1965 corvette water pump.

[August M. Heilig]

Does anyone know the pt# for the small block water pumps with the bypass for 1965 vette?

[Jack H.]

Re: 1965 corvette water pump.

GM's service replacement PN was 3957981 (don't know what the factory original PN was as the pump was installed on the engine sub-assy shipped to St. Louis). Most looking for a factory concours correct water pump don't go by the PN because that doesn't appear on the part, can't be seen, can't be judged. Judges go by the raw casting number on the waterpump housing (3859326) and visually verify the configuration of the pump agrees with known Corvette original pumps.

This is a reasonably rare pump because it was used on Corvette and Camaro Z28 ONLY. The same pump housing was used in many truck applications from that era, but secondary operations performed on the housing to take it to the final PN were different.

The one that's most visible is the upper boss was drilled and tapped for a LARGER external bypass nipple (3/4-inch NPT) in truck applications while the high performance Chevy passenger cars used a smaller (5/8-inch NPT) bypass nipple. So, judges scrutinize the pump surface surrounding the bypass nipple to see if it's been 'massaged' by the restorer (did he install a reducing sleeve?) and commensurate originality points are deducted for 'truck' water pumps obviously converted to 'Corvette' water pumps.

[Joe L.]

Re: 1965 corvette water pump.

Jack and August-----

There were 2 different waterpumps used on 1965 Corvette small blocks, depending upon which engine option was installed. Small blocks with 250 hp or 300 hp (L-75) engines used pump casting number GM #3782608. This casting has no provisions for external bypass. Cars with 350, 365, or 375 hp engines used casting GM #3859326 with a 1/2" NPT bypass fitting on the top. Castings with the 1/2" NPT fiting are rare; later castings with the 3/4" NPT fitting are not-so-rare. These can be converted indiscernably with a bushing reducer IF it's done carefully and cleverly. For the 1965 model year, the 3782608 castings were dated; 1965 3859326 castings were NOT dated.

[Jack Beebe]

64 L76 water pump

I've been following the recent water pump postings re: 326 pumps, original and service replacement. Looking at the service manual and the AIM, and the posts, I'm still not sure of the bypass question. I have a rebuilt 326 pump with a 5/8 NPT hole on the top. The pump on the car has a 5/8 or 3/4 plug but no bypass is attached. What is correct? My car is July mfg., mostly original and correct, no a/c or ps.
Thanks

[Joe L.]

Re: 64 L76 water pump

Jack-----

First of all, I can tell you this, for sure: if you have an L-76 and you have an original intake manifold there DEFINITELY SHOULD be a bypass between the front of the intake manifold and the top of the waterpump. ALL L-76, L-79 (except 1968), and L-84 engines used EXTERNAL bypass. There was a "90 degree" elbow fitting attached to the waterpump (1/2 NPT on water pump end and hose nipple on the other) and a straight fitting attached to the manifold (1/2" NPT on manifold end and hose nipple on the other). These were connected with a short section of hose and clamps. ALL of the above-referenced engines used this; NO exceptions. If your water pump fitting is plugged, then someone removed it. In doing so your engine has been "converted" to internal bypass assuming that the 3/8" holes in the block and waterpump leg are open and clear. Otherwise, you have no bypass. In that case, you don't want your thermostat to fail in the closed position. That would result in near-instantaneous and catastrophic overheating of the engine.

As far as the fitting on the top of the waterpumps, only 2 sizes were EVER used. The "early" style, such as originally used on your car, was 1/2" NPT (or, 1/2-14). Some truck pumps and most all GM SERVICE waterpumps used a 3/4" NPT (or, 3/4-14) fitting. There is NO SUCH SIZE as "5/8 NPT"; it is a non-existent size.

Now, what confuses folks is that 1/2" NPT (or, 1/2-14) DOES NOT MEASURE 1/2" in diameter. 1/2 NPT measures about 0.85" in diameter. 3/4" NPT (or, 3/4-14) DOES NOT MEASURE 3/4" in diameter. 3/4" NPT measures just a little over 1" in diameter.

[Craig S.]

Re: 64 L76 water pump

Joe - as I recall, the sizing in the pipe series relates to the internal flow size of the iron pipe, resulting in the much larger OD sizes......Craig

[Jack Beebe]

64 L76 water pump

Thanks Joe, that lifts the fog. So, if I replace my existing converted by-pass pump, should I go back to the by-pass hose?

[Joe L.]

Re: 64 L76 water pump

Jack-----

Absolutely YES.

[Jack H.]

Salute to brother, Joe!!!

Gosh, watching for that ole Easter ***** down the ***** trail (a foot of fresh snow here in the mountains outside Denver), I done shot myself in the foot....

Could have sworn we'd narrowed the '65 SB pump question to a high performance engine, but shore nuf we didn't!!!

Joe's absolutely right, amigos! There were two water pumps used in that era on SB engines (with and without external bypass) and the lower performance '208 casting pumps are relatively easy to come by.

Also, I sit corrected on the exact size of NPT bypass nipple used for the '326 housing. On Joe's assertion the bypass nipple is 90-degree angled, well, there might be some difference of opinion here (90-degree vs. 88-degree + exact supplier ID marks embossed on factory orignal vs. current reproduction nipples), but that's an esoteric, 'small potatoes' issue for judging 'purists' to yak about over cocktails.

The key point was, original post specifically asked for correct PN of the water pump. I took the bigger picture avenue that PN wasn't what judges used to guage factory authenticity because it wasn't recorded on the physical part installed on the engine and judges went by cast numbers on the water pump housings plus visible configuration of the pump.....

I think we've beaten this to death, but Joe may be one of the few sources who has the data base to actually answer the orignal poster's question....what was the factory original PN for production SB water pumps in '65 vs. the casting number on the housing and the ancillary external configuration details we judge by?

This gossamer distinction (PN vs cast # and visible configuration) would only be meaningful if someone saw a pump in GM original box or had access to some stache of untouched GM origninal service spare water pump inventory.... I know I can NOT answer that question and took my best, Easter ***** distracted, shot at supplying an answer!

[Joe L.]

Re: Salute to brother, Joe!!!

Jack-----

As far as the angle of the fitting goes, please note that I used the term "90 degrees" and not 90 degrees. The term enclosed between quotation marks means "about 90 degrees". The fittings were NOT exactly 90 degrees.

As far as the part numbers for the water pumps, I do not have the PRODUCTION part numbers as they were an internal-only GM number. The only way to determine them would be to have access to the engine assembly manuals or certain other internal GM documents and I don't have those, particular, documents.

The waterpumps available in SERVICE were not the same as PRODUCTION waterpumps. For one thing, SERVICE waterpumps usually have dual bolt pattern hubs, especially those sold after about 1969. Also, most SERVICE waterpumps sold by GM for small blocks had the external bypass regardless of engine application. In addition, this bypass was drilled and tapped for 3/4" NPT going as far back as I can find. In the early days (early to mid 60s), the SERVICE pumps were supplied with a reducing bushing for the 3/4" to 1/2" conversion for applications so requiring and a 3/4" NPT pipe plug for applications requiring no external bypass. Later, in the mid 70s, or so, the reducing bushing was eliminated from the waterpump kit and only a 3/4" square head NPT plug was supplied.

I can find no record that any SERVICE waterpump was ever available which used the 3782608 casting (no upper bypass boss) or any pump using the 3782609 OR 3859326 casting with a 1/2" NPT bypass fitting. Both the 3782609 and the 3859326 castings used for SERVICE were manufactured with 3/4" NPT fittings. Consequently, no SERVICE waterpump from GM for 55-70 models was ever the exact same piece used in PRODUCTION on ANY Corvette.

[Joe L.]

Addendum

One other thing: the reason for the single bolt pattern hubs in PRODUCTION and the dual bolt pattern hubs in SERVICE is simple. It costs extra money to drill and tap 4 extra holes in the waterpump hubs. For PRODUCTION, it's cheaper to use a single bolt pattern hub and supply the specifc pump part numbers which includes these hubs to the assembly line. For SERVICE, it's MUCH cheaper to use a dual pattern hub and stock fewer part numbers in GMSPO inventory. Plus, the slight extra cost of the dual pattern hub is passed on the the parts buyer through the list price of the part.

The situation changed for 1971-82 Corvette waterpumps. The original 1971-E72 Corvette waterpump, GM PART #3998205 (casting #3991399) was the same for PRODUCTION and SERVICE. All PRODUCTION examples of this pump had a single (large) bolt pattern hub. Most, if not all, SERVICE examples also had a single pattern hub. That's because this pump had NO other application besides Corvette. So, no second bolt pattern was necessary. Some rebuilt pumps may have a dual pattern hub installed, though. Also, this was the only non-external bypass waterpump for a small block ever available from GM in SERVICE going back to, at least, 1960.

In mid 1972 GM did something that they hadn't done before. They used, basically, a "one-size-fits-all"-type waterpump in PRODUCTION for Corvettes. The 71-82 Corvettes used a waterpump hub pattern and pilot configuration that was unique to Corvettes and some 1972+ MD and HD trucks. This pattern uses a a "large" (3-1/4") bolt circle IN COMBINATION WITH a 3/4" pilot. NO other Chevrolet car or truck used this combination. Many used the 3-1/4" bolt circle, but NONE used the 3/4" pilot. So, the 71-82 Corvette and MD/HD trucks were unique in this regard.

The trucks used exteral bypass, though, whereas the Corvette used internal bypass for all 71-82 small block applications. So, in 1972 Chevrolet apparently decided that they weren't going to keep making the 3991399 casting for Corvette applications only (this is the casting for non-external bypass). So, they changed the casting to GM #330813 and the unit to GM PART #6258551 and they used this pump for both PRODUCTION and SERVICE for mid-1972 to 1982 Corvettes and 1972 to, at least, 1978 MD/HD trucks.

That's why mid 1972 to 1982 Corvettes had that funky looking, square head 3/4" NPT plug on the top of the pump. For 1955-70 Corvettes with internal bypass this plug indicates a SERVICE waterpump, GM or otherwise. But, for mid-1972 to 1982 Corvettes, this plug is a PRODUCTION feature. Any such Corvette without one is not as originally equipped. Like it or not.