66 427 water pump back plate & shaft

[jim Larson]

Have a question regarding the correct back plate for a Nov 65 dated water pump casting # 3856284. It appears that the design of the back plate was changed sometime after the 65 396 sytle back plate. My Nov 65 water pump has the same style back plate as the 65 396, the flate style. Could this be correct for a Nov 65 pump? Pump has been rebuilt.

I also have a rebuilt sept 66 dated 284 casting and the back plate style is different; but I don't know if this is correct for either as I don't know what the back cover is suppose to look like. It is basicaly flat; but has a large diameter ring stamp into the plate and 5 of the side have a little bend one way and the 6th side is bent just the opposite, so the plate can only go on in one of two positions. Could someone send me a photo of the change in design? Or post one here? Or based on my discription tell me if they think it's possible correct and for what years?

Regarding the shaft, what was the design change from the 65 396 to the 66 427? Why the change? Did this change the mounting height of the pulley flange?

Thanks, Jim

[Jack H.]

Re: 66 427 water pump back plate & shaft

The current thinking is the 'flat' rear plate for the BB water pump was used through '66 into 'somewhere' in the 1967 MY where it changed to the 'improved' plate that's of stamped steel construction with raised 'lips' on the outside edges. Your copy of the '66 Judging Guide says:

"...The pump backing plate is flat and held in place by slot pan-head screws..."

The '67 JG book is silent on the subject of the BB water pump's rear plate as well as the style of fasteners holding it in place.

[Jack H.]

Re: 66 427 water pump back plate & shaft

The current thinking is the 'flat' rear plate for the BB water pump was used through '66 into 'somewhere' in the 1967 MY where it changed to the 'improved' plate that's of stamped steel construction with raised 'lips' on the outside edges. Your copy of the '66 Judging Guide says:

"...The pump backing plate is flat and held in place by slot pan-head screws..."

The '67 JG book is silent on the subject of the BB water pump's rear plate as well as the style of fasteners holding it in place.

[Joe L.]

Re: 66 427 water pump back plate & shaft

Jim-----

1965 big blocks used waterpump backing plate GM #3734990. This is the "flat" backing plate. It was also used for 1958-65 348 and 409 waterpumps. This plate was used in conjunction with pan head, slotted screws with captive external tooth lockwasher.

For 1966, the backing plate was changed to GM #3878262. This is the style plate with the "center ring" and "turned-up" perimeter reinforcements. I have seen NOS pumps with late 1965 casting dates that have this style plate, so I believe that it was used for most, if not all, of the 1966 model year. I suppose it's possible that VERY early 1966 might have used the 1965-style plate, though.

In any event, the plate attaching screws are incorrect on either of the 2 pumps you have pictured. All original 1965-66 pumps that I have seen, either known original or NOS with 1965 or 1966 casting dates, have the pan head screws. Also, I have 2 NOS pumps with late 1966 casting dates (for 1967 model year) and they have the pan head, slotted screws, too.

I don't know of any difference in the shaft/bearing assembly for 1965 versus 1966. It's possible, I suppose, that there was a very minor difference in length for unknown reasons, but I cannot imagine any other difference. Basically, the shaft/bearing assemblies were unchanged through 1970.