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View Full Version : On Center bore spacing: what exactly does it mean?


King_V
08-08-2005, 11:09 PM
Ok, I know when they changed from the 3.8L to the 3800, that there was a redesign. Different electronics, different type of balancer with more "ridges" for the crank sensor, etc.

One thing I'm wondering is the whole idea of "on center bore spacing" . . I figure (obviously) that it has something to do with where the bores are located in the block.

Other than that, I don't know.

So, what exactly does it mean, compared to the older 3.8L design?

Also, what other differences result? I assume that crankshafts have to be different. How about other hard parts? Cams non interchangeable? Heads/intake?

Just wondering...

Keller
08-09-2005, 01:36 AM
One thing I'm wondering is the whole idea of "on center bore spacing" . . I figure (obviously) that it has something to do with where the bores are located in the block.

Yep...you've got it.

This is not the case in the production NA and Turbo motors of the TR era. The cranks have one throw for each cylinder. Which make them somewhat weak, but the rolled fillets help. The separate throw scheme was used whem Buick went from the "odd-fire V6 motors that date from back in the 60's (!) to when they switched to the "even-fire" motors that gave a much smoother idle and revving. Of course, they weren't designed to produce gonzo horsepower originally...

For some block info, see http://www.gnttype.org/techarea/pictureguides/blocks/blockguide.html and
http://www.gnttype.org/techarea/images/guidepics/partspage.JPG which is a page right from the GM order book.

The text on that sheet describes the on-center Stage 2 block as having "cylinder bores on-center of crankshaft and connecting rod journals" - something the producion even-fire V6 did not have. Makes for a stronger setup. Which is why they used that scheme in on the Series II and Series III motors.

However, don't go looking to exchange parts with the newer motors. Nothing will fit. The Series II/III motors are all different. They have balance shafts, 6 bolt mains (four on the bottom, two on the sides), different heads, gerotor oil pumps...just plain different. But a VERY strong bottom end!

By the way, Stage 2 != Series II. The former is the 'race' version of the production TR motor. The latter is the motor still being offered in some production vehicles today.

PaCemkr86
08-09-2005, 09:05 AM
nice info Keller..
http://www.ccstreetscene.com/forums/images/smilies/t-type.gif

King_V
08-10-2005, 08:07 PM
The text on that sheet describes the on-center Stage 2 block as having "cylinder bores on-center of crankshaft and connecting rod journals" - something the producion even-fire V6 did not have. Makes for a stronger setup. Which is why they used that scheme in on the Series II and Series III motors.

Ahh . . got it, on center with regard to position of the rod on the crank. So when they went even-fire, the rods were no longer perfectly straight directly under the pistons?


However, don't go looking to exchange parts with the newer motors. Nothing will fit. The Series II/III motors are all different. They have balance shafts, 6 bolt mains (four on the bottom, two on the sides), different heads, gerotor oil pumps...just plain different. But a VERY strong bottom end!


So, anything that's a C-code 3800 or later (K and L code I think) falls into the Series II/III category then, correct?


By the way, Stage 2 != Series II. The former is the 'race' version of the production TR motor. The latter is the motor still being offered in some production vehicles today.

Ok, I think I follow . . am I correct in assuming that the on-center bore spacing of the Stage 2 block does *not* require different heads (not sure about exact valve positioning over the pistons), and will work with all other standard TR parts?

Thanks again!