes, you can use the LBZ crank for LB7-LMM engines. You cannot use LB7/LLY cranks in LBZ/LMM engines.

You should use the flexplate and balancer that is right for the crank.

In my case, I was going to put together a backup engine, and I wanted to make sure I got the right balancer and flex plate. Since I use LLY pistons, I can use either type of crank. I have 3 cranks sitting on the bench with no marks (my SOP) so I had to find a way to ID them easy.

And I write my notes in here so I don't have to figure it out again.

 

[McRat]

All kind of makes sense.

Say, dont LBZ pistons have shorter skirts than LB7s?

The LBZ has a shorter skirt coated with teflon, brass bushings in the wrist pin bore, and more material on the bottom of the wristpin, which is why the crank weights must be smaller radius.

Pic, L = LBZ (used as ashtray outside) and R = LLY piston.

 

[JoshH]

All kind of makes sense.

Say, dont LBZ pistons have shorter skirts than LB7s?

Yes and no. If you are looking at them from the wrist pin side, the LBZ is longer than the LB7 piston, but if you are looking at them 90 degrees to the wrist pin, the LB7 has a tab that hangs down farther than the skirt on the LBZ.

 

[TheBac]

Thanks guys. I was thinking the piston info would be good to see to flesh out what Pat was saying.

 

[ripmf666]

We had to get a block to replace the LB7 that was built and machined wrong by a local machine shop. The block the we bought was from Tony and it was a LBZ block and the LB7 crank works in it. The only thing Gm changed on the LCMs were the front ear mounts that were on some and not others to mount in the mid duty. And the main cap bolt threads are deeper in the block other then that not much else changed other then some say the LBZ block is stronger.