Deep groove pulleys and water pump hub spacing
Back in January we had a lively discussion about the differences between standard pulleys and deep groove pulleys, and between the two different water pump flange dimensions of 5-9/16 and 5-11/16 inches that GM used for small blocks. I have done some research and I have some information to share.
I have a 67 small block with A/C that has the standard-groove 3850838 crank pulley and 3890419 water pump pulley. I started my investigation because I wanted to substitute the 67 L79 non-A/C deep grrove pulley set which used the well known 3858533 crank pulley and 3770245 water pump pulley.
Since the deep groove pulleys have a wider groove-to-groove spacing, this introduces several alignment challenges. It appears that the mysterious water pump hub spacing is part of the puzzle.
One of the attached photos compares the two crank pulleys, showing the measured distances from the surface of the damper to the centers of the grooves. Note that the center line of the rear-most groove (bottom groove in the photo) of the deep groove pulley measures (.450-.330) = .120 inches farther forward than the standard pulley.
Now look at the rear-most grooves (top grooves in the photo) of the water pump pulleys. They measure essentially the same dimension (2.50 versus 2.48).
So, to make the deep groove water pump pulley line up with the deep groove crank pulley, the water pump pulley must be moved forward by a measured .120 inches relative to where the base water pump pulley would need to be. I think this is the 1/8 inch difference (.125 inches) that appears in the water pump hub spacing specifications.
In other words, it appears that the 5-11/16 spacing should be used for the deep groove pulley set, and the 5-9/16 spacing should be used for the standard groove pulley set.
I asked a well known water pump rebuilder how he knows whether to use 5-9/16 or 5-11/16 when he rebuilds a water pump. He said that the rebuilders all just split the difference and use 5-5/8 inches. So, I guess that all of us with water pumps rebuilt by one of the major rebuilders have a pump flange that is off by 1/16 inch in one direction or the other. Apparently that small of a difference is tolerated by most cars.
I have a 67 small block with A/C that has the standard-groove 3850838 crank pulley and 3890419 water pump pulley. I started my investigation because I wanted to substitute the 67 L79 non-A/C deep grrove pulley set which used the well known 3858533 crank pulley and 3770245 water pump pulley.
Since the deep groove pulleys have a wider groove-to-groove spacing, this introduces several alignment challenges. It appears that the mysterious water pump hub spacing is part of the puzzle.
One of the attached photos compares the two crank pulleys, showing the measured distances from the surface of the damper to the centers of the grooves. Note that the center line of the rear-most groove (bottom groove in the photo) of the deep groove pulley measures (.450-.330) = .120 inches farther forward than the standard pulley.
Now look at the rear-most grooves (top grooves in the photo) of the water pump pulleys. They measure essentially the same dimension (2.50 versus 2.48).
So, to make the deep groove water pump pulley line up with the deep groove crank pulley, the water pump pulley must be moved forward by a measured .120 inches relative to where the base water pump pulley would need to be. I think this is the 1/8 inch difference (.125 inches) that appears in the water pump hub spacing specifications.
In other words, it appears that the 5-11/16 spacing should be used for the deep groove pulley set, and the 5-9/16 spacing should be used for the standard groove pulley set.
I asked a well known water pump rebuilder how he knows whether to use 5-9/16 or 5-11/16 when he rebuilds a water pump. He said that the rebuilders all just split the difference and use 5-5/8 inches. So, I guess that all of us with water pumps rebuilt by one of the major rebuilders have a pump flange that is off by 1/16 inch in one direction or the other. Apparently that small of a difference is tolerated by most cars.
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