327 oil pump
What's the difference between the 3821979 oil pump and the later 3764547 pump. Why a change in part # and is the relief pressure correct? Thanks for the info. 
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Re: 327 oil pump
Tim-----
The GM #3764547 was actually an EARLIER oil pump. It dates back to about 1959 and was used on many small blocks, including Corvettes, through the 1962 model year. It was discontinued in October, 1963 and replaced by the GM #3821979.
From 1963 through the 1972 model year, most Chevrolet small blocks used the GM #3821979 oil pump in both PRODUCTION and SERVICE except L63-65 and 70-72 Corvette solid lifter small blocks. The 3821979 continued to be the SERVICE oil pump for 59-62 Corvette applications, too.
For the 1973 model year, things changed. The GM #3764547 "came back" and was thereafter used in PRODUCTION for all 1973 through 1985 Corvette small blocks. Also, in February, 1974, the GM #3821979 was discontinued from SERVICE and replaced by the GM #3764547, the same pump that, in October, 1963, replaced it! Proving, in this case, what goes around, comes around.
So, what's the difference between the 2 pumps? I do not know. However, I can tell you, for sure, that the pumps are functionally interchangeable AS A UNIT. As far as the installed pressure relief spring goes, it gets complicated. Both pumps use a "standard" pressure relief spring, but it was not exactly the same. At least, it was not exactly the same until the 3764547 "came back" in 1973. The 1955-62 version of the relief spring, installed in the 3764547 as well as earlier pumps dating back to 1955 was GM #3835896 (yes, this is a 1955-era part number even though it doesn't look like it). This is a standard pressure spring of 5/16" OD and 2-11/16" free length. The spring used in the GM #3821979 pump was GM #3814903. This is also a "standard pressure" relief spring and is 2-7/32" free length.
As far as I can tell, when the GM #3764547 "came back" for 1973 it was equipped with the GM #3814903 spring. The GM #3835896 spring was discontinued from SERVICE without supercession in March, 1979 indicating that GM did not consider the GM #3814903 a proper relacement.
That's all I can tell you about the 2 pumps.In Appreciation of John Hinckley -
Re: 327 oil pump
Joe, That's great information, I appreciate it. I purchased a 3764547 that looks like it's in a late 70's/80's box. I will post a pic of the relief spring when I take the lid off to check end clearance. I have read much about the bad castings Melling had with replacement pumps, you know this pump casting is good.
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Re: 327 oil pump
Timothy----Joe, That's great information, I appreciate it. I purchased a 3764547 that looks like it's in a late 70's/80's box. I will post a pic of the relief spring when I take the lid off to check end clearance. I have read much about the bad castings Melling had with replacement pumps, you know this pump casting is good.
The problem with the Melling pumps was not the defective castings, per se. It was a revised casting that I believe GM was at the root of. I believe Melling has gone back to the original design. At least, they have the original style available. I think that Melling now makes the GM pumps.In Appreciation of John HinckleyComment
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Re: 327 oil pump
I have just been through the Melling question. The 55 High Volume pump is not recommended for solid motor mount applications. The neck breaks from vibration. They do have a pump , the 155 HV for racing applications which is what I switched to.Comment
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Re: 327 oil pump
I took this new 3764547 oil pump apart to check end clearances (.006) and am wondering what this mark means on the idler gear. The 63 shop manual states put the smooth side toward cover when assembling and this one looks to me like the smooth side is in the housing.
Pic shows mark on idler gear and assembly as I took the cover off. Also, check out the mounting surface where it bolts to the main cap. Was GM quailty that bad in seventies and eighties.Comment
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